The science of low carb and keto
Furthermore, evidence also suggests that natural saturated fats are neutral with regard to health, warnings about the health dangers of red meat are based on weak evidence with very low certainty, and low-fat diets do not appear to have any special health or weight benefits beyond those of a low-carb diet.1.
There is little evidence in the scientific literature that decisively indicates any particular diet is superior for helping people lose weight for more than two years. In fact, most people who use a diet to lose weight regain most or all of it.2 Yet with those caveats in place, on average, carbohydrate restriction outperforms low-fat and other diets for weight loss in studies that follow participants for up to two years.
A number of meta-analyses of randomized controlled trials (RCTs), considered the strongest, most robust type of evidence, have come to the same conclusion: low-carb diets tend to outperform other diets for weight loss for up to two years.
Two recent examples show greater weight loss on low-carb diets compared to low-fat diets in studies that were from eight weeks to 24 months in length:
The British Journal of Nutrition 2016: Effects of low-carbohydrate diets v. low-fat diets on body weight and cardiovascular risk factors: a meta-analysis of randomised controlled trials [strong evidence] Learn more
Low carb does not just result in more weight loss than other comparison diets, it also results in more fat loss, especially when carbs are limited to 50 grams per day:
The limits of low fat
A 2015 article in the prestigious journal The Lancet summarizes all major low-fat weight loss scientific trials. The conclusion? There’s no evidence that a low-fat diet helps people lose weight, compared to any other diet advice.
This should cause us to question the commonly held belief that a low-fat diet is right for most people. In fact, the meta-analyses mentioned above show that people tend to lose significantly more weight when they are given dietary advice that includes being allowed to eat fat to satiety.
Weight loss and risk factors
A 2012 review of all major trials of low-carb diets show both reduced weight and improvement of the major risk factors for heart disease:
RCTs showing significantly more weight loss with low-carb diets vs. other diets
There are at least 31 modern RCTs that show significantly better weight loss with low-carb diets, according to the latest count by the Public Health Collaboration UK. The number of studies showing the opposite? Zero.
Here are three of the best RCTs to date:
- New England Journal of Medicine 2008: Weight loss with a low-carbohydrate, Mediterranean, or low-fat diet [moderate evidence]
This was a two-year trial in which 322 people were randomly assigned to follow a Mediterranean diet, a low-fat diet, or a low-carb diet. By the study’s end, the low-carb group had lost the most weight, even though they were allowed to eat as much low-carb food as they needed to feel satisfied; the other two groups followed calorie-restricted diets.
- Annals of Internal Medicine 2014: Effects of low-carbohydrate and low-fat diets: a randomized trial [moderate evidence]
In this one-year study, 148 people were randomized to consume a low-carb diet (less than 40 grams of carbohydrate per day) or a low-fat diet (less than 30% of energy from fat per day). In addition to losing 3.5 kg (7.7 lbs) more than the low-fat group, the low-carb group also had greater improvements in high-density lipoprotein (HDL) cholesterol, triglycerides (TG), and other cardiovascular disease risk factors.
- Journal of the American Medical Association 2007: Comparison of the Atkins, Zone, Ornish, and LEARN diets for change in weight and related risk factors among overweight premenopausal women. The A to Z weight loss study: a randomized trial [moderate evidence]
One of the most well-known weight-loss trials (often referred to as the A to Z study) involved randomizing overweight premenopausal women to eat either a low-carb (Atkins), moderate-carb (Zone), low-fat (Ornish), or low-calorie, portion-controlled (LEARN) diet for one year. At the end of the study, the women in the low-carb group had lost twice as much weight (4.7 kg, or 10.3 lbs) as the Ornish and LEARN groups and nearly three times as much as the women in the Zone group.
30 more RCTs showing more weight loss on low carb
- Nutrients 2019: Non-energy-restricted low-carbohydrate diet combined with exercise intervention improved cardiometabolic health in overweight Chinese females [moderate evidence]
- Nutrition & Diabetes 2017: Twelve-month outcomes of a randomized trial of a moderate-carbohydrate versus very low-carbohydrate diet in overweight adults with type 2 diabetes mellitus or prediabetes [moderate evidence]
- Applied Physiology, Nutrition, and Metabolism 2017: A 12-week low-carbohydrate, high-fat diet improves metabolic health outcomes over a control diet in a randomised controlled trial with overweight defence force personnel [moderate evidence]
- Obesity (Silver Spring) 2017: PROP nontaster women lose more weight following a low-carbohydrate versus a low-fat diet in a randomized controlled trial [moderate evidence]
- Journal of Medical Internet Research 2017: An online intervention comparing a very low-carbohydrate ketogenic diet and lifestyle recommendations versus a plate method diet in overweight individuals with type 2 diabetes: a randomized controlled trial [moderate evidence]
- Diabetes & Metabolic Syndrome 2017: Induced and controlled dietary ketosis as a regulator of obesity and metabolic syndrome pathologies [moderate evidence]
- Clinical Nutrition 2017: A randomized controlled trial of 130 g/day low-carbohydrate diet in type 2 diabetes with poor glycemic control [moderate evidence]
- Nutrition and Diabetes 2017: Short-term safety, tolerability and efficacy of a very low-calorie-ketogenic diet interventional weight loss program versus hypocaloric diet in patients with type 2 diabetes mellitus [moderate evidence]
- The Journal of Nutrition 2015: A lower-carbohydrate, higher-fat diet reduces abdominal and intermuscular fat and increases insulin sensitivity in adults at risk of type 2 diabet [moderate evidence]
- PloS One 2014: A randomized pilot trial of a moderate carbohydrate diet compared to a very low carbohydrate diet in overweight or obese individuals with type 2 diabetes mellitus or prediabetes [moderate evidence]
- Journal of Pediatric Endocrinology and Metabolism 2012: Metabolic impact of a ketogenic diet compared to a hypocaloric diet in obese children and adolescents [moderate evidence]
- Obesity (Silver Spring) 2011: Adiponectin changes in relation to the macronutrient composition of a weight-loss diet [moderate evidence]
- The Journal of Pediatrics 2010: Efficacy and safety of a high protein, low carbohydrate diet for weight loss in severely obese adolescents [moderate evidence]
