Why low carb can help you lose weight
Fortunately, there’s a way to lose weight that’s effective, sustainable, healthy, and even enjoyable: low carb.
Here, at Diet Doctor, one of our goals is to offer you guidance about health and nutrition issues that may seem confusing. We strive to make this guidance trustworthy by backing it with evidence — and by explaining the evidence in ways that are easy to understand.
Read on to learn why following a low-carb diet can help you lose weight.
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1. What is a low-carb/keto diet?
Carbohydrates are one of the three major nutrients found in food. Unlike protein and fat, which have other functions, their only role is providing energy for your body.
Low-carb diets — true to their name — contain very few carbohydrates (also known as carbs), which are mostly found in sugary and starchy foods. These diets also provide adequate protein and more fat than a low-fat diet.
Hearing about other people and celebrities losing weight on low-carb diets might make them sound like they’re just the latest fad, but there’s a lot more to these diets than that.
2. Calories vs. carbs: What should I be counting?
To lose weight, you need to eat fewer calories than your body requires. True or false?
This statement is, in fact, true. Please don’t shoot the messenger, though. Just because calories count, doesn’t mean you have to count them! As we’ll discuss, the way your body regulates its weight isn’t as simple as adding and subtracting calories.
Calories are units of energy that allow your body to move, breathe, and go about your daily life. If you consume more calories than you burn, the excess energy will be stored as body fat. When you take in fewer calories than needed, your body will release some of its body fat stores to be used as energy. That’s the whole point of dieting, right?
Yet trying to create a calorie deficit by intentionally restricting how much you eat often fails to achieve lasting weight loss. For one thing, it’s usually very hard to do long term.1 In fact, it may even contribute to weight gain over time, defeating the purpose of counting calories in the first place.2
It’s become increasingly clear there are many different things that influence how much we eat and whether the calories we consume are burned or stored. And one of those things is dietary carbs.
The not-so-secret trick is that reducing carb intake can trigger several changes in your body that may help it to burn more energy than it stores.
3. Eat less without counting calories? How does that work?
Rather than turning every meal into a math equation, carb restriction can help you automatically eat less without counting calories, in or out. Here’s why:
Low-carb diets have a long and well-deserved reputation for decreasing appetite. Back in the 1950s, physician A.W. Pennington demonstrated that keeping carb intake very low allowed overweight people to lose weight without going hungry or having to think about calorie restriction.3
How does this work? Lack of hunger when eating low carb has been linked to ketosis, a metabolic state in which your body runs mainly on fat — at least some of which is body fat — and fat-like compounds called ketones. Studies clearly show that these diets reduce hunger.4
Although researchers are still learning why low-carb diets reduce hunger, it seems to be connected to an increase in “fullness” hormones, like CCK, and a decrease in “hunger” hormones, like ghrelin.5
Also, a low-carb diet contains foods that make you feel full without dramatically raising your blood sugar — I’m looking at you, protein, fat, and fiber-filled veggies. Plus, carbs are often a one-way ticket to a blood sugar roller coaster ride that can actually increase hunger.6
For example, in a small study, 10 adults with obesity and type 2 diabetes, who followed a non-calorie-restricted, very-low-carb diet, ended up reducing their usual intake by 1,000 calories, on average — even though they were allowed all the fat- and protein-filled foods they wanted.7
Fewer “empty” calories
Unlike essential fatty acids in fat and essential amino acids in protein, there are no “essential” components in carbs.8 Carbs are the decorative throw pillows of food: fun to have, but unnecessary for health.
Although they provide energy, the nutritional profiles of grains and many other foods that are high in carbs aren’t uniquely impressive. But when you don’t get enough essential nutrition, especially protein, your body may keep you reaching for more food in an attempt to satisfy those needs.9
By contrast, when you eat mainly fish, eggs, cheese, and other high-quality foods, you are more likely to end up meeting those essential nutrient needs more easily and with fewer calories.
Reduced food reward
For many people, eating sweets and other refined carbs in processed foods can light up reward centers in the brain. This response can then lead to cravings, which can make it difficult to eat “just one” of anything sugary or starchy.10
But just because a low-carb diet may reduce “food reward” cravings doesn’t mean you can’t eat food that rewards you in other ways! This way of eating includes many tasty, satisfying low-carb foods — while avoiding those that are known to overactivate this food reward system.
