Before even getting into the topic of how to cook vegetables, it’s important to touch on the subject of whether you should be cooking them at all. If any kind of cooking method destroys some of the nutrients, wouldn’t it be better to eat everything raw?
Cooking does destroy some nutrients, but it makes others much more bioavailable. For example, cooking reduces the levels of some antioxidants in broccoli, but increases others. It reduces the antioxidant content of kale, but increases it in tomatoes. A 2009 study of antioxidants found that each of the cooking methods tested decreased antioxidant levels in some vegetables but increased it in others.
Fat-soluble vitamins (A, D, E, and K) tend to hold up better than their water-soluble cousins (C and some B vitamins). Vitamin C tends to be the most unstable of all, and minerals are largely unaffected, since heating doesn’t affect mineral levels.
Notably, cooking also reduces the water content of a food, decreasing its overall volume. Fill a 2-cup measuring cup with spinach, then boil the spinach and measure it again: you can fit a lot more of the cooked version onto the same “plate space.” This means that even if the nutrient content is slightly less per gram of food, the number of total grams in your meal will probably be higher. So cooking your vegetables might actually help you consume more nutrients overall.
Cooking is also an effective way to destroy some antinutrients that might otherwise prevent the absorption of vitamins and minerals. For example, one study found that blanching (briefly dropping vegetables into boiling water and pulling them out as soon as they change color) significantly reduced the tannic acid and phytic acid of vegetables. And cooked vegetables can also be easier on the stomach, since the fiber content of raw vegetables sometimes makes them more difficult to digest, especially for high- FODMAPsfoods.
Clearly, it’s a little more complicated than “cooking destroys nutrients; raw food is best.” It’s better to think of cooked vegetables and raw vegetables as two different but equally valuable additions to your plate, and eat a mix of both every day. As a general rule, cooking methods that minimize temperature, time, and liquid will maximize the nutrients in the vegetables themselves. But as the detailed breakdown of cooking methods shows, each method has its pros and cons.