Do you need to eat fruits and vegetables?
Do you need to eat fruits and vegetables to be healthy? Not necessarily.
Eating fruits and vegetables is often recommended to live longer, healthier lives. Observations show that healthy people who eat fruits and vegetables can consistently live into their 90’s and 100’s.1
While this may show that fruits and veggies can be part of a healthy lifestyle for some, it does not prove that they’re required for everyone.
Here’s the current scientific evidence on fruits and vegetables.
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1. What are fruits?
Fruits are the seed-containing portion of various flowering plants. They grow exclusively above ground.
Different types of fruit
The broad categories of fruit include pome, citrus, tropical, melons, stone fruits, and berries. Most fruits taste sweet, although citrus varieties are often sour or bitter. With the exception of bananas, most domesticated fruits are juicy due to their high water content.
Keep in mind that the fruit we have today is not the same as the fruit available to our hunter-gatherer ancestors.2
Nutritional composition of fruits
Nearly all of the calories in fruits come from sugar — not surprising, given their sweet taste. Their net carbs span a large range: 5 grams to 20 grams per 100 grams (3.5 ounces) of fruit, depending on the type. A single mid-sized orange has about 12 grams of carbs and a banana often has at least 20 grams.
Reviewing the nutrition profiles of different fruits, we see that several provide vitamin C and a few minerals. However, actual nutrient content can vary depending on the type of fruit, how and where it’s grown and stored, and how long it’s been sitting at a stand or grocery shelf.3
In addition, all fruits fall short in vitamin D and key minerals like calcium, magnesium, iron, and zinc.4
Guide What are the best and the worst fruits and berries to eat on a low-carb diet? Here’s the short version: most berries are OK low-carb foods in moderate amounts, but fruits can be seen as candy from nature (they contain quite a lot of sugar).
Low-carb fruits and berries – the best and the worst
Guide What are the best and the worst fruits and berries to eat on a low-carb diet? Here’s the short version: most berries are OK low-carb foods in moderate amounts, but fruits can be seen as candy from nature (they contain quite a lot of sugar).
2. What are vegetables?
Speaking from a botanical or gardening point of view, vegetables are the leaves, stems or roots of plants. However, many non-sweet fruits are commonly considered vegetables for eating or cooking purposes.
Different types of vegetables
Vegetables can be broadly classified into four categories:
- Above-ground vegetables: greens (spinach, lettuce, chard, et cetera), cruciferous vegetables (broccoli, brussels sprouts, cabbage, cauliflower, kale, et cetera), bulbs (onions, garlic), and fungi (mushrooms).
- Below-ground/root/starchy vegetables: beets, carrots, parsnips, rutabagas, turnips, yams, potatoes, sweet potatoes, et cetera.
- Gourds: pumpkins, hard-shelled squashes, and other winter squashes.
- Technically fruits but treated like vegetables: avocados, olives, bell peppers, eggplant, tomatoes, and zucchini. Unlike other fruits, these aren’t sweet and are often prepared and consumed with other vegetables. Avocados and olives are unique among fruits and vegetables because most of their calories come from fat rather than sugar or starch.
Nutritional composition of vegetables
Non-starchy vegetables are keto-friendly foods that provide 5 or fewer grams of net carbs per 100-gram (3.5-ounce) serving. By contrast, root and starchy vegetables have 6 to 17 grams of net carbs per serving.
Vegetables usually contain moderate to high amounts of fiber, especially avocado — which also happens to be among the lowest in net carbs.
Overall, vegetables are more nutrient-dense than fruits, but their vitamin and mineral content can also be affected by factors like growing and storage conditions. Most veggies are good to excellent sources of potassium, and bell peppers and cruciferous vegetables are also high in vitamin C.
Low-carb vegetables – the best and the worst
Guide What are the best and the worst vegetables to eat on a low-carb diet? Here’s the short version: Vegetables growing above ground are usually low carb and can be eaten freely.
3. How many fruits and vegetables should people eat per day?
Looking at official recommendations for fruit and vegetable intake in different countries, it’s clear that they’re basically all variations on “5 a day.” The US dietary guidelines, the UK National Health Service, and the World Health Organization all set minimums that are generally equivalent to two cups of fruit and two and a half cups of vegetables each day.
According to the Centers for Disease Control, only 1 in 10 adults consistently meets the US Dietary Guidelines recommendations for fruit and vegetable intake.5 But is this really a problem?