Additionally, eating nourishing low-carb meals may help you to feel pleasantly full and ready to move on when you’re done, rather than wanting “just one more bite” (and maybe a nap afterward).
4. Burning more calories without extra exercise
Although controversial and not yet accepted as settled science by all experts, there is some high-quality research suggesting that low-carb diets can help you burn more calories. This might help you maintain lost weight.
After losing weight, many people experience a decrease in their metabolic rate.11 Unfortunately, as unfair as it seems, the human body may not be designed to lose weight.
In fact, your body may think it’s in the middle of a great famine (despite passing 24-hour fast food joints every few miles!) and, as a result, it may deliberately slow down your metabolism in order to use less fuel until it senses that the famine is over.
The end result is that you have to eat even fewer calories to maintain your new weight. Yes, it’s completely unfair!
But don’t despair. There’s some interesting research suggesting that this metabolic slow-down can be minimized with low-carb eating.
Two insightful low-carb studies
In a 2012 study, 21 adults, who’d lost weight on a calorie-restricted diet, were found to burn more calories during weight maintenance if they followed a low-carb diet compared to a low-fat diet — an average of around 300 calories per day more, in fact.12 This finding has been referred to as low carb’s “metabolic advantage.”
According to Professor David Ludwig, one of the Harvard professors who conducted the study, this advantage “would equal the number of calories typically burned in an hour of moderate-intensity physical activity.”13
More recently, a similar but much larger study in 164 people appeared to confirm this effect, with participants who’d previously lost weight burning anywhere from 200 to nearly 500 more calories per day on a low-carb maintenance diet compared to a high-carb or moderate-carb maintenance diet.14
Getting regular exercise is important for many reasons. But with low carb, you may not have to spend an extra hour on the treadmill just to avoid regaining the weight you’ve lost.
Insulin is a major player
Insulin, a hormone produced by your pancreas, has several important functions, including moving glucose (sugar) out of your blood and into your cells and helping you build and maintain muscle.15
When levels are elevated, insulin also directs your body to store any excess calories as fat. Again, this is helpful if there’s a famine right around the corner, but not so helpful when you’re eating excessively or unhealthily.
Eating a low-carb diet is a powerful way to reduce your insulin levels in addition to your total calorie intake.16 This combination allows your body to more easily use stored body fat for energy, which means: fat loss.
During maintenance, though, it means you can usually increase your calorie levels somewhat without regaining weight. At that point, your body will burn dietary fat instead of body fat.17
Insulin and appetite
You may have heard that insulin suppresses appetite, which is true at least as a short-term effect.
Then maybe you’re wondering why lowering your levels of insulin won’t make you even hungrier. Good question. The difference is between the insulin that circulates in your body and the insulin that circulates in your brain.
Your appetite is regulated in part by insulin levels in your brain. Higher levels of brain insulin equal appetite suppression.
- Diabetes 2012: Postprandial administration of intranasal insulin intensifies satiety and reduces intake of palatable snacks in women [randomized trial; moderate evidence]
- Diabetes 2011: Intranasal insulin enhances postprandial thermogenesis and lowers postprandial serum insulin levels in healthy men [randomized trial; moderate evidence]
Here’s the tricky part: when insulin levels in your body remain high frequently, this may cause insulin levels in your brain to become less responsive. This means high circulating insulin in your body not only leads to fat storage, but it may inhibit appetite control.
By contrast, a low-carb diet can keep insulin levels in your body consistently low. This may allow insulin levels in your brain to remain responsive to meals, creating the kind of win-win situation that lets you lose weight — and maintain weight loss — without hunger.
- Nature Reviews Neurology 2018: Brain insulin resistance in type 2 diabetes and Alzheimer disease: concepts and conundrums [overview article; ungraded]
- Acta Diabetologia 2014: Evidence for altered transport of insulin across the blood-brain barrier in insulin-resistant humans [observational study; weak evidence]
5. The science of low carb
Although low-carb diets are still sometimes characterized as a “fad,” they’ve been studied as a tool for weight loss for decades.
The most recent studies show low carb to be at least as effective as — and often superior to — low-fat and calorie-restricted diets for losing weight. What’s more, they nearly always lead to lower blood sugar, lower blood pressure, and other health improvements, regardless of what happens weight-wise.
