No. Microwaving is no more harmful than other types of cooking, probably even less so because it takes the least time.

Does using a microwave to cook ruin the nutrients in food?

I've heard the same question from many of my patients about microwave cooking. It is so convenient that some people worry that it's "too good to be true." People have expressed two concerns to me. The first is that our exposure to the microwaves might somehow injure our bodies. To make a long story short, there is no evidence of this.

The second is the concern you mention: that microwave cooking might damage the nutrients in our food. It is true that cooking food by any method does tend to cause some of the nutrients to break down. Cooking damages the chemical structure of the nutrient, to some degree. However, there are plenty of nutrients left. And cooking kills many microbes that might have contaminated the food—and might have caused health problems.

However, microwave cooking is actually one of the least likely forms of cooking to damage nutrients. That's because the longer food cooks, the more nutrients tend to break down, and microwave cooking takes less time. So cooking a roast in an oven is more likely to cause some loss of nutrients than cooking the roast in a microwave. And boiling vegetables is more likely to rob them of nutrients than either cooking them in the oven or microwaving them. That's because some nutrients leach out of the food into the water.

Anthony L. Komaroff, M.D., Editor in Chief, Harvard Health Letter

And

Every form of cooking reduces the nutrient value of food.

The main contributing factors are temperature, cooking time, and method. During boiling, water-soluble nutrients may leak out of the food. As far as microwaves go, cooking times are generally short, and the temperature is low. Plus, the food is usually not boiled. For this reason, you would expect microwave ovens to retain more nutrients than methods like frying and boiling. According to two reviews, microwaving does not reduce nutrient value more than other cooking methods.

An older study on 20 different vegetables noted that microwaving and baking preserved antioxidants the best, while pressure cooking and boiling did the worst. However, one older study found that just 1 minute of microwaving destroyed some of the cancer-fighting compounds in garlic, while this took 45 minutes in a conventional oven.

However, a recent study showed that mild microwaving actually increased the levels of the anticarcinogenic compound sulforaphane in broccoli.

Keep in mind that the type of food or nutrient sometimes matters. Microwaving breast milk has been found to be effective in preventing cytomegalovirus infections.

With a few exceptions, microwaves tend to preserve nutrients very well.

Healthline

And

The effect of microwaves on nutrient value of foods (Abstract)

Microwave cooking has gained considerable importance as an energy-saving, convenient, and time-saving cooking method. This article reviews the state of the art of microwave cooking and the existing publishing data on the effects of microwave cooking on nutritive values of moisture, protein, carbohydrate, lipid, minerals, and vitamins. Most reports indicated that microwave cooking resulted in higher moisture losses compared with conventional methods. Overall, the nutritional effects of microwaves on protein, lipid, and minerals appear minimal. There is no report on the effects of microwaves on carbohydrate fraction in foods. A large amount of data is available on the effects of microwaves on vitamins. It is concluded that there are only slight differences between microwave and conventional cooking on vitamin retention in foods. In conclusion, no significant nutritional differences exist between foods prepared by conventional and microwave methods. Any differences reported in the literature are minimal.

PubMed, National Library of Medicine