I turned to the internet and found that there are two opposing viewpoints: 1) that microwave ovens do not destroy nutrients, and in fact, may be better than conventional cooking methods; and 2) that microwave ovens do destroy nutrients, even to the point of threatening your health. Now, keep in mind, I’m only addressing the effect of microwave ovens on nutrients. I’m not getting into whether or not microwave ovens are safe.
According to the Harvard Medical School Family Health Guide (1), some nutrients break down when exposed to heat, regardless of the source (microwave or regular oven), and the longer the exposure to heat, the more the nutrients break down. Nutrient content is probably better preserved when cooking with a microwave oven because of the shorter cooking times.
Cooking vegetables in water is problematic because many nutrients are water-soluble. Foods boiled in water will lose nutrients to the water, and the longer the foods are cooked in boiling water, the more nutrients are lost. For this reason, some say that cooking in a microwave oven is better because in addition to the shorter cooking times, there is little or no water to leach the nutrients from the food. (2, 3) The only time boiling in water is better is if you’re making a soup or stew, in which case you retain the nutrients in the broth. Studies at Cornell University looked at the effects of cooking on water-soluble vitamins in vegetables and found that folate was almost completely retained in spinach when cooked in a microwave oven, yet 77% was lost when cooked on a stove. (4)
According to Dr. Mike Roussell, PhD, microwaving can make certain nutrients more available to your body, and cited research from the University of Oslo that found that microwaving or steaming carrots, spinach, mushrooms, asparagus, broccoli, cabbage, green and red peppers, and tomatoes caused antioxidants to become more available for absorption. (5)
On the other hand, several websites cited specific studies from the scientific literature (6, 7, 8, 9), including a study published in the November 2003 issue of the Journal of the Science of Food and Agriculture that found that broccoli microwaved with a little water lost up to 97 percent of its beneficial antioxidants while steamed broccoli lost no more than 11 percent of its antioxidants. In addition, there were losses in phenolic compounds and glucosinolates. A 1999 study in Scandanavia found a reduction in vitamin C when asparagus spears were cooked in the microwave. A study of garlic found that allinase, garlic’s principle active cancer-fighting ingredient, was inactivated in as little as 60 seconds. Watanabe did a study in Japan showing that 30-40 percent of vitamin B12 in milk becomes inert after 6 minutes of microwave heating. An Australian study showed a higher degree of protein unfolding from microwaves than conventional heating, which deactivates proteins.
Microwaves excite water molecules inside food, causing the water molecules to vibrate at high frequencies, creating internal friction that heats foods wherever there’s water inside the food. That’s why microwave ovens don’t evenly heat food. According to Mike Adams at Natural News (8) microwaving results in a cell-by-cell “nuking” of food causing near-total molecular decomposition of the vitamins and phytonutrients.
I came across interesting reports that described a science fair project in which two plants were watered side-by-side, one with water that was boiled in the microwave, then cooled, and one with fresh water. (10, 11) After nine days, the one watered with the “nuked” water was dead while the one watered with the fresh water was thriving. It was assumed that the microwaved water killed the plant. Yet snopes.com repeated the experiment with three plants using water that was boiled in the microwave oven then cooled, boiled on the stove then cooled, and water that hadn’t been boiled as a control. (12) All the water used in the experiment came from the same source. In their experiment, all three plants thrived.
So where do I stand on the issue now? It’s really hard for me to say because the information I found about the effect of microwave ovens on nutrients is contradictory and I was unable to find direct evidence. Most of what I found were other people’s summaries of studies, and without seeing the actual journal articles where the original studies were published, I don’t know how accurately those studies were summarized.
I will say this, for now I will continue to do all my cooking on the stove, the grill, or in the oven, and continue to eat lots of raw veggies.