Get ready, because here comes the serious science stuff. While it may not be that exciting, it is important to see what the evidence says, so stay with us!
Results of systematic reviews of RCTs
Experts consider randomized controlled trials (RCTs) to be the highest-quality research and the “gold standard” for evidence. Systematic reviews and meta-analyses of these RCTs are graded as the strongest, most rigorous evidence.
Researchers set a high standard — such as focusing only on long-term studies — and exclude any studies that don’t meet it, such as studies lasting less than 12 weeks. Then they analyze results from studies that share these same high standards.
Within the past five years, several systematic reviews of RCTs comparing low-carb diets to low-fat diets have concluded that low carb results in greater weight loss overall:
- 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 randomized controlled trials [strong evidence] Learn more
- PloS One 2015: Dietary intervention for overweight and obese adults: comparison of low-carbohydrate and low-fat diets. A meta-analysis [strong evidence] Learn more
- The British Journal of Nutrition 2013: Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials [strong evidence]
The last review was limited to RCTs lasting at least 12 months, in order to determine whether very low-carb diets or low-fat diets were more effective for weight loss long-term. Again, low carb was the winner.
In some trials, weight loss outcomes for low-carb or low-fat diets may differ very little, while changes in body composition are more pronounced — especially when very-low-carb diets are analyzed separately from diets with more modest carb restriction.
For instance, one systematic review of 15 RCTs concluded that people who consume diets containing less than 50 grams of carbs per day seem to achieve greater fat loss than those who eat low-fat or low-carb diets containing more than 50 grams of carbs per day.18
This is a good reminder that the scale doesn’t tell the whole story. We can learn much more by following body composition.
About individual studies
Although experts think systematic reviews of RCTs are the strongest level of evidence, these reviews can’t provide details about what happened in each study. Also, the outcomes may not look as impressive when averaged among many different studies.
If we take a look at some individual low-carb weight loss studies, we can learn a little more about the diets people followed and the results they achieved.
In a two-year trial, 322 overweight adults were randomly assigned to follow a Mediterranean diet, a low-fat diet, or a low-carb diet. Those in the low-carb group consumed 20 grams of carbs per day for the first two months, which was gradually increased to as much as 120 grams of carbs, depending on each person’s weight loss and maintenance goals.
Among those who completed the study, the low-carb group lost the most weight. And here is the kicker: They ate as much low-carb food as they wanted. No restrictions! This was compared to the other groups, who had to purposely restrict their calories.19
In this study, 148 overweight men and women were randomly assigned to consume a low-carb diet (less than 40 grams per day) or a low-fat diet (less than 30% of daily calories per day) for one year. Neither group was asked to restrict calories.
In addition to losing 7.7 lbs (3.5 kg) more than the low-fat group, the low-carb group also had greater improvements in several heart disease risk factors.20
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 [randomized trial; moderate evidence]
One of the most well-known weight-loss trials involved randomly assigning overweight women to follow either a low-carb (Atkins), moderate-carb (Zone), low-fat (Ornish), or low-calorie, portion-controlled (LEARN) diet for one year.
The Zone and LEARN diets had specific calorie restriction goals; the Atkins and Ornish diets did not. The low-carb group was instructed to eat 20 grams per day initially and gradually increase their carb intake to 50 grams or less per day.
By the end of the study, the women in the low-carb group had lost twice as much weight (an average of 10.3 lbs or 4.7 kg) as those in the Ornish and LEARN groups and nearly three times as much as those in the Zone group.21
All of the results mentioned above are based on group averages, so your own mileage may vary. But in general, it is safe to say that low-carb diets outperform low-fat, calorie-restricted diets for most people.
Evidence from non-randomized, non-controlled trials isn’t as strong as RCT evidence, but these studies can provide helpful “real world” information about following a certain way of eating.
