This article was first published in The Montreal Gazette.
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 ten 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 gold standard is the double-blind, placebo-controlled, randomized trial. Suppose you want to determine if creatine supplements aid in athletic performance. There is theoretical rationale for this since creatine occurs naturally in the body and is known to play a role in regenerating adenosine triphosphate (ATP), the molecule that provides energy for many cellular processes. A test for enhanced performance would require two groups of subjects with one group being given a fixed dose of creatine every day and the other receiving a placebo. Tests for time to exhaustion on an exercise bike, or ability to bench press weights, would be determined before and after taking the creatine supplement. A placebo control is important because just the belief that a supplement can enhance energy can actually improve performance. Double-blinding is also necessary because there is a subjective element to the determination of when a subject experiences exhaustion. Such studies have been carried out and indeed have shown that creatine supplementation can help performance in some athletic events.
When it comes to dietary studies, blinding is not possible because you know whether you are eating broccoli or a hamburger. This is not a big impediment, however, because trials usually attempt to determine how dietary composition affects objective measures such as body weight, blood pressure, cholesterol levels or onset of some disease. As an example, let’s look at a trial carried out in New Zealand that aimed to investigate the effect of a plant-based diet on body weight and blood cholesterol. Sixty-five overweight subjects with at least one other cardiovascular risk factor were randomized into two groups. One group was instructed on ways to switch to a plant-based diet, and for motivation were told to watch “Forks Over Knives,” a documentary that features Cleveland Clinic physician Caldwell Esselstyn and Cornell University biochemist T. Colin Campbell. Both are staunch advocates of the health benefits of grains, fruits and vegetables and believe that animal foods, including dairy, are detrimental to health. The control group was not given any specific dietary advice. After 12 months, the control group showed no change, but the experimental group had lost more weight and managed to reduce blood cholesterol.
Such studies are interesting but can be criticized for being of short duration and having too few subjects. But long-term dietary intervention studies are extremely difficult to carry out. A clear demonstration of the benefits of fruit and vegetables would require monitoring the health of two sets of subjects for decades, one consuming minimal amounts, and the other at least five servings a day. Other factors such as activity level, socioeconomic background, calorie intake, smoking and use of medications would have to be controlled for. Clearly this represents an organizational as well as an economic challenge. That is why most of what we know about the benefits of fruit and vegetables is based on “observational studies.” These fall into two categories: case-control and 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. For example, higher intake of red meat was associated with an increased the 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.
The Hisayama study in Japan is another interesting example. Starting in 1961, roughly 1,000 healthy participants over the age of 60 in Hisayama filled out dietary questionnaires and were then followed for 24 years, by which time close to half had developed some sort of dementia. Subjects who ate the most vegetables had a 30 per cent reduced risk of dementia, including Alzheimer’s disease, when compared with those who ate the least vegetables. There was no significant association with fruit intake. Numerous other case-control and cohort studies have shown an association between increased plant-based foods 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. They claim, without any evidence, 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 fruit 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 fruit 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.
