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How Cooking Affects Nutritional Content of Foods

     Did you know that the preparation and cooking of food may alter its nutritional content? Its vitamin, mineral and fibre content can change once we prepare by peeling, chopping or cooking it. This act can actually reduce the beneficial antioxidants such as carotenoids, vitamin C, calcium, magnesium, phosphorus and potassium. Beneficial fibre may be removed during peeling. Proteins may be altered depending on if meat has been cooked in the microwave. However, there is light at the end of the tunnel, some vegetables are better cooked, steamed or paired with a fat to release their full antioxidant potential.


Most of the nutrients in root vegetables are found in or just under the skin. When we remove the peel, we lose some of the fruit or vegetables vital nutrition such as its fibre, vitamin c and folic acid. Apples have most of their fibre in the skin and peeling an apple deprives us of a third of the fruit's fibre and vitamin C. Potatoes also have most of their fibre in the peel. If possible, try and scrub the vegetable clean and cook leaving the skin on. If you must peel them, try to take away just a thin layers.


Cutting and cubing vegetables prior to roasting, boiling or steaming them does make them lose a little more of their nutritional value during the cooking process. The more surface that is exposed to heat and water, the more nutrients leach into the steam or water. A good idea would be to cook iwth a foil pack to help the veggies retain their moisture in turn their nutrients.

Boiling and Soaking

Boiling and soaking of all fruits and vegetables drastically reduces their nutritional content and natural enzymes. Also canned vegetable have many of their healthy properties leached out in the liquid before you consume them. Not to mention cans of vegetables have added salt and preservatives. If you boil your vegetables the nutrition is found in the water, so using it for soups or adding it to mashed potatoes, turnips or sweet potatoes would ensure the nutrient content is saved. Thus using a slow cooker for soups and stews is very beneficial because all of the nutrients in the food are retained and eaten.

Deep Frying

Deep frying food is a tasty treat, but the excessively hot temperatures of cooking in oil (especially plant oils) causes free radical changes to the oil that are detrimental to health. 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. If you must fry, try sauteeing. Using animal fats for cooking is beneficial because they do not oxidize with heat like plant oils do. Try tallow, marrow or butter for sautees. Click here for a list of smoking points of oils.


Microwaving protein alters it chemical structure and destroys the nutrients in most foods. Here are some studies sited by Dr. Mercola:

  • study published in the November 2003 issue of The Journal of the Science of Food and Agriculture5 found that broccoli "zapped" in the microwave with a little water lost up to 97 percent of its beneficial antioxidants. By comparison, steamed broccoli lost 11 percent or fewer of its antioxidants. There were also reductions in phenolic compounds and glucosinolates, but mineral levels remained intact.

  • A 1999 Scandinavian study of the cooking of asparagus spears found that microwaving caused a reduction in vitamin C.

  • In a study of garlic, as little as 60 seconds of microwave heating was enough to inactivate its allinase, garlic's principle active ingredient against cancer.

  • A Japanese study by Watanabe showed that just 6 minutes of microwave heating turned 30-40 percent of the B12 in milk into an inert (dead) form8. This study has been cited by Dr. Andrew Weil as evidence supporting his concerns about the effects of microwaving. Dr. Weil wrote: "There may be dangers associated with microwaving food... there is a question as to whether microwaving alters protein chemistry in ways that might be harmful."

  • A recent Australian study showed that microwaves cause a higher degree of "protein unfolding" than conventional heating.

  • Microwaving can destroy the essential disease-fighting agents in breast milk that offer protection for your baby. In 1992, Quan found that microwaved breast milk lost lysozyme activity, antibodies, and fostered the growth of more potentially pathogenic bacteria.


Steaming is the best choice if a vegetable needs to be cooked. Light steaming of vegetables retains most of their nutrients compared to boiling, microwaving or blanching. Steaming can enhance absorption in certain vegetables. For example, lycopene in tomatoes is better absorbed when cooked with an oil. A January 2008 report in the Journal of Agriculture and Food Chemistry said that steaming better preserves antioxidants, particularly carotenoid, in carrots, zucchini and broccoli, than frying, though boiling was deemed the best.


Barbequing meats have been linked to certain types of cancer due to the amino acid in meat reacting to the flame and creating carcinogens like polyaromatic hydrocarbons, advanced glycation end products, dioxins and heterocyclic amines. What do all these big words mean? In a nutshell toxic compounds that can increase free radical activity in our body that can lead to wild cells that cause certain types of cancer such as breast, colon, liver, lung and prostate. Most studies are done on animal models thus more research is required but better to err on the side of caution. Some research has found that marinating your meats and cooking them at lower temperatures on the grill (not charring them) produce less carcinogens.

Raw foods

Many foods are beneficial in their raw state because they retain the natural enzymes and many of their vitamins and minerals which are beneficial to health. Enzymes help us to digest our food better and in turn absorb more nutrients. Not to mention fresh produce is very alkaline.

Cooking food at high temperatures can alter the structure of food and cause chemical incertainty that leads to free radical damage. However some foods are better cooked for their nutrient and absorption value. It is important to consume a nice mix of both raw and cooked vegetables. Eating foods in their raw state may cause digestive upset in some sensitive individuals. Legumes are best sprouted or soaked because they can cause gas. Dehydrated foods are acceptable on most raw food diets. Some people make kale, yam, banana, plantain or sweet potato chips using this device for a nice crunchy treat.

What foods are best cooked?

  • Carrots are best cooked. A study published in the Journal of Agricultural and Food Chemistry in 2002 showed that cooking carrots actually increases their level of beta-carotene. Beta-carotene is a powerful antioxidant that belongs to the group of nutrients called carotenoids, which give fruits and vegetables their red, yellow, and orange colorings. Our liver converts beta carotene into usable vitamin A that helps us with our vision, skin, bone growth and immune system. 

  • Tomatoes have been found to increase their lycopene content when they are cooked studies show. Lycopene is a red pigment found in tomatoes, watermelon, guava, red bell pepper and papaya. A higher intake of lycopene prevents cancer and heart attacks.

  • Spinach, mushrooms, carrots, peppers, asparagus and cabbage also supply more antioxidants, carotenoids, ferurlic acid when they are cooked than raw studies show.

What can you do?

  • Eat a nice balance of raw and cooked vegetables

  • Try using foil packets to cook chopped root vegetables

  • Marinate your meat and cook on the BBQ at lower temperatures, do not char the meat

  • Consider using a crock pot to make soups and stews to retain the nutritional value of foods

  • If you cannot use a crock pot, lightly steaming vegetables has a good benefit

  • Consider eating your vegetables and fruit with the peel for better absorption of nutrients

  • If possible cook your root vegetables in their whole or partially whole state to not lose nutrition from chopping and dicing

  • Reduce the temperature and increase the time that you cook at in the oven to reduce toxic compounds from cooking and retain more nutrients

  • Limit the use of a microwave for food

  • Avoid deep frying