Some low-carb diet experts would say no; that if someone is following a diet that meets their needs for essential nutrients, eating several servings of fruits and vegetables every day isn’t necessary.
For instance, Dr. Eric Westman initially recommends two cups of leafy greens and one cup of above-ground vegetables per day — and no sweet fruit of any kind — as part of a very-low-carb diet.6
Health organizations state that their recommendations for fruit and vegetable intake are evidence-based. But as we’ve discussed previously in other guides, there are different levels of scientific evidence. And almost all these fruit and vegetable recommendations are designed for someone who is eating a standard Western diet or a low-fat, high-carb diet.8
4. Research on the benefits of eating fruits and vegetables
With few exceptions, eating more fruits and vegetables is standard weight-loss advice. But if we look at the results of experimental trials in which people actually ate more — or at least were encouraged to eat more — of these foods, that advice just doesn’t seem to work for everyone.
A 2014 systematic review of eight randomized controlled trials (RCTs) lasting between 4 and 52 weeks found that people assigned to increase their fruit and vegetable intake lost an average of only 1.5 pounds more than those assigned to eat smaller amounts of these foods.9
The same year, researchers who published a systematic review of seven different RCTs failed to find any measurable differences in weight change between people who consumed high vs. low amounts of fresh produce.10
However, the interventions differed among the RCTs included in both of these analyses. In some cases, people were provided with fresh fruits and vegetables or vouchers to purchase them; in others, they only received advice to consume more of a specific fruit or vegetable.
Furthermore, in most of these studies, researchers relied on reported produce intake from the groups rather than closely monitoring their intake. Often it just wasn’t clear whether people actually ended up eating their assigned amounts of fruits and vegetables or not.
In some cases, people who eat more fruits and vegetables may actually gain weight because they don’t compensate by cutting back on other foods — and juice seems to be especially problematic.
For instance, in one RCT, when overweight and obese people were provided with fresh fruits and vegetables to add to their diets for eight weeks, they gained twice as much weight as lean participants, who responded to increased produce intake by eating less of other foods. But all three groups gained weight after consuming the same amount of produce in juice form during a second eight-week period. Still, obese participants gained the most.11
On the other hand, replacing highly processed food with fruits and vegetables is not only a smart choice nutritionally; it might also lead to weight loss.
In a three-month study, overweight women who were given vouchers to purchase fresh produce lost 6 pounds, whereas those who were given vouchers to purchase any type of groceries gained 4 pounds by the study’s end.12 Overall, eating more fruit and vegetables hasn’t been shown to produce meaningful weight loss in most experimental studies.
Yet we often hear that doing so is the key to achieving and maintaining a healthy weight. What is the basis for this advice?
For decades, it’s mainly been larger yet lower-quality observational (also called epidemiological) research. For instance, a 2015 systematic review analyzing 17 such studies found statistically weak associations between eating a lot of produce and lower body weight and waist size.13
What were the odds ratios (ORs) in this study? They were 0.83 for high intake of fruits or vegetables alone and 0.91 for high intake of fruits and vegetables combined. These are very modest odds ratios and likely reflect the fact that *if* (taking into account much of the confounding which occurs in observational studies) there is a preventive effect of fruits and vegetables, it would be very modest. Even the study authors acknowledged that “The present meta-analysis seems to be limited by low study quality.”
Guide to observational vs. experimental studies
Guide In this guide, we discuss the differences between observational and experimental studies, the advantages and disadvantages of each, and why in nearly all cases observational research shouldn’t be used when making decisions about your diet.
Diabetes and metabolic syndrome
Fruits and vegetables are generally considered diabetes-friendly foods. In fact, nearly all types of produce make the “low GI foods” list on the American Diabetes Association website, with the exception of melons and pineapple.But how does adhering to “5 a day” or similar dietary advice affect blood sugar control and insulin resistance? The evidence from clinical trials is mixed.
One systematic review of eight RCTs examining the effects of fruit and vegetable intake in people with metabolic syndrome found that although blood pressure slightly improved in those who ate more produce, fasting blood sugar levels were no different among the groups.14
In 2017, researchers analyzed results from four RCTs with fruit and vegetable interventions in people with type 2 diabetes or other health conditions. The groups assigned to eat more fruits and vegetables improved their vitamin C and beta-carotene intake but consumed significantly more carbs and calories.16 That doesn’t sound very diabetes-friendly, does it?