For instance, sometimes these studies allow participants to select between two types of diets. If you’re allowed to choose your own diet, you’ll probably be more likely to stick with it, enjoy it, and achieve better results.22
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 [non-randomized trial; weak evidence]
Adults enrolled in an ongoing study at Virta Health were given the choice to follow a very-low-carb diet that included frequent nutrition coaching or to receive standard diabetes care. After two years, those who selected the low-carb intervention and remained in the study had lost an average of 26 pounds (11.9 kg). Three-quarters of the group had reduced their body weight by 5% or more, in addition to improving their blood sugar control.23
The results above are all impressive in their own right. But here are some other dramatic weight loss outcomes, although they did not have a self-select option or a control group.
In 2011, researchers studied the effects of a 12-week ketogenic diet in 22 people with metabolic syndrome. The diet provided less than 30 grams of carbs per day in the form of nonstarchy vegetables. By the end of the study, participants had lost an average of 32 pounds (14.5 kg).
That’s impressive, but what’s even more important is that all of them no longer met the criteria for metabolic syndrome due to significant improvements in waist circumference, blood pressure, lipids, and fasting blood sugar.24
In a slightly older, larger study, 83 obese adults followed a ketogenic diet restricted to less than 30 grams of carbs per day. After 24 weeks on the diet, participants had lost 31.5 pounds (14.3 kg), on average. They also achieved a number of health improvements, including lower triglycerides, LDL cholesterol and blood sugar, and higher HDL cholesterol.25
Compliance and intention-to-treat studies
Ideally, all participants follow their assigned diet until the end of the study. However, this rarely, if ever, happens. Noncompliance in diet studies is very common, as is dropping out altogether.26
Traditionally, researchers report results for those who follow the diet they’re instructed to eat and complete the trial. However, this practice can set up overly optimistic expectations for outcomes in real life, where many people find it difficult to stick to a diet.
In order to address this issue, researchers often conduct intention-to-treat (ITT) analyses of RCTs in which they report data for everyone. This includes those who didn’t follow the diet they were assigned to, dropped out of the study, or otherwise didn’t follow the study protocols.
However, ITT analysis can end up underestimating weight loss and other benefits that occur in people who do follow the dietary advice they’ve been given. In fairness, this applies to both low-carb and low-fat studies — and compliance is roughly equal for both diets.27
We know dietary preferences vary a lot among different people. If a certain diet isn’t a good fit for someone – if it feels hard to stick to or if weight loss is very slow – it’s likely that the person won’t end up following the diet.
Fortunately, low-carb diets are a good fit for many — and this may very well include you.
Let’s briefly review the features and benefits of low-carb diets for weight loss:
- Delicious, nourishing foods that help you feel full and satisfied while eating less
- Weight loss at your own pace without consciously restricting calories
- Freedom from the “food reward” cycle triggered by high-carb foods
- Potential long-term health benefits
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The British Journal of Nutrition 2018: A carbohydrate-reduced high-protein diet acutely decreases postprandial and diurnal glucose excursions in type 2 diabetes patients [randomized trial; moderate evidence]
Nutrients 2019: Using the avocado to test the satiety effects of a fat-fiber combination in place of carbohydrate energy in a breakfast meal in overweight and obese men and women: a randomized clinical trial [moderate evidence] ↩
Annals of Internal Medicine 2005: Effect of a low-carbohydrate diet on appetite, blood glucose levels, and insulin resistance in obese patients with type 2 diabetes [non-randomized trial; weak evidence] ↩
“The lower limit of dietary carbohydrate compatible with life apparently is zero, provided that adequate amounts of protein and fat are consumed.”
Institute of Medicine of the National Academies 2005:Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids [textbook chapter; ungraded] ↩
This is called the “protein leverage hypothesis.” The following RCT provides evidence that this hypothesis may be true.
For reduced calorie intake, see references above. The following studies demonstrate that a low-carb diet powerfully lowers insulin levels:
American Journal of Clinical Nutrition 2010: Lack of suppression of circulating free fatty acids and hypercholesterolemia during weight loss on a high-fat, low-carbohydrate diet [RCT; moderate evidence]
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. [RCT; moderate evidence]
Annals of Internal Medicine 2005: Effect of a low-carbohydrate diet on appetite, blood glucose levels, and insulin resistance in obese patients with type 2 diabetes [non-randomized study; weak evidence]
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] ↩
This may not always happen, though. One trial found that people who were allowed to choose between a low-carb or low-fat diet lost no more weight than people who were randomly assigned to one diet or the other:
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 [non-randomized trial; weak evidence] ↩