Any botanist will tell you that a lot of the foods nutritionists label as "vegetables" are really fruits. Here's what you need to know about fruits and vegetables from a nutritional perspective.
Q. "You suggest that we should eat more vegetables than fruit. But a lot of so-called vegetables (such as zucchini, pumpkin, peppers, cucumbers, tomatoes, eggplant, and green beans) are actually fruits. Is there some reason these are less good for you than true vegetables like spinach and carrots?"
A. You're absolutely right: Nutrition terminology takes little heed of botanical designations. Technically, a vegetable is the leaf, stem, or root of a plant, and any seed-bearing thing that develops out of the flower is considered a fruit. To a botanist, a tomato is clearly a fruit. (Actually, its a berry!) But in nutrition, we tend to define things more by their nutritional profile than their botanical classification.
We generally use "fruit" to describe plant foods that taste sweet, such as apples and grapes, and "vegetable" to describe plant foods that are lower in sugar, such as lettuce and broccoli. But even that isn't an air-tight rule. Lemons and cranberries, which are very low in sugar, are still considered fruits. Carrots, which are relatively high in sugar, are considered vegetables.
And that's just the beginning of the insanity. I usually put avocados, which are technically fruit, in the same category as fats and oils and count corn and potatoes, which are both vegetables, as starches.
See also: Is Corn Really Bad for You?
My advice to eat more vegetables than fruit is mostly about sugar content. Although they can be a good source of nutrients, things like apples, bananas, grapes, and mangoes are also higher in sugar and calories. While I'd rather have you eating fruit than jelly beans, it is still possible to overdo it. Foods like zucchini, peppers, and green beans, on the other hand, boast all the nutrients but a fraction of the sugar.
Botanically speaking, a salad made with cucumbers, peppers, and tomatoes could correcty be described as a fruit salad! But, in my world, those are vegetables. Eat up!