Vegetables and fruits are the cornerstones of a healthy diet. They are rich sources of vitamins, minerals, complex carbohydrates, and fiber. Some, such as peas and corn, are also good sources of protein. Moreover, vegetables and fruits are generally inexpensive, contain no cholesterol, have little or no fat, and are low in calories. A high intake of vegetables, particularly of the Brassica family (broccoli, cabbage, cauliflower, and Brussels sprouts) can sharply reduce the risk of cancer.10 These vegetables contain compounds that can help the body detoxify and clear potential carcinogens. In addition, fruits and vegetables are rich sources of antioxidant nutrients, such as beta carotene and vitamin C, that may also protect against cancer and heart disease.21,22
Until recently, vegetables and fruits tended to be available on a regional and seasonal basis: asparagus in the spring, tomatoes in the summer, and cabbages in the fall. But today, worldwide distribution has made most food available all year round. This greater availability, however, has come at a price. Large-scale mechanized growing and harvesting methods, combined with a need for foodstuffs to withstand the rigors of long-distance transport and storage, have led to an emphasis on hardiness and a long 'shelf-life' as opposed to flavor, freshness, and nutrient content. The nutritional value of much of today's produce is further reduced by modern intensive agriculture that depletes the soil of important minerals (such as zinc and selenium) so that plants grown on these soils are less nutritious. Furthermore, vegetables and fruits can lose most of their vitamins, particularly fragile ones like riboflavin and vitamin C, when carelessly stored.8 Fresh produce should ideally be stored in a cool dark place; nutrient losses are accelerated when produce is exposed for long periods to light, heat, or air.
Many nutrients are concentrated in or just beneath the skin of produce. For example, nearly all the fiber in an apple is contained in the peel, and much of the vitamin C in potatoes is concentrated just beneath the skin. If apples, pears, potatoes, and other produce are agri-chemical-free, they should be washed thoroughly and the skin left on. The rules for maintaining micronutrient content when cooking vegetables are simple: minimal water, a covered pot, and the shortest possible cooking time.
To get the most micronutrients from fruits, eat them in their fresh, raw state. Some vegetables are healthier if thoroughly cooked, whereas others are much healthier if eaten raw. Levels of oxalic acid, a substance present in spinach and other greens that can block absorption of calcium and iron,23 are reduced by cooking. Also, natural toxins found in cabbages, cauliflower, and mushrooms are heat labile and destroyed by cooking. Mushrooms, beets and beet greens, spinach, cabbage, broccoli, cauliflower, brussel sprouts, peas, beans, and eggplant are all healthier if cooked. On the other hand, most other vegetables, including onions and garlic, are more nutritious when consumed raw.
How nutritious are canned and frozen vegetables and fruits? Most frozen produce is processed without cooking, so most of the micronutrient content is conserved. But canned vegetables and fruits undergo a heating process that destroys much of the vitamin C and B vitamins.8 Also, minerals leach out of canned food into the water, and unless the liquid in the can is used in food preparation, the minerals will be lost. Large amounts of sodium are added during the processing and canning of vegetables. Canned fruit is often conserved in heavily sugared water. A fresh peach has about 70 calories; a canned peach, with the added sugar, contains about 180 calories. When available, fruit that is conserved in its own juice is preferable.
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