- Lipids 2009: Carbohydrate restriction has a more favorable impact on the metabolic syndrome than a low fat diet [moderate evidence]
- Obesity 2009: Effects of a low carbohydrate weight loss diet on exercise capacity and tolerance in obese subjects [moderate evidence]
- The American Journal of Clinical Nutrition 2008: Effects of weight loss from a very-low-carbohydrate diet on endothelial function and markers of cardiovascular disease risk in subjects with abdominal obesity [moderate evidence]
- Nutrition & Metabolism 2008: The effect of a low-carbohydrate, ketogenic diet versus a low-glycemic-index diet on glycemic control in type 2 diabetes mellitus [moderate evidence]
- American Journal of Clinical Nutrition 2007: Low- and high-carbohydrate weight-loss diets have similar effects on mood [moderate evidence]
- Diabetic Medicine 2007: A low-carbohydrate diet is more effective in reducing body weight than healthy eating in both diabetic and non-diabetic subjects [moderate evidence]
- Diabetic Medicine 2006: Short-term effects of severe dietary carbohydrate-restriction advice in Type 2 diabetes–a randomized controlled trial [moderate evidence]
- Journal of the American Dietetic Association 2005: Perceived hunger is lower and weight loss is greater in overweight premenopausal women consuming a low-carbohydrate/high-protein vs high carbohydrate/low-fat diet [moderate evidence]
- Diabetologia 2005: Comparison of high-fat and high-protein diets with high-carbohydrate diet in insulin-resistant obese women [moderate evidence]
- The Journal of Clinical Endocrinology and Metabolism 2005: The role of energy expenditure in the differential weight loss in obese women on low-fat and low-carbohydrate diets [moderate evidence]
- Archives of Internal Medicine 2004: The national cholesterol education program diet vs a diet lower in carbohydrates and higher in protein and monounsaturated fat. A randomized trial [moderate evidence]
- Nutrition & Metabolism 2004: Comparison of energy-restricted very low-carbohydrate and low-fat diets on weight loss and body composition in overweight men and women [moderate evidence]
- Annals of Internal Medicine 2004: A low-carbohydrate, ketogenic diet versus a low-fat diet to treat obesity and hyperlipidemia. A randomized, controlled trial [moderate evidence]
- The Journal of Nutrition 2004: Very low-carbohydrate and low-fat diets affect fasting lipids and postprandial lipemia differently in overweight men [moderate evidence]
- The Journal of Clinical Endocrinology & Metabolism 2003: A randomized trial comparing a very low carbohydrate diet and a calorie-restricted low fat diet on body weight and cardiovascular risk factors in healthy women [moderate evidence]
- The New England Journal of Medicine 2003: A low-carbohydrate as compared with a low-fat diet in severe obesity [moderate evidence]
- The Journal of Pediatrics 2003: Effects of a low-carbohydrate diet on weight loss and cardiovascular risk factor in overweight adolescents [moderate evidence]
Comment on RCTs
All of the studies above show significantly more weight loss for the group that was advised to eat a low-carb diet (the Atkins diet, in many cases). To the best of our knowledge, the opposite has never been shown: Low carb has never significantly lost a weight loss trial. This means that low carb’s victory record over low-fat, calorie-restricted advice is 31-0.
Feel free to let us know of any exceptions (or more examples) by emailing firstname.lastname@example.org.
RCTs with non-significant results
According to the Public Health Collaboration UK, this is the detailed record for 62 RCTs comparing low-carb to low-fat or low-calorie diets:
- In 53 of the 62 studies, people in the low-carb group lost more weight than those in other diet groups – including the 31 cases above where the difference was statistically significant.
- In 7 of the 62 studies, people in the low-fat or low-calorie diet group lost slightly, but insignificantly, more weight than those in the low-carb diet group.
- In 2 studies, weight loss was equal in both groups.
Here are a few examples where there were no major differences in weight loss between low-carb and other diets:
Journal of the American Medical Association 2018: Effect of low-fat vs low-carbohydrate diet on 12-month weight loss in overweight adults and the association with genotype pattern or insulin secretion: the DIETFITS randomized clinical trial [moderate evidence]
This recent 12-month study found that people who followed a low-carb or low-fat diet (both with low amounts of sugar or processed carbohydrates) lost a similar amount of weight, on average, regardless of the type of diet they were assigned to and whether it “matched” their genetic insulin response. However, there was a minor, non-significant trend towards more weight loss with low carb, even in this study.
- The American Journal of Clinical Nutrition 2015: Comparison of low- and high-carbohydrate diets for type 2 diabetes management: a randomized trial [moderate evidence]
In this trial of people with type 2 diabetes, both low-carbohydrate and high-carbohydrate diets led to similar weight loss of about 10 kg (22 pounds) within one year.
- Journal of the American Medical Association 2005: Comparison of the Atkins, Ornish, Weight Watchers, and Zone Diets for weight loss and heart disease risk reduction [moderate evidence]
Interestingly, two years before the A to Z study showed that low carb was clearly more effective for weight loss than other diets, a similar study found that weight loss was roughly the same after one year for each of the four diets studied: low-carb, low-fat, Weight Watchers, or Zone.
All 62 RCTs can be found on the Public Health Collaboration website.
Metabolic risk factors
Low-carb diets improve all the main features of metabolic syndrome: obesity, high blood sugar, high blood pressure, and a cholesterol profile that indicates metabolic dyslipidemia, which is characterized by low HDL and high TG.
Metabolic risk factors are strongly linked to chronic diseases like type 2 diabetes, heart disease, stroke, dementia, and even cancer.3 Because low-carb diets can improve risk factors related to these diseases, it may be an important tool in preventing their development.
Overview of randomized controlled trials
The overview below of dozens of studies and meta-analyses comparing low-carb or low-fat diets shows that weight and other risk factors are more likely to be improved with a low-carb diet. The table shows studies up to 2018. More recent trials are listed individually after the table.
The blue (low-carb) or red (low-fat) squares signify a statistically significant advantage in improving the indicated risk factor. The pale blue or red squares signify a non-significant trend toward low carb or low fat being better.
