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Do Vegetables Lose Their Nutritional Value When Heated?

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Frying these foods will give you a major boost in important disease-fighting vitamins

While frying can reduce the nutritional value of most foods, there’s an exception to the rule:

It’s a group of foods that contain significant amounts of the organic pigments called carotenoids, which studies indicate can help reduce the risk of several chronic diseases in humans, like heart disease, eye disease, and certain cancers.

When you expose carotenoids to high temperatures, energy from the heat breaks them down. This makes it easier for the body to absorb into your bloodstream, where it goes to work against disease.

And if you fry those foods in oil, as opposed to steaming or baking them, you absorb even more because carotenoids are fat soluble.

Where to find carotenoids

Carotenoids are prevalent throughout nature, but the three that are most common in our foods are:

  • Pro-vitamin A carotenoids like alpha carotene and beta carotene, which gives carrots and sweet potatoes that iconic orange color and has been shown to reduce the risk of age-related macular degeneration — a leading cause of vision loss in people over 50.
  • Lycopene, which provides most red-hued fruits, like tomatoes, pink grapefruit, and red peppers, their color. About 80% of the lycopene we ingest comes from tomato-based products, and over the last decade, studies have found that the lycopene in tomatoes could be linked to a reduced rate of prostate, lung, and stomach cancers.
  • Lutein, which is found in dark leafy greens like kale and Brussels sprouts and has also been shown to reduce the risk of eye diseases, like age-related macular degeneration.

A major nutritional boost

So just how much more of these disease-fighting carotenoids do you get by pan frying them in oil (not to be mistaken for deep frying, which is an entirely different method of cooking)?

We asked Guy Crosby, who has spent 30 years in the commercial food industry business and is now the editor for America’s Test Kitchen and teaches a food science course at the Harvard School of Public Health. You can learn more about him on his site “The Cooking Science Guy.”

As Crosby explains:

“In the fresh tomato most of these [carotenoid] pigments are all tied up with proteins,” Crosby told Business Insider. “If you cook the tomato you break down the bonds between the proteins and the pigments — the lycopene — and you absorb about four times more lycopene into your blood from cooked tomatoes than from fresh tomatoes.”

But wait, there’s more: Carotenoids fall under a class of vitamins called fat soluble vitamins, as opposed to water soluble vitamins like Vitamin C and some types of Vitamin B. This means that carotenoids will dissolve in fats, for example the fat in frying oil, just like the Vitamin B-6 in broccoli dissolves in water when you boil it.

“Since lycopene is soluble in oil, if you cook your tomato in olive oil, you’ll absorb two times more again above and beyond from what you absorb from cooking tomatoes without the oil,” Crosby said.

Now, if you’re watching your waistline, it’s important to limit the amount of fat you ingest daily. And frying anything is certainly going to up the fat content.

However, you don’t need very much oil to get this boost in nutrition — about three to five grams of fat is enough, which is equivalent to one teaspoon of olive oil.

Serving Vegetables Makes You More Thoughtful, Less Boring

Fact or Fiction: Raw veggies are healthier than cooked ones
Do vegetables lose their nutritional value when heated?

By Sushma Subramanian  March 31, 2009

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 tells ScientificAmerican.com 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 plant’s 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 carotenoids, 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.

Jessica Orwig    Feb. 1, 2016

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