In one RCT published in Diabetes Care — a journal of the American Diabetes Association — researchers assigned overweight adults to consume two, four, or seven portions of fruits and vegetables per day for 12 weeks. Ultimately, none of the groups experienced any improvement in insulin resistance.17
However, results from another RCT found that a powder derived from broccoli sprouts lowered insulin levels and improved insulin sensitivity in adults with type 2 diabetes.18
Of course, neither the experimental nor control group were consuming low-carb or keto diets. It’s unknown whether adding broccoli or other green vegetables to a low-carb or keto diet would provide any further benefit on insulin resistance.
Since low-carb diets by definition eliminate two likely offenders behind insulin resistance — sugar and other high-carb foods — there may already be a maximal benefit which may not increase by adding more vegetables. Controlled trials exploring this would need to be done.
What about lower-quality observational studies that suggest eating plenty of fruits and vegetables can help protect against type 2 diabetes? Large meta-analyses of these studies have shown very weak associations between fruit and vegetable intake and type 2 diabetes risk.19
As you can see, the data is far from conclusive given the variety of study weaknesses. And, as usual, the comparative diet make s a big difference.
Videos about diabetes
Are fruits and vegetables heart-healthy? Although some experimental research suggests that eating more produce might reduce some cardiovascular disease (CVD) risk factors, the evidence to date is inconclusive with respect to clinical outcomes.
For instance, in 2013 researchers conducted a systematic review of 10 RCTs investigating whether eating more fruits and vegetables led to improvements in heart health markers. They reported that the trial designs differed significantly and often included other nutrition and lifestyle interventions that may have contributed to the beneficial effects on CVD risk.20 These data suggest that if there is a disease preventive effects, it is likely to be small.
Their conclusion? Further trials investigating higher fruit and vegetable intake as the sole intervention are needed.
Another systematic review of RCTs found that high potassium intake seemed to be beneficial for the arteries, while the effects of high fruit and vegetable intake on arterial function weren’t clear.21
More recent RCTs suggest that boosting produce intake may increase blood levels of antioxidants that might improve HDL function and reduce inflammation in those at high risk for CVD, such as those with diabetes.22
Yet others found no reduction in CVD risk factors in overweight people who consumed seven servings of produce per day compared to those who consumed two servings per day for 12 weeks.23 In these studies, blood levels of lutein (an antioxidant found in vegetables) were measured to confirm compliance among the different groups.
The evidence from observational studies in this area is very weak. For instance, a large 2017 meta-analysis of 95 studies reported only an 8 percent reduction (RR of 0.92) in CVD risk for every 200 grams of fruits and vegetables people reported consuming per day, up to a maximum of 800 grams per day (roughly 10 servings).24
It’s also worth remembering that these studies are confounded by the fact that most people who actually eat 10 servings of produce on a daily basis probably also engage in other habits known to protect heart health, such as working out regularly and avoiding junk food, excessive alcohol consumption, and smoking.Despite the very weak association and low quality of evidence, major medical organizations cite this paper as conclusive support that fruits and vegetable reduce heart disease events.
It’s important to recognize that very few single dietary changes – whether increasing fruits or vegetables or adding or reducing carbohydrates have on their own a pronounced effect on CVD incidents. This is why most organizations promote a pattern of eating.
Despite the relatively small association between fruit or vegetable intake and CVD prevention, fruits and vegetables are promoted because this relationship is consistent over well-designed observational studies. What tends to be important in how data is interpreted is how consistent the “direction” of effect is. With fruits and vegetables, there tends to be a consistent, if small protective effect. So at an individual level, there is a minuscule effect, if any. But across the population, especially with the standard baseline diet, there is a reduction.
You can think of the way organizations make dietary recommendations as a kind of “precautionary principle”. For example, the thinking goes something like this: Since fruits and vegetables 1) *might* help prevent CVD a bit, 2) are also good sources of key micronutrients including folate and potassium; and 3) provide different fibers which are associated with a “healthy” microbiome, organizations promote their consumption.
Nevertheless, as we have seen, the evidence behind many of these statements is not particularly strong (and to be honest, “strong” evidence in nutrition is hard to come by); and there is also data from better quality controlled trials that adding fruit into the diet can cause weight gain!
Is it possible that you can greatly decrease your risk of heart disease solely by eating 10 servings of fruits and vegetables every day? We simply don’t have the data to answer this question, and observational studies certainly cannot give us this level of precision. For some people, increasing their fruit and vegetable consumption might help prevent CVD; in other people, there may be no effect at all.