LF %kh = Percent carbohydrate intake in the low-fat group
HDL = High density lipoprotein
TG = Triglycerides
BP = Blood pressure
A1c = HbA1c
fGluc = Fasting glucose
fIns = Fasting insulin level
InsSens = Insulin sensitivity
TC = Total cholesterol
LDL = Low density lipoprotein
However, ratios of cholesterol measurements that may have a higher predictive value than LDL, such as TG/HDL or TC/HDL ratios, also favor low carb because HDL and TG are more likely to be improved on a low-carb diet.4
In addition, low carb is more likely to improve other important risk factors: blood pressure, blood sugar, weight, and waist circumference, a measure of abdominal obesity.
Links to references
Here are links to all the studies above, ordered from oldest to newest:
Brehm -03, Foster -03, Samaha -03, Sondike -03, Aude -04, Volek -04, Meckling -04, Yancy -04, Stern -04, McAuley -05, Nickols-R -05, Dansinger -05, Daly -06, Truby -06, Gardner -07, Ebbeling -07, Halyburton -07, Dyson -07, Westman -08, Keogh -08, Shai -08, Tay -08, Davis -09, Bradley -09, Volek -09, Sacks -09, Brinkworth -09, Jenkins -09, Frisch -09, Elhayany -10, Iqbal -10, Lim -10, Hernandez -10, Yancy -10, Foster -10, Krebs -10, Goldstein -11, Summer -11, Guldbrand -12, Partsalaki -12, Ruth -13, Yamada -14, Saslow -14, Bazzano -14, Tay -15, Saslow -17, Gardner -18.
Gardner 2018. DIETFITS study compared 30% carb vs 48% carbs. The low carb eaters had a greater reduction in TG and increase in HDL
JAMA 2018: Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial [randomized trial; moderate evidence]
Saslow 2017. This was a 12mo RCT of a Low-carb diet (20-50g carbs) vs a diet with 45-50% carb. Low carb showed TG decreased 10mg/dl, compared to an increase in control, and HDL was also better for low carb.
Nutrition and Diabetes 2017: Twelve-month outcomes of a randomized trial of a moderate-carbohydrate versus very low-carbohydrate diet in overweight adults with type 2 diabetes mellitus or prediabetes [randomized trial; moderate evidence]
Tay 2015. RCT compared 14% carbs vs 53% carbs. Low carb was better for glycemic variability, TG reduction and HDL increase
American Journal of Clinical Nutrition 2015: Comparison of low- and high-carbohydrate diets for type 2 diabetes management: a randomized trial [randomized trial; moderate evidence]
Bazzano ’14. RCT comparing a diet of <40g carbs to a low-fat diet with <30% fat. Low carb was better for TG and HDL. Annals of Internal Medicine 2014: Effects of low-carbohydrate and low-fat diets: a randomized trial [randomized trial; moderate evidence]
Saslow ’14. RCT of a diet with 57g of carbs vs 138g. Greater carb reduction improved TG, glucose and insulin
PLoS One 2014: A randomized pilot trial of a moderate carbohydrate diet compared to a very low carbohydrate diet in overweight or obese individuals with type 2 diabetes mellitus or prediabetes [randomized trial; moderate evidence]
Yamada ‘14,. RCT comparing calorie restriction vs low carb. Low carb was better for A1c and TG
Internal Medicine 2014: A non-calorie-restricted low-carbohydrate diet is effective as an alternative therapy for patients with type 2 diabetes [randomized trial; moderate evidence]
Ruth ’13. RCT comparing 40g carbs vs 200g. Low carb was better for TG, HDL, and systolic BP
Metabolism 2013: Consuming a hypocaloric high fat low carbohydrate diet for 12 weeks lowers C-reactive protein, and raises serum adiponectin and high density lipoprotein-cholesterol in obese subjects [randomized trial; moderate evidence]
The effect of low-carb diets on the cholesterol profile is demonstrated in the following systematic review and meta-analysis of RCTs showing improved HDL cholesterol and TG levels, as well as improved blood pressure:
Dong 2020. They evaluated 12 RCTs and found low-carb diets led to a decrease in TG and systolic BP and an increase in HDL.
Hellon 2019. They evaluated 8 RCTs concluding low-carb diets do not lead to a significant increase in LDL but did increase HDL and lower TG.
Huntris 2017. They included 18 RCTs on low-carb diets and found improved HbA1c, TG and HDL.
European Journal of Clinical Nutrition 2017: The interpretation and effect of a low-carbohydrate diet in the management of type 2 diabetes: a systematic review and meta-analysis of randomised controlled trials [strong evidence]
Mansoor 2016. They evaluated 11 RCTs and found low carb led to greater weight loss, lower TG and higher HDL compared to low-fat diets.
British Journal of Nutrition 2016: Effects of low-carbohydrate diets v. low-fat diets on body weight and cardiovascular risk factors: a meta-analysis of randomised controlled trials
[systematic review of randomized trials; strong evidence]
Sackner 2015. They evaluated 17 trials of low carb vs. low fat and found better reduction in TG and systolic BP and increase in HDL for low-carb diets.
Hession 2009. They reviewed 13 RCTs and found low carb better than low fat for HDL, TG and systolic BP.
Obesity Reviews 2009: Systematic review of randomized controlled trials of low-carbohydrate vs. low-fat/low-calorie diets in the management of obesity and its comorbidities
[systematic review of randomized trials; strong evidence]
Nordman 2006. They evaluated 5 RCTs of low-carb vs. low-fat diets. Low carb had better effects on TG and HDL.
