Does Heat Destroy Nutrients in Fruits & Vegetables?

A dinner plate may contain both a salad and cooked vegetables in addition to a protein such as fish or chicken, and perhaps a serving of rice. While you may not be thinking about nutrition at that point, one consideration for you might be whether raw or cooked vegetables are a better choice from the standpoint of nutrients.

Cooking has been used for thousands of years to help soften food such as whole grains and dried legumes, to decrease the risk of parasites in meat and to help people digest food more easily. But research of cooking process has produced some interesting facts. For example, lycopene is an antioxidant found in watermelon, red bell peppers and tomatoes. Rui Hai Liu, an associate professor of food science at Cornell University, reported in the 2002 issue of the "Journal of Agriculture and Food Chemistry" that cooking increased lycopene in tomatoes by 35 percent.

Some other nutrients are better supplied from cooked food than raw, according to Liu. Cooked carrots, mushrooms, asparagus, cabbage and peppers supply more antioxidants if they're boiled or steamed. And cooking method matters -- carrots, zucchini and broccoli had higher levels of carotenoid when boiled or steamed than when fried. Deep fried foods are sources of free radicals, which can injure body cells. Broccoli, on the other hand, may be better for you in the raw state, as cooking damages an enzyme that helps break down compounds in the vegetable that seem to have cancer-fighting properties.

However, some vitamins and nutrients are heat-sensitive. The University of Michigan says that when fruits or vegetables are cooked at high temperatures or for long periods of time, heat-sensitive nutrients such as B vitamins, vitamin C and folate are more likely to be destroyed. The University of Illinois Department of Food Science and Nutrition compared a variety of fresh, canned and fresh cooked foods and says that while vitamin C may be lost during the process of cooking or canning, it dissolves in the cooking liquid. In canned foods, the remaining vitamin C is stable for two years. And thiamine, another heat-sensitive B vitamin found in beans, also survives the canning process well.

While most forms of fruits and vegetables will provide a variety of nutrients, the Oregon State University Extension program recommends some strategies for maximum nutrient preservation. Refrigerate raw vegetables and use them as soon as possible. Buy local produce to prevent nutrient loss in shipping, and cook raw vegetables in a small amount of water. Canned foods should be stored in a cool dark place, and the brine or syrup should also be consumed. Frozen foods should be kept from temperature fluctuations and packaged properly.