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How does cooking method alter a food's nutrition?

A display of fruits and vegetables in the new grocery section of the new Wal-mart Supercentre in Stouffville, Ontario, Canada, opening Wednesday November 08, 2006.

Deborah Baic/Deborah Baic /The Globe and Mail

The question

Does cooking method (such as microwave versus stove top) affect how nutritious food is?

The answer

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This is certainly the case for vegetables - how you cook them definitely makes a difference to their nutrient content.

Contrary to popular belief, cooked vegetables actually have a few advantages over raw. Cooking vegetables increases their content of disease-fighting antioxidants such as beta-carotene, lycopene and lutein. That's because heating vegetables releases these antioxidants by breaking down cell walls.

Cooking also releases minerals in vegetables that are bound to a natural compound called oxalic acid. You'll get more calcium, magnesium and iron if you eat your vegetables cooked rather than raw. Three cups of raw spinach, for example, has 90 milligrams of calcium whereas one cup of cooked has nearly triple the amount (259 milligrams).

However, when it comes to cooking water is your enemy. Boiling vegetables causes a significant amount of nutrients that dissolve in water - vitamin C, folate and thiamin (vitamin B1) - to be leached away. The longer you cook your veggies, and the more water you use, the more vitamins you'll lose.

Steaming is much gentler on nutrients because vegetables don't come in contact with cooking water. Dry cooking methods like grilling, roasting, stir-frying and microwave cooking without water also preserve a greater amount of nutrients than boiling or pressure cooking.

If you do use the microwave, you preserve the most antioxidants if you cook them without water and only until tender.

Bottom line: Steam vegetables or stir-fry briefly until tender, but crisp. If you cook vegetables in water, some vitamins will be leached into the cooking liquid. So use the liquid that's left over to make sauces or add extra flavour and nutrients to soups, stews and pasta sauces.

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