Archives of Internal Medicine 2006: Effects of low-carbohydrate vs low-fat diets on weight loss and cardiovascular risk factors: a meta-analysis of randomized controlled trials
[systematic review of randomized trials; strong evidence]
Heart disease, cholesterol and saturated fats
Despite half a century of research, there is still no high-quality evidence that natural saturated fat (from foods like butter and eggs) is anything but neutral from a health perspective.5
In a recent analysis, 19 leading researchers concluded that it’s wrong to maintain the general advice to reduce saturated fat intake, as evidence does not support it.6 A similar article was published in the Journal of the American College of Cardiology. 7
Another recent analysis of the field published in BMJ Evidence-Based Medicine concluded that: “The preponderance of evidence indicates that low-fat diets that reduce serum cholesterol do not reduce cardiovascular events or mortality. Specifically, diets that replace saturated fat with polyunsaturated fat do not convincingly reduce cardiovascular events or mortality. These conclusions stand in contrast to current opinion.”8
Finally, a recent overview of the evidence linking saturated fat to heart disease, written by four prominent experts in the area, noted that when any fat, including saturated fat, replaces carbohydrate in the diet, there is improvement in important lipid markers related to risk of heart disease.9
A 2009 systematic review of cohort studies and RCTs looking at potential relationships between dietary factors and heart disease concluded that “Insufficient evidence of association is present for intake of … saturated or polyunsaturated fatty acids; total fat … meat, eggs and milk”:
Similarly, a 2010 review of cohort studies found “…no significant evidence for concluding that dietary saturated fat is associated with an increased risk of CHD or CVD”:
Meta-analyses of observational studies in 2015 and 2017 reached the same conclusions about the lack of association between saturated fat and heart disease:
British Medical Journal 2015: Intake of saturated and trans unsaturated fatty acids and risk of all cause mortality, cardiovascular disease, and type 2 diabetes: systematic review and meta-analysis of observational studies [moderate evidence]
The most recent systemic review of RCTs published in 2020 found eating less saturated fat was associated with a small reduction in cardiovascular events, but with no difference in risk of death. However, when analyzed based on cholesterol and study quality, any deleterious effect vanished.10
Two other systematic reviews of RCTs, also showed no convincing evidence that saturated fat is harmful.
In these reviews, researchers concluded that replacing saturated fats with unsaturated fats might slightly reduce the risk of heart attack and other cardiovascular events by about 14 to 19 percent.
However, in the first of these review, the reduction in risk was seen only in studies of at least two years duration and in studies of men, but not of women; furthermore, these changes appeared to have no effect on total or heart disease mortality:
In the second systematic review, for five of the eight studies analyzed there was insufficient evidence to conclude that the group outcomes were statistically significantly different from each other:
In addition, other more recent systematic reviews of RCTs haven’t shown any reduction in heart diease risk as a result of substituting saturated fats with unsaturated fats:
When it comes to butter and other high-fat dairy specifically, there is some observational evidence that people consuming it might be, if anything, thinner and healthier than others.
In 2012, researchers exploring the relationship between full-fat dairy, heart health, and weight concluded that “the observational evidence does not support the hypothesis that dairy fat or high-fat dairy foods contribute to obesity or cardiometabolic risk”:
In fact, follow-up on the PURE study with over 177,000 subjects revealed no association between saturated fat intake and heart disease but did show saturated fat was linked to decreased risk for all-cause mortality and stroke. While this is weak observational data, it does make it much less likely that saturated fat in dairy is intrinsically harmful harmful
Learn more: A user guide to saturated fat
Polyunsaturated fats and the Sydney Diet Heart Study
Advice to replace saturated fats with polyunsaturated fats – to reduce heart disease risk – is controversial, and the evidence supporting this guidance comes mostly from older studies with many weaknesses.
A re-evaluation of one of these, the Sydney Diet Heart Study, uncovered previously unpublished data showing a trend toward increased risk of death and heart disease in the group who got polyunsaturated omega-6 fats as a replacement for saturated fat in their diets.
When this data is included in a meta-analysis of other studies where saturated fats are replaced with polyunsaturated omega-6 fats, the almost-significant trend towards increased heart disease and death from heart disease shows that this replacement does not appear to have health benefits.
British Medical Journal 2013: Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis [strong evidence]
Similar results were seen in the Minnesota Coronary Experiment, showing no health benefits to replacing saturated fats with polyunsaturated fats.
However, there is much conflicting data ranging from mechanistic studies to RCTs. Read more in our evidence-based guide to vegetable oils.
The PURE study
A recent large observational study published in The Lancet shows that when diverse global populations are surveyed, diets with lower fat and higher carbohydrate content do not confer any health advantage.The PURE study followed over 135,000 people in 18 countries from five continents for over seven years.
The researchers found that people who ate the most carbohydrate died earlier. A higher intake of fat, on the other hand, was linked to longer life, regardless of whether the fat was unsaturated or saturated.
This is an observational study and therefore cannot prove that eating fewer carbs and more fat extends life. However, it does call our current dietary advice into question, as the authors note: “Global dietary guidelines should be reconsidered in light of these findings.” Learn more
Hard endpoints on low carb
Most low-carb studies just track risk factors for heart disease. They are not large enough to track actual health outcomes, such as heart attacks. However one RCT tracked signs of carotid atherosclerosis during two years of advice to eat a low-carb, high-fat diet based on the Atkins diet. The study also followed participants eating low-fat and Mediterranean diets.
The result? There was significant regression of measurable carotid vessel wall volume seen in all three diet groups, implying a reduction of atherosclerosis. One possible explanation, according to the researchers, was the loss of weight and reduced blood pressure resulting from the change in diet.
What this tells us is that a higher-fat, lower-carb Atkins-type diet not only does not cause thickening of carotid arteries, it can help reduce signs of atherosclerosis.
Similarly, an 18-month RCT showed no difference in carotid IMT between a low-carb diet and a traditional diabetes diet for those with type 2 diabetes
PLos One 2020: Effect of a 90 g/day low-carbohydrate diet on glycaemic control, small, dense low-density lipoprotein and carotid intima-media thickness in type 2 diabetic patients: An 18-month randomised controlled trial [moderate evidence]
In addition, a small non-randomized trial showed a keto diet can improve endothelial function based on flow-mediated dilatation (FMD). Although more data are needed for a stronger conclusion, this suggests keto diets may be beneficial for vascular health.
Endocrine, Metabolic & Immune Disorders Drug Targets 2020: Effects of a low carb diet and whey proteins on anthropometric, hematochemical and cardiovascular parameters in subjects with obesity. [non-randomized, weak evidence]
Fats, cholesterol and dietary guidelines
Type 2 diabetes
No improvements through conventional treatment
Conventional treatment, based on intensive medication regimens and the usual lifestyle advice to eat less and exercise more, has shown that, although it can help people manage type 2 diabetes, it does not ultimately improve their health outcomes. No less than seven randomized controlled trials have demonstrated that diabetes drugs do not help reduce heart disease, the major killer of people with diabetes.
