"Eat your fruits and vegetables!" You've probably heard that nagging scold more often than any other nutritional advice - not only from your mother but from doctors, health organizations, and governments over the last four decades.
Upping your fruit and veggie consumption has been recommended so often, for so long, as the best way to live longer, healthier lives, that to question that rationale seems very controversial.
But how many fruits and vegetables do we really need to eat? Might we perhaps be better off eating fewer of them than recommended? More provocatively, do we actually need any at all? In a world where fruit bowls and green smoothies are seen as virtuous and bacon is viewed as sinful, it may seem difficult to even consider these possibilities.
Yes, there's been a lot of research on the health benefits of fruits and vegetables, but is it rigorous enough to recommend a standard minimum daily intake for both healthy people and those with medical issues? Read on to learn where the scientific evidence currently stands on fruits and vegetables.
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, fruits are juicy due to their high water content.
Nutritional composition of fruits
Nearly all of the calories in fruit come from sugar - not surprising, given their sweet taste. Their net carb counts span a large range: 5 grams to 20 grams of carbs per 100 grams (3.5 ounces) of fruit, depending on the type. A single juicy, mid-sized orange would have about 12 grams of carbs and a banana at least 23 grams.
Reviewing the nutrition profiles of different fruits, we see that several are good sources of vitamin C and a few minerals. However, the 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. In addition, all fruits fall short in key minerals like calcium, magnesium, iron and zinc.
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, turnip, yams, potatoes, sweet potatoes etc.
- 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
Above-ground vegetables are keto-friendly foods that provide 5 or fewer grams of net carbs per 100-gram (3.5-ounce) serving. Not so for the root and starchy vegetables, however, which range from 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, although 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.
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 meet the US Dietary Guidelines recommendations for fruit and vegetable intake. But is this really a problem?
Some low-carb and ketogenic diet experts would say no; that if someone is following a diet that meets their needs for essential nutrients, consuming 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 fruit of any kind - as part of a very-low-carb diet.
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Other doctors have gone on record as saying that consuming plants is entirely optional - and in some cases may be problematic - including the Paleomedicina Group and Dr. Georgia Ede.
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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 recommendation are designed for someone who is eating a standard Western, or low-fat, high-carb diet.
Let's explore the high-quality evidence available to date to see whether eating more fruits and vegetables has actually been proven to improve health.
4. Research on the benefits of eating fruits and vegetables
Obesity
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 and meta-analysis of eight randomized controlled trials (RCTs) - considered the strongest level of evidence - found that people assigned to increase their intake of fruits and vegetables lost an average of only 1.5 pounds more than people assigned to eat smaller amounts of these foods. That's hardly a significant difference in studies that lasted anywhere from four weeks to 12 months.
The same year, researchers who published a 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.
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 added fresh fruits and vegetables to their diets for eight weeks, they gained twice as much weight as lean participants, who responded by eating less of other foods. However, people from all three groups gained weight after consuming fruits and vegetables in juice form during a second eight-week period, although obese participants gained the most.
On the other hand, replacing highly processed refined 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 provided with vouchers to purchase only 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.
Overall, though, 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 been predominantly larger yet lower-quality observational (also called epidemiological) research. For instance, a 2015 systematic review analyzing 17 epidemiological studies found statistically weak associations between eating a lot of produce and lower body weight and waist size.
In observational nutrition studies, a hazard ratio (HR), odds ratio (OR) or relative risk (RR) that is close to 1 means there is almost no observable difference. That means any OR less than 2 or greater than 0.50 strongly suggests that any association between a behavior (e.g., eating produce) and an outcome (e.g., weight loss) is likely random and false. What were the ORs in this study? Pretty underwhelming: 0.83 for high intake of fruits or vegetables separately, and 0.91 for high intake of fruits and vegetables combined. Indeed, even the study authors acknowledged that "The present meta-analysis seems to be limited by low study quality."
Diabetes and metabolic syndrome
Fruits and vegetables are generally considered diabetes-friendly foods. In fact, all types of produce make the " low GI foods " list on the American Diabetes Association website. 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 and meta-analysis of eight RCTs examining the effects of fruit and vegetable intake in people with metabolic syndrome found that although blood pressure improved in people who ate more produce, fasting blood sugar levels were no different among the groups. The same held true for waist circumference, triglycerides and HDL cholesterol levels - all of which are considered markers of insulin resistance when outside the normal range.
In 2017, Scottish 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 overall vitamin and mineral intake but consumed significantly more carbs. That doesn't sound very diabetes-friendly, does it?
