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There are people who avoid preparing their food in microwave ovens for various health-related reasons. The claims most often stated are:
Is there any scientific evidence to suggest that microwaved food is less healthy compared to food prepared in more conventional ways?
TL;DR: No, food cooked via a microwave oven is generally not less healthy than food cooked by other methods.
In general, cooking by any method destroys or reduces nutrient value1,4. This is due to a number of factors:
Solubility. Water-soluble nutrients (like Vitamin B, C) are highly prone to leach out if cooking in water2 (boiling, for example). Fat-soluble nutrients are at a similar risk when cooking in a fat medium or when the fat is lost in the process (e.g., frying, grilling)5.
Heat. Both fat- and water-soluble nutrients are susceptible to heat from any cooking source, while mineral nutrients are less so5.
Duration. Methods with shorter cook times (e.g., stir-frying, blanching, microwaving) reduce the effects of heat degradation because the food is not exposed to heat for as long5.
(It's not even always a loss: some nutrients are more available after cooking, because heat breaks down thick cell walls that our digestive system has trouble with, which means that the nutrients are easier for our bodies to uptake3.)
Each cooking method has a different combination of the above factors, and so has different effects on nutrients in food. Microwave cooking has the advantage of not cooking in water, and being able to reach a higher heat in a shorter time. Depending on the food, this means that microwaving is sometimes better at preserving nutrients. For example, boiling spinach causes a much higher loss (77% vs. almost none) of folate compared to microwaving4.
Of course, when people make claims about microwaved food being less healthy, they are usually referring to a fear that the act of microwave heating (dielectric heating) is 'unnatural' in some way, or is subjecting the food to dangerous radiation (which may then be ingested). This is simply not supported9 by scientific literature, which indicates that microwave ovens, when used properly, are safe and effective -- with two caveats:
Human milk. The CDC6 does not recommend heating human milk in the microwave, because of the risk of uneven heating of the liquid (which could scald a baby), and because there is some evidence7 that it decreases its anti-infective properties.
Superheating of liquids. If water is heated in a microwave in a smooth container, it can pose a risk of scalding due to superheating9. From wikipedia:
Superheating can occur when an undisturbed container of water is heated in a microwave oven. When the container is removed, the water still appears to be below the boiling point. However, once the water is disturbed, some of it violently flashes to steam, potentially spraying boiling water out of the container. [...] There are ways to prevent superheating in a microwave oven, such as putting a popsicle stick in the glass or using a scratched container.
In conclusion, microwaving food is a safe alternative to cooking via other methods9. Any cooking method will decrease the nutritional value of food, but this is usually an acceptable tradeoff because of the benefits gained from cooking5. Microwaves do not cause food to be radioactive or acutely dangerous in any way.
Heat in a microwave oven is generated by the rotation of water molecules by bombarding them with electromagnetic radiation in the microwave spectrum (non-ionizing electromagnetic waves between common radio waves and infrared frequencies) causing polarized molecules in the food to rotate or collide and subsequently thermal energy is built up (dielectric heating).
One 1998 study shown that microwaving food can lead to faster breakdown of vitamin B121998. This is because microwaved food can get locally hotter than the boiling point of water (100°C/212°F).
Another study from 2007 by J. Agric. Food Chem. show a general decrease in the levels of all the studied compounds except for mineral nutrients which were stable under all cooking conditions:
Vitamin C showed the greatest losses mainly because of degradation and leaching, whereas losses for phenolic compounds and glucosinolates were mainly due to leaching into water. In general, the longest microwave cooking time and the higher volume of cooking water should be avoided to minimize losses of nutrients.
So it seems that the most heat-sensitive nutrients such as folic acid, vitamins B and C (mainly water-soluble vitamins)are the most affected when microwaving food.
One study published in The Journal of the Science of Food and Agriculture in 2003 found that microwave cooking destroyed more flavonoids than other methods. When steamed or cooked without water, the broccoli retained most of its nutrients and antioxidants.
Loss of flavonoids in Broccoli Study:
According to Dr Cristina García-Viguera, leader of the study:
During microwave heating they leach into the cooking water, removing their nutritional benefits from the foodstuff.
However other studies didn't show a high loss of nutrients with microwave cooking, in fact two studies indicate that microwave cooking helps retain flavonoids better than other methods.
Source: Healthy Microwave Cooking of Vegetables
Loss of flavonoids from Tomatoes:
Source: Healthy Microwave Cooking of Vegetables
Loss of flavonoids from Potatoes:
To minimize phenolic losses in potatoes, microwaving should be done at 500W2008.
Study at Cornell University showed that spinach retains nearly all its folate when cooked in a microwaveNYT.
Study at Cornell University found that bacon cooked by microwave has significantly lower levels of cancer-causing nitrosamines than conventionally cooked baconNYT.
Acrylamide is considered a potential occupational carcinogen by U.S. government agencieswiki and it can be found in starchy foods, such as potato chips/crisps, French fries and bread that had been heated higher than 120°C (248°F)2002. Acrylamide levels appear to rise as food is heated for longer periods of time.
Unlike frying and baking, based on FDA studies2008, microwaving whole potatoes with skin on to make “microwaved baked potatoes” does not produce acrylamide, although unlike deep-frying, it has limited effectiveness in reducing glycoalkaloid (i.e. solanine) levels1999.
However acrylamide can been found in other microwaved products such as popcorn.
Microwave ovens are frequently used for reheating leftover food, and bacterial contamination may not be repressed if the safe temperature is not reached, resulting in foodborne illness.
The radiation produced by a microwave oven is non-ionizing (in comparison to X-rays and high-energy particles which are associated with the cancer risks).
Long-term studies to assess cancer risk have so far failed to identify any carcinogenicity microwave radiation (2.45GHz) even with chronic exposure levels1998, 1998.
However, with the oven door open, the radiation may cause damage by heating, although the modern design of microwave ovens is equipped with safety switches that turn off the radiation when the door is open and ovens are sufficiently insulated to allow only "minimal leakage" when the oven is working.
Any form of cooking will destroy vitamins and other nutrients in food, but it depends how much water is used in the cooking, how long the food is cooked and at what temperature2006.
However comparative cooking method studies generally find that, if properly used, microwave cooking does not affect the nutrient content of foods to a larger extent than conventional heating1995.
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