On one occasion, a trial had to be stopped because the drugs meant to control blood sugar not only resulted in more weight gain and more adverse side effects, but also resulted in a greater risk of death.11
Low carb: systematic reviews
The advantage of a low-carb diet in type 2 diabetes has been recognized in several systematic reviews and meta-analyses of RCTs:
BMJ Open Diabetes Reserach and Care 2020: Systematic review and meta-analysis of dietary carbohydrate restriction in patients with type 2 diabetes [systematic review of randomized trials; strong evidence]
Nutrients 2020: Impact of a ketogenic diet on metabolic parameters in patients with obesity or overweight and with or without type 2 diabetes: A meta-analysis of randomized controlled trials [systematic review of randomized trials; strong evidence]
Diabetes Research and Clinical Practice 2018: Effect of dietary carbohydrate restriction on glycemic control in adults with diabetes: A systematic review and meta-analysis [strong evidence]
The American Journal of Clinical Nutrition 2018: Effects of low-carbohydrate- compared with low-fat-diet interventions on metabolic control in people with type 2 diabetes: a systematic review including GRADE assessments [strong evidence]
European Journal of Clinical Nutrition 2017: The interpretation and effect of a low-carbohydrate diet in the management of type 2 diabetes: a systematic review and meta-analysis of randomised controlled trials [strong evidence]Another review of the evidence for low-carb diets in the treatment of type 2 diabetes, written by experts who are generally in favor of the idea, suggests that a low-carb diet should be offered as the first approach in managing type 2 diabetes:
The following RCTs demonstrate the benefits of low-carbohydrate diets in people with type 2 diabetes:
PLos One 2020: Effect of a 90 g/day low-carbohydrate diet on glycaemic control, small, dense low-density lipoprotein and carotid intima-media thickness in type 2 diabetic patients: An 18-month randomised controlled trial [moderate evidence]
Nutrition & Diabetes 2017: Twelve-month outcomes of a randomized trial of a moderate-carbohydrate versus very low-carbohydrate diet in overweight adults with type 2 diabetes mellitus or prediabetes [moderate evidence]
A study of a ketogenic, low-carbohydrate diet done by Virta Health and involving about 330 people found that, at the one-year mark, 97 percent of the participants had reduced or stopped their insulin use. Furthermore, 58 percent no longer had a diabetes diagnosis; in other words, they had reversed their disease.12
These results indicate that type 2 diabetes is not inevitably progressive and irreversible. When low-carbohydrate diets are chosen by individuals and supported by healthcare professionals, type 2 diabetes can be reversed and potentially put into remission. Read more
Frontiers in Endocrinology 2019: Long-term effects of a novel continuous remote care intervention including nutritional ketosis for the management of type 2 diabetes: A 2-year non-randomized clinical trial [weak evidence]The longest published study yet on low carb for type 2 diabetes is a non-randomized intervention trial that compared a 20% carbohydrate diet (daily intake of carbohydrate was 80-90 grams) to usual care. Although the study was quite small, with only 23 participants in the final low-carb group, a follow-up assessment at 44 months showed continued improvement in HbA1c, weight, and reduction of diabetes medications:
Learn more: How to reverse type 2 diabetes
Type 2 diabetes
Type 1 diabetes
The authors of a recent systematic review of low-carbohydrate diets in type 1 diabetes concluded that there is an urgent need for additional randomized controlled trials and other high-quality studies in this area:
Compared with the robust evidence base supporting low-carb and ketogenic diets for type 2 diabetes, research on carb restriction for type 1 diabetes is greatly lacking. However, as we await the results of RCTs currently in progress, the results from existing studies on low-carb diets in people with type 1 diabetes are encouraging.
Controlled studies have shown that people with type 1 diabetes who limit carbs to 50 to 100 grams per day experience more stable blood sugar and fewer episodes of hypoglycemia compared to people with type 1 who eat higher-carb diets — with a bonus of increased weight loss in those who are overweight:
Diabetes, Obesity and Metabolism 2017: Short-term effects of a low-carbohydrate diet on glycaemic variables and cardiovascular risk markers in patients with type 1 diabetes: a randomized open-label crossover trial [moderate evidence]
Asia Pacific Journal of Clinical Nutrition 2016: A randomised trial of the feasibility of a low-carbohydrate diet vs standard carbohydrate counting in adults with type 1 diabetes taking body weight into account [moderate evidence]
Some non-controlled studies, while considered weak evidence, demonstrate excellent “real world” results for people with type 1 diabetes who follow a low-carbohydrate diet long term.
This study shows that patients with type 1 diabetes who use a lower-carb, higher-protein diet achieve, on average, excellent results, with low rates of major complications. This study also shows that children who followed a carbohydrate-reduced diet did not show any signs of impaired growth, although the duration of time on diet was short (1.2 +/- 0.8 years).
Type 1 diabetes
Long-term safety of low carb
Some health experts have raised concerns about the long-term effects of low carb. All diets can be done well or poorly, and risk of of nutritional inadequacy exists with all ways of eating. However, a well-planned, nutritionally adequate low-carb diet has no evidence of risk, to the best of our knowledge, as carbohydrate is not a part of essential nutrition.
Some concerns have been raised about the perceived risks of protein intake on the acid-base balance of the body and how that could affect bones. No evidence supports this concern. The internal pH of the body is firmly regulated and is not significantly affected by diet.14
Regarding bone health, the latest meta-analysis endorsed by the International Osteoporosis Foundation makes the situation very clear: “There is no evidence that diet-derived acid load is deleterious for bone health. Thus, insufficient dietary protein intakes may be a more severe problem than protein excess in the elderly.”
The above supportive evidence is far stronger than evidence suggesting harm, as we detail in a news post about a study showing increased markers of bone turnover after 3 weeks on a keto diet on highly trained race walkers.
Ketogenic diets have been studied extensively in people with epilepsy. In fact, they are often recommended for those who either fail to respond to anti-seizure medications or can’t tolerate their side effects.
