Do vegetables lose their nutritional value when heated?
By Sushma Subramanian
Cooking is crucial to our diets. It helps us digest food without expending huge amounts of energy. It softens food, such as cellulose fiber and raw meat, that our small teeth, weak jaws and digestive systems aren't equipped to handle. And while we might hear from raw foodists that cooking kills vitamins and minerals in food (while also denaturing enzymes that aid digestion), it turns out raw vegetables are not always healthier.
A study published in The British Journal of Nutrition last year found that a group of 198 subjects who followed a strict raw food diet had normal levels of vitamin A and relatively high levels of beta-carotene (an antioxidant found in dark green and yellow fruits and vegetables), but low levels of the antioxidant lycopene. Lycopene is a red pigment found predominantly in tomatoes and other rosy fruits such as watermelon, pink guava, red bell pepper and papaya. Several studies conducted in recent years (at Harvard Medical School, among others) have linked high intake of lycopene with a lower risk of cancer and heart attacks. Rui Hai Liu, an associate professor of food science at Cornell University who has researched lycopene, says that it may be an even more potent antioxidant than vitamin C.
One 2002 study he did (published in the Journal of Agriculture and Food Chemistry) found that cooking actually boosts the amount of lycopene in tomatoes. He reports that the level of one type of lycopene, cis-lycopene, in tomatoes rose 35 percent after he cooked them for 30 minutes at 190.4 degrees Fahrenheit (88 degrees Celsius). The reason, he says: the heat breaks down the plants' thick cell walls and aids the body's uptake of some nutrients that are bound to those cell walls.
Cooked carrots, spinach, mushrooms, asparagus, cabbage, peppers and many other vegetables also supply more antioxidants, such as carotenoids and ferulic acid, to the body than they do when raw, Liu says. At least, that is, if they're boiled or steamed. A January 2008 report in the Journal of Agriculture and Food Chemistry said that boiling and steaming better preserves antioxidants, particularly carotenoid, in carrots, zucchini and broccoli, than frying, though boiling was deemed the best. The researchers studied the impact of the various cooking techniques on compounds such as carotenoids, ascorbic acid and polyphenols.
Deep fried foods are notorious sources of free radicals, caused by oil being continuously oxidized when it is heated at high temperatures. These radicals, which are highly reactive because they have at least one unpaired electron, can injure cells in the body. The antioxidants in the oil and the vegetables get used up during frying in stabilizing the cycle of oxidation.
Another study published in the Journal of Agricultural and Food Chemistry in 2002 showed that cooking carrots increases their level of beta-carotene. Beta-carotene belongs to a group of antioxidant substances called carotenoids, which give fruits and vegetables their red, yellow, and orange colorings. The body converts beta-carotene into vitamin A, which plays an important role in vision, reproduction, bone growth and regulating the immune system.
The downside of cooking veggies, Liu says: it can destroy the vitamin C in them. He found that vitamin C levels declined by 10 percent in tomatoes cooked for two minutes—and 29 percent in tomatoes that were cooked for half an hour at 190.4 degrees F (88 degrees C). The reason is that Vitamin C, which is highly unstable, is easily degraded through oxidation, exposure to heat (it can increase the rate at which vitamin C reacts with oxygen in the air) and through cooking in water (it dissolves in water). Liu notes, however, that the trade-off may be worth it since vitamin C is prevalent in far more fruits and vegetables than is lycopene. Among them: broccoli, oranges, cauliflower, kale and carrots. Besides, cooked vegetables retain some of their vitamin C content.
That said, research shows that some veggies, including broccoli, are healthier raw rather than cooked. According to a study in the Journal of Agricultural and Food Chemistry in November 2007, heat damages the enzyme myrosinase, which breaks down glucosinates (compounds derived from glucose and an amino acid) in broccoli into a compound known as sulforaphane. Research published in the journal Carcinogenesis in December 2008 found that sulforaphane might block the proliferation of and kill precancerous cells. A 2002 study in The Proceedings of the National Academy of Sciences also found that sulforaphane may help fight the bacterium Helicobacter pylori, which causes ulcers and increases a person's risk of stomach cancer.
On the other hand, indole, an organic compound, is formed when certain plants, particularly cruciferous vegetables such as broccoli, cauliflower and cabbage, are cooked. According to research in The Journal of Nutrition in 2001, indole helps kill precancerous cells before they turn malignant. And while boiling carrots was found to increase carotenoid levels, another study found that it leads to a total loss of polyphenols, a group of chemicals found in raw carrots. Specific polyphenols have been shown to have antioxidant properties and to reduce the risk of cardiovascular disease and cancer, according to a 2005 report in The American Journal of Clinical Nutrition.
Comparing the healthfulness of raw and cooked food is complicated, and there are still many mysteries surrounding how the different molecules in plants interact with the human body. The bottom line, says Liu, is to eat your veggies and fruits no matter how they're prepared. "We cook them so they taste better," Liu says. "If they taste better, we're more likely to eat them." And that's the whole idea. (From Scientific American Online, March 31, 2009)
Exercise 7. What fruits and vegetables are mentioned in the text? Speak about each one and give detailed information about the healthy substances it contains and what happens to it after cooking.
Exercise 8. Divide into three groups. Each group should read one text (either Text A, B or C) about various effects of diet on human health. Then tell other students what you have read about.