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).