A large review of 16 studies in adults with uncontrolled epilepsy found that ketogenic diets were well-tolerated long term and typically resulted in significantly fewer seizures or, in a minority of cases, complete freedom from seizures:
Epilepsy Research 2016: Evaluation of a simplified modified Atkins diet for use by parents with low levels of literacy in children with refractory epilepsy: a randomized controlled trial [moderate evidence]
Can low carb help reverse fatty liver disease? A team of Swedish researchers published a 2017 study in the journal Cell Metabolism that showed the effects of a low-carbohydrate diet with increased protein and no reduction in calories on obese subjects with fatty liver disease. The research plan was meant to separate the effects of caloric reduction and weight loss from those of carbohydrate restriction.
Lead author Jan Boren of the University of Gotheburg reported the effects of carbohydrate restriction on liver fat:
“We observed rapid and dramatic reductions of liver fat and other cardiometabolic risk factors and revealed hitherto unknown underlying molecular mechanisms.”
This effect was also shown in a more recent study:
Proceedings of the National Academy of Science U.S.A 2020: Effect of a ketogenic diet on hepatic steatosis and hepatic mitochondrial metabolism in nonalcoholic fatty liver disease [nonrandomized study, weak evidence]
The most recent RCT showed a Mediterranean keto diet was more effective than a low-fat diet for improving signs of fatty liver.
Given the strong links between excess weight, high insulin levels, and other metabolic problems, a low-carb diet should be ideal for reversing polycystic ovary syndrome (PCOS). A low-carb diet consistently and reliably lowers insulin levels.15 This may indicate that low-carb diets can help reverse metabolic issues that include PCOS.
Although long term, high quality studies have not been done, and not all studies agree, much of the limited available science shows promise:
- One small 2005 study followed 11 women with PCOS as they went on a ketogenic low-carb diet for six months. The five women who completed the study lost weight, improved their hormonal status, and reduced the perceived amount of body hair. Two of them became pregnant despite previous infertility problems:
Nutrition & Metabolism 2005: The effects of a low-carbohydrate, ketogenic diet on the polycystic ovary syndrome: a pilot study [weak evidence]
- In a 2015 study, 24 women with PCOS who ate diets containing roughly 70 grams of net carbs per day for 12 weeks had significant reductions in insulin levels, insulin resistance, TG, and testosterone, along with losing an average of 19 pounds (8.6 kg) by the end of the study:
- A 2020 study showed 14 women with PCOS showed a significant improvement in hormone levels and markers of insulin resistance.
- Studies in 2006 and 2013 showed that even a very modest reduction in carbohydrates (from 55 to 41 percent of energy) can result in significant improvements in weight, hormones, and risk factors for women with PCOS:
Fertility & Sterility 2006: Role of diet in the treatment of polycystic ovary syndrome [weak evidence]
- Finally, a 2017 review of relevant studies notes that low-carbohydrate diets tend to “reduce circulating insulin levels, improve hormonal imbalance and resume ovulation to improve pregnancy rates.”
Nutrients 2017: The effect of low-carbohydrate diets on fertility hormones and outcomes in overweight and obese women: a systematic review [systematic review of randomized and nonrandomized trials; weak evidence]
Beyond the scientific evidence cited above, the clinical experience of doctors using carbohydrate restricted diets strongly supports these diets as an effective treatment for PCOS:
Irritable bowel syndrome
In 2009, researchers examined a very low-carb diet (less than 20 grams of carbs a day) for treatment of irritable bowel syndrome (IBS). During the study, 13 people with diarrhea-predominant IBS started with a standard American diet for two weeks, then switched to a very-low-carb diet for four weeks; 10 out of the 13 subjects (77%) had significant positive changes, with the very-low-carb diet reducing abdominal pain and diarrhea and improving their quality of life:
In recent years, much research has been done on a diet low in FODMAPs. FODMAP is an acronym for “fermentable oligosaccharides, disaccharides, monosaccharide’s and polyols.” That unwieldy name describes types of short-chain carbohydrates found in many fruits, vegetables, legumes, grains, dairy products and some processed foods. At least one study has shown that 75 per cent of people with diagnosed IBS had their symptoms improve on a low FODMAP diet.
European Journal of Nutrition 2016: Does a diet low in FODMAPs reduce symptoms associated with functional gastrointestinal disorders? A comprehensive systematic review and meta-analysis [moderate evidence]
Inflammatory bowel disease
Anecdotal reports suggest that a low-carb or low-carb Paleo diet may reduce symptoms of inflammatory bowel diseases, such as Crohns’ disease or ulcerative colitis.16
To our knowledge, there are not yet any controlled trials investigating whether carbohydrate restriction can improve inflammatory bowel disease. However, this published case report suggests that this is an area in need of further investigation:
Reflux disease / heartburn
At least two studies show that low-carb diets may be promising treatments for reflux disease.
In a small study of eight obese people with gastroesophageal reflux disease (GERD), participants had esophageal pH tested before and after consuming a ketogenic diet containing less than 20 grams of carbs per day for six days. The second test showed normalization of esophageal pH levels, which indicates a less acidic environment in the esophagus. In addition, participants reported improvements in chest burning and discomfort, belching, and other symptoms of reflux.
Alimentary Pharmacology and Therapeutics 2016: Dietary carbohydrate intake, insulin resistance and gastro‐oesophageal reflux disease: a pilot study in European‐ and African‐American obese women [weak evidence]
Is it possible to reduce or even completely eliminate migraines by reducing dietary carbohydrate? Studies and anecdotal evidence suggest it might be. There are at least two studies so far that show that carbohydrate restriction can help those who suffer from migraines:
- Euroupean Journal of Neurology 2015: Migraine improvement during short lasting ketogenesis: a proof-of-concept study [weak evidence]
- Journal of Headache and Pain 2016: Cortical functional correlates of responsiveness to short-lasting preventive intervention with ketogenic diet in migraine: a multimodal evoked potentials study [weak evidence]
Limited evidence from animal studies and case reports suggests that low-carb diets might be helpful for some people with attention deficit and hyperactivity disorder (ADHD) and other related conditions, such as obsessive-compulsive disorder (OCD) and Tourette’s syndrome.
