Does Cooking Destroy Vitamins and Minerals?

Does Cooking Destroy Vitamins and Minerals?

When you eat vegetables, are you maximizing the health benefits you get from them?

Vegetables are one of the best sources of natural antioxidants and a good source of vitamins and minerals – not to mention rich in heart-healthy fiber. Most people enjoy eating vegetables more when they’re cooked – but what does cooking do the nutritional content of the veggies you’re eating? Most importantly, what can you do to maximize the nutritional value of vegetables when you prepare them?

Nutrient Loss from Cooking

Does cooking destroy vitamins and minerals? Some vitamins and minerals in vegetables are more heat sensitive than others. Among the most fragile vitamins are vitamin C and the B-vitamin folate. Even though some vegetables are a good source of vitamin C, you lose a significant portion of this antioxidant vitamin when you expose it to heat. Unlike vitamins, minerals tend to be more stable when heated with the exception of potassium.

Although minerals are pretty stable when you heat them, they can leach out into the water you use to cook them. When you pour the water into the sink, you’re dumping minerals like calcium, magnesium and zinc down the drain along with some water-soluble vitamins. There goes the good stuff!

What can you do to minimize nutrient loss from cooking? Cook vegetables using a method that minimizes their exposure to high heat and water or save the liquid you cook them in. Use the liquid in soup or stew so you can still benefit from the vitamins and minerals that leached into the water.

Do You Lose Antioxidants and Phytochemicals When You Cook Vegetables?

One reason we’re so fond of vegetables is because they’re rich in natural chemicals called phytochemicals that reduce inflammation and may reduce the risk of some forms of cancer. The question is how do the phytochemicals in veggies hold up under heat?

One study looked at the effects of different cooking methods on anti-cancer compounds called glucosinolates in broccoli. The cooking methods they tested including boiling, steaming, microwaving and stir-frying. Unfortunately, all of these cooking methods led to loss of these anti-cancer compounds with the exception of steaming.

Another study showed that steaming preserves the glucosinolates in cruciferous vegetables including cabbage, cauliflower and Brussels sprouts. Therefore, to maximize the amount of anti-cancer compounds you get from broccoli and other cruciferous vegetables, steaming them is the best option.

Microwaving with only a small amount of water also doesn’t significantly reduce the anti-cancer benefits of broccoli. The key is to limit the amount of water you use. Boiling is the worst way to cook cruciferous vegetables.

Nutrient Loss from Cooking: The Role Heat Plays

Time under heat is a major factor that affects nutrient loss. Some nutrients break down because they’re unstable when heated. In other cases, the nutrient remains stable but leaches out into the water you use to prepare it. Even though roasting exposes vegetables to a high temperature, there’s no water for the nutrients to leach into. That’s an advantage, but there can still be some nutrient loss due to the high heat, although less than if you boil vegetables.

Lightly sautéing vegetables in olive oil is another method to prepare vegetables that minimizes nutrient loss. The healthy fats in the olive oil can actually increase absorption of the fat-soluble vitamins and antioxidants like carotenoids in veggies.

What about Eating Raw Vegetables?

You might expect that eating raw vegetables would be the most nutritious of all. Not necessarily. Cooking makes some nutrients more available to your body. For example, tomatoes are a good source of heart-healthy nutrients called lycopenes, but your body doesn’t get maximal benefits unless the tomatoes have been cooked or processed. The same goes for carrots, a good source of carotenoids. Your body can better use the carotenoids in carrots if they’re cooked.

There are other problems with eating certain vegetables raw. Cruciferous vegetables like broccoli contain compounds called compounds called goitrogens that block the action of thyroid hormone, the hormone that controls your metabolism. Cooking reduces goitrogens by about two-thirds. Goitrogens in foods are mainly a problem for people who have an underactive thyroid gland.

If you have a history of an under-active thyroid or take medication for it, limit the amount of raw cruciferous vegetables you eat. If you don’t have a thyroid problem, it’s fine to eat moderate amounts of raw cruciferous vegetables, but cooking them lightly is better.

How Not to Destroy Vitamins and Minerals When You Cook

  • Lightly steam or sauté veggies whenever possible. We use a vegetable steamer to do this at home. If you microwave vegetables, use as little water as possible. Don’t boil or deep-fry vegetables.
  • Don’t cut vegetables up into small pieces before cooking them. This increases the surface area exposed to heat and water. You’ll retain more nutrients if you keep the pieces larger.
    If you use a slow cooker or microwave to cook vegetables and add water, save the water and use it in a soup or stew so you’ll get the benefits of the nutrients that leached out into the water.
  • Don’t count on cooked vegetables to be a reliable source of vitamin C. Vitamin C is too heat unstable. Get your vitamin C by eating a piece of citrus fruit or other fruits high in vitamin C like kiwi that you don’t have to expose to heat.
  • You can enhance the anti-cancer benefits of steamed broccoli by eating them with uncooked broccoli sprouts. Broccoli sprouts contain significant amounts of an enzyme called myrosinase that activates the anti-cancer compounds in broccoli.
  • Whatever you do keep eating vegetables!

References:

J. Zhejiang Univ. Sc. B. 2009. August: 10(8): 580-88.
World’s Healthiest Foods website.
J Food Sci. 2009 Apr;74(3):H97-H103.

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About Dr. Kristie

Dr. Kristie Leong and Dr. Apollo Leong are physicians helping you to lead a healthy lifestyle by sharing nutrition and fitness tips and keeping you abreast of the latest health news.
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