Q: To what extent does heating (boiling, baking) foods like vegetables destroy vitamins?
A: It’s true that cooking methods alter the nutritional composition of fruits and vegetables, but that’s not always a bad thing. Several studies have shown that while cooking can degrade some nutrients, it can enhance the availability of others. As a result, no single cooking or preparation method is best, and that includes eating vegetables raw.
Many people believe that raw vegetables are packed with more nutrition than cooked vegetables, but, again, it depends on the type of nutrient. One study of 200 people in Germany who ate a raw food diet found that they had higher levels of beta carotene, but their plasma lycopene levels were well below average. That’s likely because fresh, uncooked tomatoes actually have lower lycopene content than cooked or processed tomatoes. Cooking breaks down the thick cell walls of many plants, releasing the nutrients stored in them.
Water-soluble nutrients like vitamin C and vitamin B and a group of nutrients called polyphenolics seem to be the most vulnerable to degradation in processing and cooking. Canned peas and carrots lose 85 to 95 percent of their natural Vitamin C. After six months, another study showed that frozen cherries lost as much as 50 percent of anthocyanins, the nutrients found in the dark pigments of fruits and vegetables. Cooking removes about two-thirds of the vitamin C in fresh spinach.
Depending on the method used, loss of vitamin C during home cooking typically can range from 15 percent to 55 percent, according to a review by researchers at the University of California, Davis. Interestingly, vitamin C levels often are higher in frozen produce compared with fresh produce, likely because vitamin C levels can degrade during the storage and transport of fresh produce.
Fat-soluble compounds like vitamins A, D, E and K and the antioxidant compounds called carotenoids fare better during cooking and processing. A report in The Journal of Agriculture and Food Chemistry concluded that over all, boiling was better for carrots, zucchini and broccoli than steaming, frying or serving them raw. Frying vegetables was by far the worst method for preserving nutrients.
But when it comes to cooking vegetables, there are always tradeoffs. A method may enhance the availability of one nutrient while degrading another. Boiling carrots, for instance, significantly increases measurable carotenoid levels compared with raw carrots. However, raw carrots have far more polyphenols, which disappear once you start cooking them.
And while many people think microwaving is bad for food, vegetables cooked in a microwave may have a higher concentration of certain vitamins. A March 2007 study looked at the effects of boiling, steaming, microwaving and pressure cooking on the nutrients in broccoli. Steaming and boiling caused a 22 percent to 34 percent loss of vitamin C. Microwaved and pressure-cooked vegetables retained 90 percent of their vitamin C.
The bottom line is that no one cooking or preparation method is superior for preserving 100 percent of the nutrients in a vegetable. And since the best vegetables are the ones you will actually eat, taste should also be factored in when deciding on a cooking method. The best way to get the most out of your vegetables is to enjoy them in a variety of ways — raw, steamed, boiled, baked and grilled. If you eat a variety of fruits and vegetables on a regular basis, you don’t have to worry about the cooking method.