Vegetables are an excellent source of vitamins, minerals, phytochemicals, and fiber, and health-conscious consumers naturally want to know how to get the most nutritional impact from these powerful foods. “Nutritionally, there are pluses and minuses to cooking vegetables,” says Helen Rasmussen, PhD, RD, a senior research dietitian at Tufts’ Jean Mayer USDA Human Nutrition Research Center on Aging. For example, cooking carrots reduces levels of vitamin C (which plays an important role in maintaining collagen, the glue that holds cells together) but increases availability of beta-carotene, a precursor of vitamin A (which plays an important role in vision, reproduction, bone growth, and regulating the immune system).
Reduced Concentration. Some nutrients will be lost in any cooking method. “Some vitamins are not very stable,” says Rasmussen. “The longer a food is exposed to heat, the more vitamin C levels are reduced, for example.” The concentration of some nutrients is particularly affected by cooking in water. “Vitamin C and B vitamins are water soluble, as are certain phytochemicals, like flavonoids,” says Jeffrey B. Blumberg, PhD, a research professor at Tufts’ Friedman School of Nutrition Science and Policy. “They leach out into water when the vegetables are boiled.” Blumberg recommends eating produce high in these nutrients (like broccoli, kale, and bell peppers) raw. “When you do cook them, try methods like steaming, blanching, sauting, roasting, or microwaving, which use little water,” says Blumberg. If you do boil your vegetables in excess water, Rasmussen recommends using that water to make broths or sauces, rather than pouring nutrients down the drain.
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Increased Availability. Eating a raw diet to avoid losing nutrients to cooking is not the answer. A study published back in 2008 in the British Journal of Nutrition found that following a strict long-term raw food diet was associated with relatively high blood levels of beta-carotene and normal levels of vitamin A, but low levels of prostate-cancer fighting lycopene. “Heat breaks down the cell matrix and allows some nutrients to be released from the cell walls,” says Rasmussen. This explains why studies find cooked tomato products like tomato sauce and ketchup have higher levels of available lycopene than raw tomatoes. Other studies have found vegetables like carrots, spinach, broccoli, mushrooms, zucchini, asparagus, cabbage, and peppers supply more carotenoids to the body when cooked than when eaten raw.
Method Matters. How vegetables are cooked and eaten does make a difference in nutrient availability and absorption. In general, steaming preserves nutrients best, since this method avoids the leaching of water-soluble compounds and limits exposure to heat that can degrade some vitamins. Regardless of cooking method, some nutrients will be better absorbed if veggies are prepared or eaten with some healthy fat. “Fat soluble vitamins (A, D, E, and K) and phytonutrients (especially beta-carotene, lycopene, and other carotenoids) are better absorbed when cooked with vegetable oil,” says Blumberg. “Chopping also helps release these health-promoting compounds.” When eating vegetables raw, such as in a salad, add some dressing, avocado, or nuts to help with absorption of fat-soluble nutrients.
“The best choice is to eat vegetables however they most appeal to you,” says Rasmussen. Blumberg agrees. “Taste and texture matter. If you don’t like them, you won’t eat them at all,” says Blumberg. “More important than raw versus cooked is to eat your veggies. Getting 50 percent of a nutrient is still better than getting nothing.”