A study in the November 2003 Journal of the Science of Food and Agriculture caused a stir because it found that microwaved broccoli loses much of its nutritional punch. The researchers steamed, pressure-cooked, boiled, and microwaved about 2 cups of the vegetable with 10 tablespoons of water and then compared the flavonoid content by cooking method. Flavonoids are substances in fruits and vegetables with antioxidant properties that may lower your risk for heart disease, stroke, and some cancers. Steamed broccoli lost 11% of its flavonoids; pressure-cooked, 53%; boiled, 81%; and microwaved, a whopping 97%.
Heat from any sort of cooking lowers the levels of some vitamins. But water exposure is another major factor. Many nutrients dissolve in it, so when vegetables are prepared in water, some of the healthful elements leach out and get thrown away with the (veggie) bath water. That's probably why steamed broccoli, which didn't come into direct contact with water, came out on top.
But you don't have to microwave broccoli in 10 tablespoons of water, as the researchers in this study did. Just a couple will do — and frozen vegetables don't need any extra water. You can have your microwave and get most of your veggie nutrients, too.
June 2004 Update