There's a common public perception that microwave ovens "irradiate" food in a manner analogous to that of a nuclear reaction. This perception is reflected in the common perception that microwaved food is low in nutrients compared with conventionally-cooked food. However, in reality, microwaves are a healthy and effective way to cook and heat foods.
Radiation
In common parlance, "radiation" is taken to mean a kind of energy capable of causing cancer and other cellular changes. In fact, some kinds of radiation -- specifically ultraviolet light, X-ray radiation and radiation from certain nuclear reactions -- do cause cancer. Scientifically, however, "radiation" simply refers to a kind of energy, some forms of which can cause damage to living tissue and biological molecules, and some forms of which can't. Visible light, for instance, is a kind of radiation, but has no potential to cause harm.
Microwaves
Microwaves are, in fact, a kind of radiation. They're even lower in energy than visible light, however, and as such, have no potential to cause cancer, damage living tissue or destroy molecules of any kind. Specifically, microwave ovens function by causing water molecules in your food to spin -- this creates friction, which produces heat, just as you produce heat when you rub your hands together. The heat spreads through your food, warming and cooking it.
Nutrients
Some nutrients, specifically vitamins, are sensitive molecules that are easily destroyed. While microwave radiation can't destroy molecules of any kind, heat destroys certain vitamins -- regardless of whether the source of heat is a microwave oven or a conventional oven. A 2004 study published in the scholarly journal "Food Chemistry" says that when you cook vegetables, you reduce the vitamin content relative to raw vegetables. The study examined both conventionally-cooked and microwaved vegetables, and found the same results in either case.
Benefits of Microwaving
It's not simply heat that damages nutrients in food; it's exposure to heat over time. Because microwaving is faster than conventional cooking, in some cases, microwaving will actually leave more of the vitamin content of a food intact relative to conventional cooking. A 2007 article in the scholarly publication "The Journal of Food Quality" notes that some foods retain more nutritional value when microwaved -- this is particularly true with vegetables.
Denaturation
One final concern that some individuals have regarding microwaving is that it will somehow damage enzymes in food. It's important to understand, first and foremost, that while humans rely on enzymes to run chemical reactions, these enzymes are made in the body and are not procured from food. Enzymes are proteins, and cooking -- via microwave or conventional methods -- does denature proteins, meaning it renders them nonfunctional. However, this doesn't affect the nutritional value of the food in any way because humans don't depend on the function of proteins that are consumed -- the body denatures them in the stomach whether or not they're denatured by cooking beforehand.
References
- "Food Chemistry"; Phenolics, ascorbic acid, carotenoids and antioxidant activity of broccoli and their changes during conventional and microwave cooking; D. Zhang et al; December 2004
- "Journal of Food Quality"; Sensory Attributes and Nutrient Retention in Selected Vegetables; M. Schnepf et al; May 2007
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