A randomized controlled study published in Diabetes Care - a journal of the American Diabetes Association - found that overweight adults who consumed four to seven fruits and vegetables per day didn't experience any improvement in insulin resistance compared with those consuming less produce.
However, results from another RCT suggest that including low-carb vegetables like broccoli may help lower insulin levels and improve insulin sensitivity in adults with type 2 diabetes. 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 low-carb or keto diets would provide any further benefit on insulin resistance. Since low carb diets by definition eliminate the biggest offenders behind insulin resistance, 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 the lower-quality observational studies health organizations reference in support of claims that eating plenty of fruits and vegetables can help protect against diabetes? Large meta-analyses of these studies have shown very weak associations between fruit and vegetable intake and diabetes risk.
Finally, although results from observational nutrition studies often have such weak correlations that they're likely due to chance, occasionally there are exceptions. For example, a 2017 prospective cohort study in pregnant women found that those who reported consuming the highest amount of fruit during their second trimester had a 400% greater risk (OR of 4.82) of developing gestational diabetes than women with the lowest reported fruit intakes.
Controlled studies exploring this relationship are needed. However, it certainly seems possible that eating large amounts of fruit ("nature's candy") during pregnancy - a time of dramatic hormonal fluctuations and insulin resistance - could increase a woman's likelihood of developing gestational diabetes.
Videos about diabetesHeart disease
Are fruits and vegetables heart-healthy? Although some experimental research suggests that eating more produce might reduce some cardiovascular disease (CVD) risk factors, the bulk of evidence to date is inconclusive.
For instance, in 2013 researchers conducted a systematic review of 10 RCTs investigating whether increased fruit and vegetable consumption 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 observed beneficial effects on CVD risk. 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 arterial health, while the effects of high fruit and vegetable intake on arterial function weren't clear.
RCTs conducted after the 2013 meta-analysis suggest that eating more fruits and vegetables may increase blood levels of antioxidants that improve HDL function and might also reduce inflammation in those at high risk for CVD, such as those with diabetes.
By contrast, RCTs in 2015 and 2016 study 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. 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 per cent reduction (relative risk reduction of 0.68) 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).
Is it realistic that you can greatly decrease your risk of heart disease solely by eating 10 servings of fruits and vegetables every day? Never mind that food-frequency questionnaires and other methods for reporting dietary intake in observational studies are notoriously inaccurate. An RR of 0.68 suggests that this association has a high chance of being random and false.
In all likelihood, anyone who actually eats 10 servings of produce on a daily basis also engages in other habits known to protect heart health, such as working out regularly and avoiding junk food, excessive alcohol consumption and smoking.
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" 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 really 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 cellular DNA 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. Similar results have been shown for cruciferous vegetables like broccoli and cabbage. However, other studies haven't shown any improvement in DNA repair in people who increased their fruit and vegetable intake.
Can healthy people reduce their chance of developing cancer by consuming antioxidants found in plants? At this point, we don't really know. After all, our bodies have their own built-in antioxidant systems in place that, when functioning normally, can help repair cell damage. Additional trials exploring the effects of fruits and vegetables on DNA repair would give us more information about this.
Cruciferous vegetables contain compounds that might help reduce cancer risk in several ways, such as decreasing inflammation and improving cell signaling. Yet it appears that there's a threshold of these protective compounds that can be absorbed, so that very high intakes offer no further benefit than consuming more modest amounts.
Many fruits and vegetables contain other phytochemicals (literally "plant chemicals") with potential anti-cancer activity, such as resveratrol and sulforaphane. 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 a limited amount of experimental research, plenty of observational studies explore the relationship between fruit and vegetable intake and cancer risk. Researchers who conduct systematic reviews and meta-analyses of these studies often conclude that people who eat the most fruits and vegetables decrease their risk of developing cancer. Yet these analyses reveal very 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.
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.
Other conditions
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 assigned to consume the "Scarborough Fare" diet - which included several 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.
- Psychological health: Only a single two-week study has investigated the potential psychological 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.
4. Modern-day fruits: larger, sweeter and widely available
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 simple 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 as a result of advanced farming methods and hybridization.
Fruits in particular have become much bigger and sweeter than anything paleolithic humans could have imagined:
Diet Doctor: What fruits looked like before we domesticated themColossal apples, oranges and other fruits deliver more sugar with each bite - delighting our taste buds but ushering more glucose and fructose into the bloodstream than our bodies may be able to handle effectively.