For people who want to consume a low-carbohydrate diet, the best advice we can give is to rely on the “precautionary principle” in a way that works for you. For many people this would mean avoiding the higher-carb fruits and starchy veg; and incorporating low-carb plant-based items into your diet – from spinach to mushrooms to nuts and seeds to avocados.
Videos about heart disease
“Eating lots of fruits and vegetables can help reduce your cancer risk.” This message is taken verbatim from the website of the American Cancer Society. But is there strong scientific evidence that eating large amounts of fruits and vegetables will help protect you from cancer?
Although fruits and vegetables are whole foods that contain beneficial nutrients, it’s too early to make that claim for them. In fact, it’s too early to make that claim for any foods with certainty, because very little high-quality experimental research has examined how specific foods affect cancer risk and progression.
It’s known that damage to DNA in cells may raise the risk of developing cancer in the future. One randomized cross-over study in healthy people found that consuming kiwifruit for three weeks led to increased antioxidant activity that helped repair DNA, regardless of whether small or large amounts were consumed.25
Similar results were seen in young male smokers who consumed three servings of broccoli per day for 10 days.26
However, other studies haven’t shown any improvement in DNA repair in people who increased their fruit and vegetable intake.27 And one even suggested that cruciferous vegetables may temporarily damage DNA, although this effect seems to disappear within several hours.28 Can healthy people reduce their chance of developing cancer by consuming antioxidants found in plants? At this point, we don’t know. After all, our bodies have their own built-in antioxidant systems in place that can help repair cell damage.29 Additional trials exploring the effects of fruits and vegetables on DNA repair would give us more information about this.
Cruciferous vegetables like broccoli and cabbage contain compounds that might help reduce cancer risk in several ways, such as decreasing inflammation and improving cell signaling.30
Yet it appears there’s probably a threshold of these protective compounds that can be absorbed, so that very high intakes offer no further benefit than consuming more modest amounts.31
Many fruits and vegetables have other phytochemicals (literally “plant chemicals”) with potential anti-cancer activity, such as resveratrol and sulforaphane.32 However, their effects have been studied mainly in test tubes and animals. High-quality human research is needed before any conclusions can be made about their use in cancer prevention or treatment.
In contrast to the limited amount of experimental research, plenty of observational studies have explored the relationship between fruit and vegetable intake and cancer risk. Researchers who conduct systematic reviews of these studies often conclude that people who eat the most fruits and vegetables decrease their risk of developing cancer.
Yet these analyses reveal weak associations (RRs of 0.78 to 0.92) between eating a lot of produce and being diagnosed with any type of cancer, including breast, lung, colon, bladder, and non-Hodgkin’s lymphoma.33
In summary, eating fruits and vegetables — especially cruciferous types — may very well decrease your risk of cancer, but much more rigorous research is needed before we can say this for sure.
Diet and cancer: What we know and what we don’t
Guide In this guide, we’ll look at what we know — and what we don’t know — about food and cancer.
Nearly all research on the benefits of fruit and vegetables for other conditions is observational, but a couple of experimental studies suggest potential benefits:
- Bone health: Results from one RCT found that increasing intake of certain fruits, vegetables, and herbs may improve bone health. Postmenopausal women who ate the “Scarborough Fare” diet — which included daily servings of leafy greens, cabbage, onions, mushrooms, prunes and, of course, parsley, sage, rosemary, and thyme (remember the old Simon and Garfunkel song?) — experienced less bone breakdown and calcium loss than women who consumed their usual diets or added other types of plants to their diets.34 By contrast, a meta-analysis of both observational studies and RCTs found no association between bone health and intake of fruits and vegetables.35
- Psychological health: A two-week study investigated the potential mental-health benefits of increasing fruit and vegetable intake in young adults who typically consume very little of these foods. The study participants reported improvements in well-being and motivation but no changes in depressive symptoms or overall mood.36
5. Can eating fruits and vegetables cause health problems in some people?
Most of us can eat a moderate portion of vegetables without any problems. Because they’re higher in sugar, fruits are generally best enjoyed in smaller quantities. However, for some people, both fruits and vegetables may cause issues.
Although there isn’t much published research about sensitivities to specific compounds found in fruits and vegetables, it is possible a small minority of people may have an unpleasant reaction to fruits and vegetables. This might be related to:
- Salicylates: Salicylates are found naturally in Some fruits and vegetables37 Fruits and vegetables high in salicylates include berries, oranges, pineapples, apricots, broccoli, cucumbers, and zucchini.
