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You Are What You Ate

The effects of cooking on nutrition

by Sara Garduno Diaz

Seasonal and imported foods on display at Sandal castle (including a purple carrot), photo courtesy of You Are What You Ate

The effects of cooking on nutrition

Nutrient loss is a consequence of nearly every cooking process. Exposure to heat, light or oxygen will alter the nutrients found in food, and methods that involve water often reduce the amounts of nutrients as these get ‘washed out’ and left behind. However, cooking foods also has its advantages, including a reduction of the number of possible malignant microbes, an increase in digestibility and the increased availability of certain phytonutrients.Image of an egg in an egg box and a whisk (source: Stock.Xchg)

Heating affects mostly the vitamin and fat content of foods, as well as the texture that they present. Some fats are able to tolerate higher temperatures than others, before reaching their 'smoke point', at which their chemical structure is modified. These changes have been associated with health risks, unpleasant odours, impaired flavour and reduced vitamin content. Similarly, the structure of proteins changes with heat too. For example, an egg looks very different from its raw form to its cooked form; this is mostly due to a rearrangement of its structural proteins.

On the other hand, cooking processes that involve heating also make certain nutrients more available for the body to use. For example, the amount of total carotenoids content in carrots and other vegetable-based dishes is higher in boiled versions.

Have you ever used frozen vegetables? If so, you might have noticed how different they look when they are frozen and after they have been reheated. This is partially due to water crystals. When foods with high amounts of water (such as vegetables) are frozen, the water forms crystals which may puncture the cell wall. This will cause the shape of the cell to change and give a different ‘look’ to the food. In the food industry, processes involving a change of temperature (usually to increase shelf life) are carried out using extreme temperatures in order to avoid the formation of crystals and alterations in the structure of the products.

Loss of water
Certain nutrients, particularly sodium, potassium and calcium, may be lost when cooking in water. This process is called leaching; nutrients lost during leaching are usually discarded with the cooking waters. To reduce the loss, it is advisable to cook with as little water as possible (steaming is even better) to reduce the surface contact of the foods (think big chunks of food, not tiny diced cubes), and to use the cooking water whenever possible (as in soups).

Light and oxygen
When exposed to the atmosphere, foods lose some of their nutrients. This affects most greatly the vitamin C content and it may be reduced by exposing fresh or recently cooked foods as little as possible to light and oxygen before consumption. Just as when cooking with water, reducing the surface contact will reduce the damage.

Grilling (barbecuing)
Image of a plate of barbecued food (source: Stock.Xchg)Outdoor grilling is a very popular cooking method due greatly to the distinctive flavors obtained during the process. However, grilling also presents some health risks as two types of carcinogenic compounds are produced: HCAs (heterocyclic amines) and PAHs (polycyclic aromatic hydrocarbons). These are mostly produced when the fatty meat is in direct contact with the smoke or flame of the grill and can be reduced by employing certain techniques when grilling. Try selecting leaner cuts as there will be less fat to drip on the grill and produce smoke. You can also marinate the meat before grilling as this is believed to form a protective barrier against the reactions that produce HCAs. Finally, grill at lower temperatures and avoid flare-ups and overcooking.

Eating raw food
Due to all these alterations in the composition of our foods as a result of cooking and processing, some health-conscious people have taken to consuming raw foods. While in terms of nutrient loss this might sound like a good and logical idea, it has its drawbacks. Cooking processes may in some cases increase the availability of certain nutrients. Such is the case of lycopen and carotenoids, both of which are easier for our bodies to use once the foods sources rich in these nutrients have been exposed to heat. Cooking may also make our foods safer by killing microbes or making it difficult for them to grow and multiply (as in salted meats, which have a longer life due to the lack of water available for bacteria to grow). And, simply put, cooking usually turns plain ingredients into a wonderful meal!

The exact amount of nutrients lost during cooking will vary depending on the characteristics of the foods and the processes followed. As long as they are safe to eat, try consuming your foods raw as often as possible or cook them in a way that will reduce the nutrient loss to a minimum.

Further reading
SELFNutrtion Data. Nutritional effects of food processing. Available from: (accessed 23.08.12)
Khokhar et al. 2012. Carotenoid and retinol composition of South Asian foods commonly consumed in the UK. Journal of Food Composition and Analysis 25, 166-172
Smoke points of various fats. Cooking for engineers. Available from: (accessed 23.08.12)

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