Should we really be consuming much of these modern-day fruits if we're concerned about our weight, blood sugar and general health?
Indeed, following a very-low-carb diet containing no fruit at all (other than perhaps minimal amounts of berries on occasion) has been repeatedly shown to help people lose body fat and get their diabetes or pre-diabetes under optimal control.
5. Can eating fruits and vegetables cause health problems in some people?
Most of us can eat a moderate portion of vegetables without any difficulty. Fruits, because they're higher in sugar, 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, plenty of personal stories can be found online about some of the more common ones, including:
- Salicylates: Although harmless for most of us, salicylates are chemicals found naturally in certain foods and also synthetically produced for use in medications like aspirin and other products. Ingesting them can lead to asthma, nasal discharge, and digestive issues in susceptible people. Fruits and vegetables high in salicylates include berries, oranges, pineapples, apricots, broccoli, cucumbers and zucchini.
- Oxalate: People who suffer from kidney stones and women with vulval pain syndrome may find that their symptoms worsen after consuming fruits and vegetables high in oxalate. 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 people with excessive levels or an inability to break it down, histamine can cause symptoms like hives, abdominal pain, asthma and headaches, among other symptoms. High-histamine foods include eggplant, spinach, tomatoes and avocados.
- FODMAPS: This is an acronym for fermentable oligosaccarides, disaccarides, monosaccarides and polyols. Essentially these are all fermentable sugar molecules, found most often in fruits, vegetables and grains, that can cause digestive upset in people with Irritable Bowel Syndrome. People with IBS often find reducing or eliminating FODMAP foods greatly improves their symptoms.
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. Moreover, your symptoms might be due to another health problem. Make sure to see a doctor for a full work-up to rule out other causes.
6. Will a diet devoid of 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 provide a range of micronutrients, other real foods can help meet your needs. Meat and fish provide as many (or more) essential vitamins and minerals as fruits and vegetables do, and organ meats like chicken liver and beef kidney are excellent sources of vitamin C.
In fact, there is evidence that we need far less vitamin C on a low-carb diet because glucose competes with vitamin C receptors; the more sugar or carbs you eat, the more vitamin C you need in your diet. That said, it may be difficult to consistently meet your nutrient needs for vitamin C, potassium, folate and other key nutrients on an entirely produce-free diet.
What about fiber? Although fiber isn't an essential nutrient, most health organizations, including the Dietary Guidelines for Americans, recommend a daily minimum of 25 grams per day for adults. Of course, fruits and vegetables aren't the only source of dietary fiber; nuts and seeds also provide some. 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, studies demonstrating fiber's health benefits have been in people eating higher-carb diets.
Recently, ketogenic experts Dr. Steve Phinney and Jeff Volek PhD, wrote a post on their Virta Health blog explaining that although high fiber intake may be important for people who don't follow a ketogenic diet, fiber needs may be greatly reduced for those of us in nutritional ketosis. That is because during ketosis, the ketone body beta hydroxy butyrate (BHOB) protects the intestinal lining and is very similar to the chemical, butyrate, that is produced by the gut microbiome when it digests fiber.
Virta Health blog: Fiber and colon health on a well-formulated ketogenic diet.7. Summary - fruits and vegetables: Optional, optimal, or essential?
In summary, 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 a tasty sauce. Think creamed spinach, zucchini noodles with alfredo sauce, or berries with high fat cream.
Moreover, there's no question that replacing processed foods high in added sugar and refined carbohydrates 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. However, there's a limited amount of high-quality evidence on this, and much of it is conflicting. The observational research is much larger in volume but can't be relied upon due to extremely weak correlations and a likely "healthy user" bias.
Aside from phytochemicals (and vitamin C, if organ meats aren't consumed), fruits and vegetables don't contain any nutrients that can't be found in other whole low-carb foods. And since there's a lack of high-quality evidence on the role that phytochemicals play in human health, at this time we can't say with certainty that consuming fruits and vegetables is absolutely necessary for everyone.
On the other hand, including vegetables - and in some cases low sugar fruit like berries - probably makes sense for most people. Yet the optimal amounts to consume are unknown, and it's likely that they vary from person to person. Some of us thrive on a diet that includes several servings of produce a day, while others seem to do best with minimal amounts.
On a keto or low-carbohydrate diet, low-sugar fruit and vegetables should be consumed based on personal preference, metabolic goals, and individual tolerance. Across-the-board recommendations to include five or more servings of produce per day, especially when the types aren't specified, simply isn't based on strong evidence.
/ Franziska Spritzler, RD