Your grandmother, if you were lucky enough to have one, probably told you to eat your fruits and veggies. And now it seems that those grandmothers who meddled with our dietary habits and urged kids to eat their peas and carrots were bang on. Grandmas have been joined by a plethora of scientists who tell us that we should be eating anywhere between five and 10 servings of fruits and vegetables a day. Grandmothers went by instinct, but science progresses through studies. So what evidence do the scientists have for providing their advice?
The strongest evidence would come from randomized, controlled trials in which two groups of people are followed for years with one group being subjected to some sort of intervention. In this case, one group would eat a limited amount of fruits and vegetables, the other would consume the recommended amount. All other lifestyle factors such as exercise, smoking, total calorie intake, exposure to pollutants and medications would have to be the same. Ideally the average age of the two groups would be the same and they would be drawn from the same socio-economic background. Such studies are expensive and very difficult to organize and none for fruit and vegetable intake have been carried out.
The closest is the study that compared the DASH diet (Dietary Approaches to Stop Hypertension) to a typical North American diet. The DASH diet featured more fruits and vegetables but it was also lower in refined carbohydrates, higher in fibre, higher in low-fat dairy products, nuts and poultry at the expense of red meat. Blood pressure decreased with the DASH diet but fruits and vegetables were not the only distinguishing difference. So for the overall benefits of fruits and vegetables we are left with “observational studies.” These fall into two categories: case-control or cohort studies. In either case there is no intervention by the researchers, they just observe a group of subjects and note their exposure to some factor of interest and record disease outcomes.
In a case-control study subjects with a certain disease are compared to a group that is matched in every way except for the presence of the disease in question. A classic example is smoking. When groups of lung cancer patients were compared to healthy people, it became clear that the cancer patients were much more likely to be smokers. Cohort studies furthered this link. In these studies, groups of smokers and groups of non-smokers were recruited and were followed for years. The smokers were more likely to develop lung cancer.
Another type of cohort study involves following a large group of subjects for many years, evaluating their lifestyles usually through elaborate questionnaires, and recording cases of disease. A classic example is the Nurses Health Study that began in 1976 by recruiting over 120,000 registered nurses. Some interesting findings have emerged in terms of diet. For example, higher intake of red meat was associated with an increased risk of premenopausal breast cancer. A Mediterranean type of diet with vegetables, nuts and fish reduced the risk of heart disease and stroke, a high intake of green leafy vegetables reduced the risk of cognitive impairment, and high intakes of folate, vitamin B6, calcium and vitamin D reduced the risk of colon cancer. Numerous other case-control and cohort studies have shown an association between increased fruit and vegetable intake and reduced risk of various diseases.
Still, there are people who want no part of fruits or vegetables and champion a “carnivore diet,” meaning they eat only meat. The claim, without any evidence, is that such a diet reduces inflammation as well as the risk of food intolerances. Proponents ignore studies that have linked meat consumption, particularly processed meat, to cancer, and run the risk of scurvy due to the lack of vitamin C and bowel problems due to a lack of fibre. To say nothing of the numerous phytochemicals that have antioxidant and other beneficial properties.
True, one can always argue that when it comes to fruits and vegetables observational associations can never prove cause and effect and that only a randomized controlled trial can do that. However, we have such an overwhelming number of observational studies that show the benefits of fruit and vegetable intake that it would be a waste of energy and money to organize randomized trials. Grandmas were right. Eat those fruits and veggies. That doesn’t mean meat has to be avoided but best leave the carnivore diet to tigers and lions. They have no need of vitamin C or fibre.