- Oxalate: People who suffer from kidney stones may find that their symptoms worsen after consuming fruits and vegetables high in oxalate. 38 Spinach contains much more oxalate than any other vegetable, but rhubarb, beet greens, kale, and other leafy greens also contain large amounts.
- Histamines: Your body releases histamine on its own as part of an immune response, but in rare conditions people can have excessive levels or an inability to break it down, and histamine can cause symptoms like hives, abdominal pain, asthma, and headaches.39 High-histamine foods include eggplant, spinach, tomatoes, sauerkraut, and avocados.
- FODMAPS: This is an acronym for fermentable oligosaccarides, disaccarides, monosaccarides and polyols. These sugar molecules — found most often in fruits, vegetables and grains — can cause digestive upset in people with irritable bowel syndrome (IBS). People with IBS often find reducing or eliminating FODMAP foods greatly improves their symptoms.40
If you suspect you may have one of these sensitivities, keep in mind that processed, canned, and fermented foods also contain some of these compounds. Your symptoms might also be due to another health issue. Be sure to see a doctor for a full work-up to rule out other causes.
6. Will avoiding fruits and vegetables lead to nutrient deficiencies?
Many people feel that fruits and vegetables are indispensable for health because they provide important vitamins and minerals. While it’s true that they contain several micronutrients and phytonutrients which could play important roles in the maintenance of good health; it’s important to recognize other minimally processed foods can help meet your needs.
Meat, fish, eggs, and dairy provide many essential vitamins and minerals. For example, dairy is a great source of potassium and magnesium. Organ meats like chicken liver and beef kidney are excellent sources of vitamin C.41 Hence, by eating a varied, low-carb diet – with a mix of low-carb fruits and vegetables – will help you meet your nutrient needs for vitamin C, potassium, folate, and other key nutrients.
What about fiber? Although fiber isn’t an essential nutrient, many health organizations recommend a daily minimum of 25 grams per day for adults. Of course, fruits and vegetables aren’t the only sources of dietary fiber; nuts and seeds avocado, berries coconut and many non-starchy veggies are all high fiber foods you can enjoy on a low-carb diet.
However, the amount of fiber we actually need has been a hotly debated topic within the low-carb and keto community for some time. And to date, in studies demonstrating fiber’s health benefits, people were eating higher-carb diets.42 You can read more about the science behind fiber intake in our guide on whole grains.
7. Summary – fruits and vegetables: optional, optimal, or essential?
Fruits and vegetables are whole foods that provide vitamins, minerals and fiber, along with other potentially beneficial compounds like phytochemicals.
In addition, many people — although certainly not all — genuinely enjoy the taste and texture of keto-friendly fruit and veggies, especially when combined with a fat or tasty sauce. Think creamed spinach, zucchini noodles fettuccine with olive oil and butter, or berries with cream.
Moreover, there’s no question that replacing high-carb processed foods with fresh fruits and vegetables is a healthy move.
It’s possible that fruits and vegetables may play an independent role in reducing disease risk. In the case of cruciferous and leafy green vegetables, it seems likely.
As always, we advise considering fruits and vegetables in the context of the whole diet. If you’re one of those people that find following a very-low-carb keto diet helps you lose body fat and get your diabetes or prediabetes under optimal control, this is going to have the greatest impact on your overall health. And there’s no reason you can’t add a wide variety of low-carb vegetables and some fruits into your diet to “hedge your bets” and get the potential benefits – whether lower blood pressure; potentially lower cancer and CVD risk – from whole plants.
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These observations were made in the so-called “Blue Zones,” areas where people live into their 90’s and 100’s much more often than the world-wide population.
However, these were not results from a scientific study. Plus, these individuals are typically thin, routinely physically active, eat fewer calories than the general population, eat mostly home-prepared local ingredients, and have other healthy lifestyle habits. ↩
Looking back at the diets of our hunter-gatherer ancestors, it’s clear that humans have been consuming plants (along with animals) for over a million years. Roots, leaves, berries and other fruits were readily eaten, but always based on seasonal availability.