One small study has found improvement in memory in people with mild cognitive impairment who were using a ketogenic diet:
Ketogenic diets provide ketones as an alternative fuel source for the brain, which may be of benefit to people with dementia or other cognitive issues. Clinical trials in patients with AD or cognitive impairment have shown that, even though the brains of these individuals can’t use glucose effectively, they may have a similar capacity to use ketones as the brains of healthy older people:
Annals of the New York Academy of Sciences 2016: Can ketones compensate for deteriorating brain glucose uptake during aging? Implications for the risk and treatment of Alzheimer’s disease [overview article]
An eight-week RCT found that people with Parkinson’s disease who ate a ketogenic diet had greater improvement in symptoms than those who ate a low-fat diet:
The following small pilot study of five people with Parkinson’s disease showed an improvement of symptoms on a ketogenic diet. There was no control group, so a placebo effect can’t be ruled out:
One RCT found exogenous ketone esters improved exercise time by 24% in patients with Parkinson’s, suggesting ketones may have a metabolic benefit.
Otherwise, aside from a few case studies, there is not much more published research in this area.
Similar to research on Parkinson’s disease, high-quality evidence for benefits of carbohydrate restriction in multiple sclerosis (MS) is extremely limited. Beyond occasional positive anecdotal reports, there is some emerging clinical research.
A 2015 study reported that MS patients who followed a ketogenic diet or fasted for several days experienced significant improvements in quality of life scores and TG levels:
European Committee For Treatment and Research in Multiple Sclerosis 2015: Ketogenic diet and prolonged fasting improve health-related quality of life and lipid profiles in multiple sclerosis – a randomized controlled trial [moderate evidence]
A small pilot study in 20 people with relapsing MS demonstrated lack of progression of their disease for 6-months while following a ketogenic diet. While this is preliminary data that needs larger confirmatory studies, it raises the possibility that a ketogenic diet might have a role in MS treatment:
Preliminary evidence suggests a connection between fibromyalgia and insulin resistance, high blood sugar, and type 2 diabetes. Since low-carb diets often are effective in treating the latter conditions, a positive effect on fibromyalgia could be possible. Learn more
It’s not unusual for people to combine a low-carb diet with intermittent fasting, either for convenience or to improve the effectiveness of weight loss or diabetes reversal.
A common variant of intermittent fasting, also called time restricted eating when it is for less than 24 hours, is called 16:8, meaning you fast for 16 hours per day and consume all of your daily food during an eight-hour window. This is often achieved by skipping breakfast. This can sometimes improve weight loss results.17
Intermittent fasting appears to be a promising alternative to continuous calorie restriction:
In addition, as little as 14 hours of fasting for 12 weeks appears to be enough to see significant metabolic improvements.
And a six-week trial in women over age 60 showed 16 hours of daily fasting improved weight loss and metabolic health.
International Journal of Environmental Research and Public Health 2020: Effect of a six-week intermittent fasting intervention program on the composition of the human body in women over 60 years of age [randomized trial; moderate evidence]
Low carb and the environment
Low-carb diets are often described as being ecologically unsustainable, due to concerns about the impact of meat production on the environment. However, these concerns may be largely unfounded.
First of all, low-carb and keto diets are moderate in protein; they are not “high” protein diets. Protein as a percentage of calories may increase because carbohydrate calories are kept low, but this does not necessarily indicate an increase in amounts of absolute protein, or meat, intake. Recommendations for protein intake on carbohydrate restricted diets fall well within the acceptable macronutrient distribution ranges (AMDR) recommended by the U.S. government. 18 It’s even possible to eat a vegetarian or vegan low-carb diet.
Secondly, the environmental impact of meat production is highly variable. Even the production of beef – usually considered by far the worst from a climate perspective – can be handled in an environmentally friendly way.
A 2018 study demonstrates that properly managed livestock can be made climate neutral or even carbon-negative, meaning more carbon is stored in soil than is released into the atmosphere:
Learn more: The green keto meat eater, part 1
Red meat and health
Warning are often raised regarding the negative health effects of red meat. However, these claims are supported only by weak nutrtional epidemiology studies, with low statistical associations and high possibility of confounding and bias. Most epidemiological findings like these turn out to be false when they are tested in intervention trials.19
Most recently, a review of RCTs comparing lower vs. higher red meat consumption used the GRADE system to quantify the strength of the evidence. The authors concluded that restricting red meat may have little or no effect on whether a person will develop heart disease or cancer.
Annals of Internal Medicine 2019: Effect of Lower Versus Higher Red Meat Intake on Cardiometabolic and Cancer Outcomes: A Systematic Review of Randomized Trials. [systematic review of randomized trials; strong evidence]
This same group of researchers showed that evidence from observational studies trials does not support dietary guidance to limit red meat consumption.
Annals of Internal Medicine 2019: Reduction of Red and Processed Meat Intake and Cancer Mortality and Incidence: A Systematic Review and Meta-analysis of Cohort Studies. [meta-analysis of cohort studies; weak evidence]
Not surprisingly, when the theory that red meat causes cancer is tested in an interventional trial, it is not confirmed. A good example is the Polyp Prevention Trial, which tested a low-fat, low-meat diet versus usual diet on over 2,000 people for 8 years to see if it could prevent colorectal cancer. Although the experimental group changed their diets significantly, they did not have lower rates of cancer recurrence.
- Cancer Epidemiology, Biomarkers & Prevention 2007: The polyp prevention trial continued follow-up study: no effect of a low-fat, high-fiber, high-fruit, and -vegetable diet on adenoma recurrence eight years after randomization [moderate evidence]
- Journal of the American Medical Association 2006: Low-fat dietary pattern and risk of colorectal cancer [moderate evidence]
Salt and health
In order to prevent heart disease, the American Heart Association recommends that everyone should restrict sodium intake to less than 1500 mg per day, which equals about 3/4 teaspoon of salt.
Yet, for individuals on a low-carb diet, sodium needs may actually increase, due to increased losses via the kidneys.21
As a result of these increased needs, individuals using low-carbohydrate diets are not typically asked to restrict sodium intake. Concerns have been raised about the safety of this practice for these individuals, many of whom may also belong to specific groups — such as African Americans or people with diabetes, chronic kidney disease, or heart disease — for which very-low-sodium diets have been recommended in the past.
