IN nutrition, there is sometimes a tendency to identify a food by a single outstanding nutrient -- milk for calcium, meat for iron, orange juice for vitamin C. But in looking at foods this way, the forest may be lost for the trees.
Foods are certainly more than one single component -- and this is especially true for fruits and vegetables, which contain literally thousands of substances called phytochemicals, which have wide-reaching effects on health.
They act as antioxidants, defending against damage in the body's cells that results from normal metabolism. They keep inflammatory processes in check and protect our genetic material.
Many of these beneficial compounds provide fruits and vegetables with their beautiful colors, none more than the carotenoids, nature's most widespread pigments. They include lycopene (which provides the red color to tomatoes, pink grapefruit and watermelon), beta carotene (the orange in carrots, pumpkin, papaya and cantaloupe) and yellow-green lutein (found in spinach, corn and avocado).
Each fruit and vegetable has its own unique phytochemical profile and level of antioxidant activity. There is no recommended daily intake for antioxidants and other phytochemicals -- just the advice that we consume them in five to nine servings of fruits and vegetables a day.
The advice to eat produce instead of taking supplements makes sense, because attempts to pinpoint the specific, healthful "magic bullets" in fruits and vegetables have been, for the most part, disappointing. Scientists know, for instance, that the risk of cancer goes down with an increased consumption of deep green and yellow vegetables and fruits. Because these foods are rich in beta carotene, it has been tempting to assume that this single compound could be responsible for cancer protection. Yet clinical studies have been unable to bear this out.
Also, it is becoming increasingly clear that whole fruits and vegetables, with their full complement of phytochemicals, are more bioactive than any single substance they might contain.
Logically, it makes sense that whole foods should trump single nutrients when it comes to keeping us healthy. After all, from a genetic standpoint, we are still hunter-gatherers; we're just clad in jeans these days instead of loincloths. Our bodies are designed to receive a range of phytochemicals, not different ones in isolation, and we have complex enzymatic machinery to metabolize them.
But eating whole foods is only part of the equation. Adding new fruits and vegetables in new combinations and varieties may be more healthful still.
Phytochemicals seem to work cooperatively to exert their effects so that the benefits, when they are taken together, are often greater than the sum of the parts. For example, it has been shown that the antioxidant effects of a combination of red apples, blueberries, grapes and oranges are much larger than when any of the fruits are taken individually.
Even different varieties of the same fruit or vegetable can provide different compounds to the body. For example, a deep-purple blood orange shares phytochemicals in common with berries, while a navel orange doesn't, so the two offer somewhat different benefits. The pairing of different foods may affect how well these nutrients are absorbed. Colorful carotenoids, for example, are all fat-soluble, so small amounts of fat eaten at the same time (a teaspoon or less in each meal) helps the body efficiently use them.
Scientists have shown that the healthful fat in avocado helps the body absorb the carotenoids in romaine, carrots and tomato. Thus, guacamole and salsa eaten together may pack a better punch than either one eaten alone.
Finally, while this may not be welcome news to raw food enthusiasts, cooking carotenoid-rich spinach, carrots and tomatoes releases the carotenoids from the whole food, making them up to six times more bioavailable than when the foods are eaten fresh.
Cooked or raw, increasing your fruit and vegetable servings is a great first step toward reaping the health benefits of literally thousands of beneficial phytochemicals. Adding new foods, new varieties and new combinations may be even better.
Susan Bowerman is a registered dietitian and assistant director of the UCLA Center for Human Nutrition.