Today, a quick trip to the grocery store can present us with hundreds of produce options 365 days a year — many of them larger and more visually appealing than ever due to advanced farming methods and hybridization. Some domesticated fruits have become much bigger and less bitter compared to their earlier counterparts, and most have smaller seeds, thinner peels, and a higher water content, making them easier to eat:
However, it’s important to note that there is more to nutrition than vitamins and minerals and fruits and vegetables also contain a variety of compounds – from fiber to phytonutrients like polyphenols – which may also have important health effects, although the research is not definitive at this point. ↩
This is based on opinion of individual practitioners and is not assigned a level of evidence. ↩
We are not aware of any studies examining the RDA of fruits and vegetables in the setting of a low-carb or keto diet. ↩
Health Promotion Practice 2011: Pass the fruits and vegetables! A community-university-industry partnership promotes weight loss in African American women [randomized trial; moderate evidence] ↩
PloS One 2015: Fruit and vegetable consumption and changes in anthropometric variables in adult populations: a systematic review and meta-analysis of prospective cohort studies [observational cohort study with ORs < 2; very weak evidence] ↩
International Journal of Food Sciences and Nutrition 2015: Effect of fruits and vegetables on metabolic syndrome: a systematic review and meta-analysis of randomized controlled trials [strong evidence] ↩
British Journal of Nutrition 2017: The effect of increased fruit and vegetable consumption on selected macronutrient and micronutrient intakes in four randomised-controlled trials [moderate evidence] ↩
International Journal of Food Sciences and Nutrition 2012: Effect of broccoli sprouts on insulin resistance in type 2 diabetic patients: a randomized double-blind clinical trial [moderate evidence] ↩
In one meta analysis, they found green leafy veggies, yellow veggies, cruciferous veggies and blueberries all had weak associations with reduced diabetes risk, with RRs between 0.72 and 0.93:
In another meta analysis, fruit and vegetable intake combined had a very low association with reduced diabetes (RR of 0.93) but when taken individually, vegetable intake was statistically significant while fruit intake was not:
Finally, this meta analysis showed no overall association between fruit and vegetable intake and risk of DM, but when separated, the data did start to show weak associations:
European Journal of Clinical Nutrition 2017: Fruit and vegetable intake and risk of incident of type 2 diabetes: results from the consortium on health and ageing network of cohorts in Europe and the United States (CHANCES) [observational study; very weak evidence] ↩
Cochrane Database of Systematic Reviews 2013: Increased consumption of fruit and vegetables for the primary prevention of cardiovascular diseases [systematic review of randomized trials; strong evidence] ↩
If you are interested in boosting your potassium intake, foods such as mushrooms, spinach, and broccoli are good sources of potassium.
Cardiovascular Diabetology 2014: A randomised controlled trial of increasing fruit and vegetable intake and how this influences the carotenoid concentration and activities of PON-1 and LCAT in HDL from subjects with type 2 diabetes [moderate evidence]
American Journal of Clinical Nutrition 2014: Flavonoid-rich fruit and vegetables improve microvascular reactivity and inflammatory status in men at risk of cardiovascular disease–FLAVURS: a randomized controlled trial [moderate evidence] ↩
Journal of Nutrition 2015: Increasing fruit and vegetable intake has no dose-response effect on conventional cardiovascular risk factors in overweight adults at high risk of developing cardiovascular disease [randomized trial; moderate evidence]
Nutrition, Metabolism and Cardiovascular Diseases 2016: Increasing fruit and vegetable intake has no effect on retinal vessel caliber in adults at high risk of developing cardiovascular disease [randomized trial; moderate evidence] ↩
International Journal of Epidemiology 2017: Fruit and vegetable intake and the risk of cardiovascular disease, total cancer and all-cause mortality-a systematic review and dose-response meta-analysis of prospective studies [observational study; very weak evidence] ↩
European Journal of Nutrition 2018: Effect of increasing fruit and vegetable intake by dietary intervention on nutritional biomarkers and attitudes to dietary change: a randomised trial [moderate evidence]
Cancer, Epidemiology, Biomarkers and Prevention 2003: No effect of 600 grams fruit and vegetables per day on oxidative DNA damage and repair in healthy nonsmokers [randomized trial; moderate evidence] ↩
Cancer Prevention Research 2016: Harnessing the power of cruciferous vegetables: Developing a biomarker for brassica vegetable consumption using urinary 3,3′-diindolylmethane [randomized trial; moderate evidence] ↩
But bear in mind liver is also a very rich source of vitamin A – and a single serving contains twice the “upper tolerable limit” for vitamin A. Vitamin A toxicity is very rare and usually occurs following a rapid and acute increase in vitamin A intake. So this is why eating a variety of foods to get your vitamin C is a good idea.
It’s possible that people who eat low-carb diets need less vitamin C because glucose competes with vitamin C receptors. At this point this is based on mechanistic studies without human data showing a decreased requirement.
We are unaware of any studies examining an RDA for fiber in the setting of a low-carb or keto diet ↩