Although very-low-sodium diets may be beneficial for some individuals who are also on high-carb diets, it is difficult to determine if this is a reasonable concern for those who are on low-carb diets given that support for significant sodium reduction for all individuals is mixed.
In fact, a 2013 US Institute of Medicine report concluded that reducing sodium to very low levels is not recommended for the general population. The report also found that current science “provides some evidence for adverse health effects of low sodium intake” among individuals with diabetes, chronic kidney disease, and heart disease.
In addition, a 2014 Cochrane review of RCTs that tested interventions to reduce sodium intake found no support for dietary advice to restrict salt intake:
Those reports stand in contrast to another review of RCTs that found reduced salt intake did improve lifespan and reduce deaths from heart disease:
Annals of Internal Medicine 2019: Effects of nutritional supplements and dietary interventions on cardiovascular outcomes: An umbrella review and evidence map [systematic review of RCTs; strong evidence]
The inconsistent data makes it difficult to draw strong conclusions or make definitive recommendations for the general population to significantly restrict sodium intake.
In addition to the two reports mentioned above, population-wide advice to cut salt intake to very low levels has also been brought into question by seven observational studies that show no links between very low salt intake and health benefits or longevity.
Although these studies do not show causality, they seem to indicate that intakes of sodium below about 3500 mg per day may be associated with a shorter lifespan, especially for older adults.
- European Heart Journal 2020: Sodium intake, life expectancy, and all-cause mortality [very weak evidence]
- Journal of the American Medical Directors Association 2018: Association between sodium excretion and cardiovascular disease and mortality in the elderly: a cohort study [very weak evidence]
In this study in an elderly population, lower intakes of salt were associated with shorter lifespans.
- Journal of the American Medical Association 2016: Sodium excretion and the risk of cardiovascular disease in patients with chronic kidney disease [very weak evidence]
This study showed an increased risk of cardiovascular events for sodium intakes above about 4000 mg per day in people with mild to moderate kidney disease. People with kidney disease may be more sensitive to higher salt intakes, but very low intakes were not associated with increased health benefits.
- Lancet 2016: Associations of urinary sodium excretion with cardiovascular events in individuals with and without hypertension: a pooled analysis of data from four studies [very weak evidence]
This study shows, on the one hand, consuming more than 7000 mg of sodium per day was associated with increased risk of heart attack and premature death in people with hypertension. On the other hand, consuming less than 3000 mg of sodium per day was associated with increased risk in both people with normal blood pressure and those with hypertension.
- Journal of the American Medical Association 2011: Fatal and nonfatal outcomes, incidence of hypertension, and blood pressure changes in relation to urinary sodium excretion [very weak evidence]
Overall, this study suggests that lower sodium intake is associated with increased risk of death from heart disease.
- Diabetes Care 2011: Dietary salt intake and mortality in patients with type 2 diabetes [very weak evidence]
In a population of people with type 2 diabetes, higher sodium intakes were associated with decreased risk of premature death from all causes and from death from heart disease.
- Diabetes Care 2011: The association between dietary sodium intake, ESRD, and all-cause mortality in patients with type 1 diabetes [very weak evidence]
In this study in people with type 1 diabetes, both highest and lowest intakes of sodium were associated with increased mortality. Furthermore, individuals with the lowest sodium intake had the highest risk of end-stage kidney disease.
- Hypertension 1995: Low urinary sodium is associated with greater risk of myocardial infarction among treated hypertensive men [weak evidence]
As the title of the study indicates, low intakes of sodium were associated with greater risk of heart attacks in men being treated for high blood pressure. This effect was largest in the group of men over 55 years old.
External resources about the science of low carb
The science of low carb
Dr. William Yancy, a researcher who investigates the effects of carbohydrate-restricted diets, shares his views on the science of low carb.
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The Lancet. Diabetes and Endocrinology 2015: Effect of low-fat diet interventions versus other diet interventions on long-term weight change in adults: a systematic review and meta-analysis [strong evidence]
A review published in the mainstream cardiology journal JACC reviewed the lack of evidence supporting limits on saturated fat intake. Even the Academy of Nutrition and Dietetics has publicly stated that saturated fat should no longer be considered a nutrient of concern, based on the lack of evidence connecting it to heart disease. ↩
Journal of the American College of Cardiology 2020: Saturated fats and health: A reassessment and proposal for food-based recommendations: JACC State-of -the-Art Review [overview article; ungraded] ↩
Some disagree with the use of the word “reverse” when it comes to type 2 diabetes. The concern is that it may imply the disease is completely gone, never to return. At Diet Doctor, we use the term “reverse” to indicate that the diagnosis of diabetes is no longer present at that moment. However, we acknowledge that the diagnosis of diabetes will likely return if a patient goes back to their prior high-carb eating habits. Therefore, “reverse” does not imply a forever cure. ↩
Evidence strength downgraded due to lack of high-quality studies. ↩
This includes diets in which very few carbs are consumed, such as ketogenic diets.
Nutrition and Metabolism 2006: Comparison of isocaloric very low carbohydrate/high saturated fat and high carbohydrate/low saturated fat diets on body composition and cardiovascular risk [randomized controlled trial; moderate evidence] ↩
However, not every study on breakfast-skipping shows increased weight loss. Here’s one negative trial:
In LCHF studies that have shown health benefits, people are usually advised to eat a moderate amount of protein and as much fat as needed to feel satisfied.
Applied Physiology, Nutrition and Metabolism 2017: A 12-week low-carbohydrate, high-fat diet improves metabolic health outcomes over a control diet in a randomised controlled trial with overweight defence force personnel [moderate evidence] ↩
Another publication details how there was also no benefit in preventing breast cancer:
Journal of the American Medical Association 2006: Low-fat dietary pattern and risk of invasive breast cancer: the Women’s Health Initiative Randomized Controlled Dietary Modification Trial. [moderate evidence] ↩
Although the precise mechanism for this isn’t entirely known, some researchers have suggested that when insulin levels are very low (as they typically are on a low-carb diet), the kidneys may absorb less sodium and excrete more into the urine.