The USDA and most nutritionists recommend eating a diet with lots of fruits and vegetables. In the old guideline, the recommendation was to “strive for five” servings of fruits and vegetables. The new guideline advises us to fill half of our plates with fruits and vegetables. Food writer Michael Pollen (see essay here) tells us to “Eat food. Not too much. Mostly plants.”
The question then is why? Food consists of the three macronutrients – fat, carbohydrates and protein, water, fibre, minerals, vitamins, other micronutrients (more on this later), and ash (non-digestible stuff). You need water everyday. Your energetic requirements come out of the macronutrients. There are a number of essential amino acids, fatty acids, vitamins and minerals you need. Fibre is thought to be important for digestion but you could live without it. In principle, you could obtain all of your nutritional requirements by eating raw animal products (cooking and drying may destroy some vitamins like vitamin C). You could also just eat hamburgers and a multi-vitamin supplement but most health conscious people would recoil from such a diet. Thus there seem to be three dietary situations: 1) malnutrition, where you miss some of the essential nutrients, 2) nutritional sufficiency, where you obtain all the essential nutrients, and 3) healthful eating, which is 2) plus lots of fruits and vegetables (and perhaps other things like olive oil, etc).
Whether there is a real difference between 2) and 3) is not proven. My emotional reaction is that a balanced diet rich in vegetables is healthier than one consisting of pizza and soda but this is not based on science. Every generation has it’s nutritional fad – be it vitamin C, anti-oxidants, or lycopenes but the evidence that these actually make people healthier in the long term is scant. There have been epidemiological studies (e.g. see here) that have found a correlation between eating fruits and vegetables to living longer but correlation, as they say, is not causation. It is basically impossible to study the effect of some factor on humans in a controlled way.
I can think of three plausible reasons for why 3) is different from 2). The first is that there are nonessential and potentially unknown molecules in plant derived foods that are healthful. These are potentially toxins that are beneficial in minute doses. The second is that the combination of nutrients in fruits and vegetables are optimal to promote health. We evolved eating plant matter so perhaps our systems are optimized to obtaining food in that form. The third explanation is that what really matters is eating fewer calories and since fruits and vegetables have low caloric content, they displace higher calorie foods.
I’m leaning more towards three at this point. It’s pretty hard to overeat if you’re munching on celery sticks. You also get more water and people may be dehydrated. I don’t think two is very plausible since diets are highly varied around the world and the digestive system mixes everything we eat and then dispenses the various constituents over multiple time scales. However, Pollen argues that there could be unknown reactions taking place when we consume “real foods” as opposed to processed stuff and this may have some validity. Reason one is intriguing although very difficult to test. It follows the hormetic hypothesis that mild stresses to our system like exercise can be beneficial. In any case, as with most things, moderation may be the best course.