Microwaves are very convenient for cooking or heating foods quickly. It's not unusual, however, to wonder whether they're safe to use on food, and whether they leave nutrients intact. As it turns out, microwaves do adversely affect the vitamin content of foods, but no more so -- and possibly less so -- than conventional ovens.
Microwave ovens, though very convenient, are the subject of many misconceptions. Many of these misconceptions stem from the false perception that microwaves cook by exposing food to nuclear radiation. In fact, the kind of radiation released by microwaves is far more closely related to visible light than to nuclear radiation, and as such, it doesn't have any negative effect whatsoever on nutrient compounds, including vitamins, in your food.
While microwave radiation can't destroy vitamins, microwaves also produce something else that can: heat. When exposed to heat, vitamins can degrade, explain Drs. D. Zhang and colleagues in a 2004 article published in the journal "Food Chemistry." Heat, and subsequent vitamin destruction, isn't unique to microwave cooking, however. Conventional oven and stove-top cooking also destroys vitamins through exposing them to heat.
Microwaves Vs. Stoves
In general, the longer a food is exposed to heat, the more of its vitamin content will be destroyed, explain Zhang and colleagues. In fact, a 2007 study published in the "Journal of Food Quality" by Dr. M. Schnepf and colleagues found that you'll actually destroy more vitamins by cooking a food in the oven or on the stove than you will by microwaving it, simply because microwave cooking typically takes less time to achieve the desired result.
In general, you can assume that when you microwave a food, you're losing some vitamin content relative to the raw food. Zhang and colleagues, for instance, note that microwaving broccoli results in a loss of as much as two-thirds of the vitamin C content because it is a particularly heat sensitive vitamin. However, you're generally not losing as much as you would if you cooked the food on the stove or in the oven, meaning that microwaving is a safe and healthy alternative to conventional cooking.