Microwaves are produced inside the oven by an electron tube called a magnetron. The microwaves are reflected within the metal interior of the oven where they are absorbed by food. Microwaves cause water molecules in food to vibrate, producing heat that cooks the food. That's why foods high in water content, like fresh vegetables, can be cooked more quickly than other foods. The microwave energy is changed to heat as it is absorbed by food, and does not make food “radioactive” or "contaminated."
Although heat is produced directly in the food, microwave ovens do not cook food from the "inside out." When thick foods are cooked, the outer layers are heated and cooked primarily by microwaves while the inside is cooked mainly by the conduction of heat from the hot outer layers.
Microwave cooking can be more energy efficient than conventional cooking because foods cook faster and the energy heats only the food, not the whole oven compartment. Microwave cooking does not reduce the nutritional value of foods any more than conventional cooking. In fact, foods cooked in a microwave oven may keep more of their vitamins and minerals, because microwave ovens can cook more quickly and without adding water.
The subject of the safety of microwave oven use can cause heated debate. It seems that most people I speak with in the holistic nutrition field decry the use of these ovens, citing studies and articles that support their contentions that microwave cooking destroys nutrients and is dangerous to our health. The "studies" that virtually every book and Internet site use to condemn cooking in microwave ovens emanate from two sources that for all intents and purposes don't exist.
One is early microwave oven research by a Swiss scientist named Hans Hertel, who found negative changes in the blood of people who ate microwave-cooked food. This study has never actually been seen by anybody, as far as can be ascertained through searches on the Internet, and according to some, it was neither peer reviewed nor published. Though it is not available anywhere for scrutiny, its “information” is widely reproduced, all based solely on hearsay.
The second study has to do with alleged Russian research originating right after World War II and culminating, in 1976, with the banning of microwave ovens because of their supposed dangers. Not only is this particular study also unavailable for viewing, the author has purportedly changed his name and disappeared. Not only are microwave ovens currently in use everywhere in Russia, it appears that they were never banned.
Since microwave ovens often use less heat than conventional methods and involve shorter cooking times, they generally have the least destructive effects. The most heat-sensitive nutrients are water-soluble vitamins, like folic acid and vitamins B and C, which are common in vegetables. When it comes to vegetables, adding water can greatly accelerate the loss of nutrients.
In studies at Cornell University, scientists looked at the effects of cooking on water-soluble vitamins in vegetables and found that spinach retained nearly all its folate when cooked in a microwave, but lost about 77 percent when cooked on a stove. They also found that bacon cooked by microwave has significantly lower levels of cancer-causing nitrosamines than conventionally cooked bacon.
In a 1992 article in the journal Pediatrics, a team of researchers from Stanford University reported that re-heating human breast milk in a microwave oven – even at a low setting – can destroy some of its important disease-fighting capabilities. Specifically, it lost lysozyme, which protects against bacterial infection. The fact that adverse changes were found at very low temperatures suggested to the researchers that “microwaving itself may in fact cause some injury to the milk above and beyond the heating”.
Vitamin B-12 is another nutrient that can be destroyed by microwaving. Japanese research reported in Science News in 1998 found that as little as six minutes of microwave cooking destroyed half of the vitamin B-12 in dairy foods and meat, a much higher rate of destruction than other cooking techniques. This kind of breakdown took about 25 minutes of boiling with conventional heat.
A Spanish study published in The Journal of the Science of Food and Agriculture in 2003 found that broccoli cooked by microwave — and immersed in water — loses about 74 percent to 97 percent of its antioxidants. When steamed or cooked without water, the broccoli retained most of its nutrients - losing about 8 to 11 percent of the very same antioxidants.
The studies with findings that indicate large losses of nutrients tend to be older ones, where in review, researchers were found to have used large amounts of water and to have cooked study foods to death. Nutrients are lost in water, no matter the cooking method, the more water and the longer the cooking time, the greater the nutrient loss. Other studies, more rigorously controlled (no overcooking), are often finding FEWER nutrient losses all the way around from microwave cooking versus steaming (we're talking mainly vegetables here, most often cruciferous).
Another benefit is that there are no heterocyclic amines (carcinogens) in microwaved meats versus those cooked directly on heat sources, such as through barbequing, broiling, or pan-frying.
An exception to lower nutrient loss has been shown to occur with beneficial bacteria, which may undergo more extensive destruction due to the inside-out nature of this cooking method, which often results in regionally very high temperatures. On the other hand, pathogenic bacteria have been found to be inadequately heated in some instances, causing outbreaks of illness. These both, however, are functions of temperature and cooking time, not of microwave cooking specifically. Clearly, we now know that unhealthful effects can be created when foods are microwave cooked or heated in materials that can leach toxins, such as plastics, but this can also occur from cooking in non-stick cookware on a stovetop.
Microwave ovens are also not dangerous to use. Their "radiation" is not high frequency; it is somewhere in the range of AM radio frequencies, and cordless and cellular phones, and these waves are easier to avoid than those from telephones and cell towers. As such, there are also no radiolytic or radioactive compounds released into food, as the ovens produce neither of these substances. And the doors of microwave ovens let almost none of their waves escape, anyway. Standing just 20 inches from a newer microwave oven prevents any effects. This has been well studied, is federally regulated, and a search on Google Scholar will turn up some high quality studies documenting this.
» Snopes: "It's just water, plain and simple."
» Center for Science in the Public Interest: Microwave Myths (PDF)