A growing body of research shows that when it comes to vegetables, it’s not only how much we eat, but how we prepare them, that influences the amount of phytochemicals, vitamins and other nutrients that enter our body.
The benefits are significant. Numerous studies show that people who consume lots of vegetables have lower rates of heart disease, hypertension, diabetes, eye problems and even cancer. The latest dietary guidelines call for 5 to 13 servings — that is, 2 1/2 to 6 1/2 cups a day. But how should they be served?
Surprisingly, raw and plain vegetables are not always best. “There is a misperception that raw foods are always going to be better,” said Steven K. Clinton, a nutrition researcher and professor of internal medicine in the medical oncology division at Ohio State University. “For fruits and vegetables, a lot of times a little bit of cooking and a little bit of processing actually can be helpful.”
No single cooking or preparation method is best. Water-soluble nutrients such as vitamins C and B and a group of nutrients called polyphenolics are often lost in processing. For instance, studies show that after six months, frozen cherries have lost as much as 50 percent of anthocyanins, the healthful compounds found in the pigment of red and blue fruits and vegetables. Fresh spinach loses 64 percent of its vitamin C after cooking. Canned peas and carrots lose 85 to 95 percent of their vitamin C, according to data compiled by the University of California at Davis.
Fat-soluble compounds such as vitamins A, D, E and K and the antioxidant compounds called carotenoids are less likely to leach out in water. Cooking also breaks down the thick cell walls of plants, releasing the contents for the body to use. That is why processed tomato products have higher lycopene content than fresh tomatoes.
In January, a report in The Journal of Agriculture and Food Chemistry concluded that overall, boiling was better for carrots, zucchini and broccoli than steaming, frying or serving them raw. Frying was by far the worst.
Still, there were trade-offs. Boiling carrots, for instance, significantly increased measurable carotenoid levels but resulted in the complete loss of polyphenols compared with raw carrots.
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That report did not look at the effects of microwaving, but a March 2007 study in The Journal of Food Science looked at the effects of boiling, steaming, microwaving and pressure-cooking on the nutrients in broccoli. Steaming and boiling caused a 22 to 34 percent loss of vitamin C. Microwaved and pressure-cooked vegetables retained 90 percent of their vitamin C.