When it comes to making healthy vegetables, cooked can sometimes be better than raw. Cooking can free up more nutrients for your body to absorb.
"Common wisdom says cooked food has lower nutritional value compared with fresh produce, but that's not always true," says Rui Hai Liu, a professor in the department of food science at Cornell University, who has studied how heat affects food. "Many nutrients in fruits and vegetables are bound in the cell walls," he explains. "Cooking helps release them, so they're more bioavailable and absorbed by the body."
Here are five healthy vegetables that you should heat before eating, plus tips on how to unleash their full potential in terms of nutrition and taste:
A 2009 study in the International Journal of Food Science & Technology found that cooking these stalks raised the level of six nutrients, including cancer-fighting antioxidants, by more than 16 percent. Another 2009 study in the International Journal of Molecular Sciences found that cooking asparagus more than doubled the level of two types of phenolic acid, which some studies have linked to lower cancer rates.
Try this: Steaming asparagus is a good method to keep spears crisp and prevent nutrients leaching into the cooking water. Another option is this flash-cook microwave method from Pamela Braun, a recipe developer for MyMansBelly.com: Soak four paper towels in water and lemon juice, then wring them out and roll spears in them. Microwave for 3 to 4 minutes on high
Cooking ignites this veggie's cancer-fighting carotenoids. A 2008 study in the Journal of Agricultural Food Chemistry found that boiling carrots until tender boosted their concentration of carotenoids by 14 percent. But hold the fry pan. Pan frying caused a dip in carotenoid levels by 13 percent.
Try this: To maximize the nutritional benefits, boil carrots whole before slicing. Cooking them that way keeps valuable nutrients from escaping into the cooking water. Bonus: Once cooked, they'll be easier to cut. Top with a tiny bit of honey or maple syrup to bring out the natural sweetness of carrots, says Catherine Jones, co-author of "The Calories In, Calories Out Cookbook" (The Experiment, 2014).
A cup of cooked white mushrooms has about twice as much muscle-building potassium, heart-healthy niacin, immune-boosting zinc, and bone-strengthening magnesium as a cup of raw ones. That's according to the Department of Agriculture's nutrient database. Even mushrooms considered edible can sometimes contain small amounts of toxins that can be destroyed through cooking.
Try this: Mushrooms are like sponges when it comes to soaking up fat, so go easy on the oil, Braun says. Because they release a lot of water when cooking, don't overcrowd the pan, and let them cook down. For a flavor boost, Braun sautées mushrooms with garlic and sprigs of fresh thyme. They make a tasty side dish and are a great topper for pasta or burgers, she says.
This leafy green is packed with nutrients, but you'll absorb more calcium and iron if you eat it cooked. The reason: Spinach is loaded with oxalic acid, which blocks the absorption of iron and calcium but breaks down under high temperatures. One study found that cooking spinach quickly in boiling water, then plunging it into cold water, reduced oxalate content by 40 percent, on average, which was more effective than pan or pressure cooking.
Try this: Blanch a bunch of fresh spinach leaves in boiling water for 1 minute, then plunge in ice water for a few more. Drain well and keep wrapped in the fridge, ready to add to omelets, soup, and other dishes. It should keep for a few days.
Cooking tomatoes—whether they're baked, fried, or even puréed into spaghetti sauce—increases a phytochemical, lycopene, that has been linked to lower rates of cancer and heart disease. It also gives red tomatoes their rosy color. According to a landmark study in 2002 by Liu, heating tomatoes for 30 minutes at 190.4° F (the temperature of soup simmering on a stove) boosted the levels of absorbable lycopene by 35 percent. Though cooking reduced the vitamin C content, Liu's study found that it raised the total power of the disease-fighting antioxidant by 62 percent.
Try this: Instead of serving raw tomatoes cut up in a salad, try roasting them in the oven. Roasting concentrates their flavor, says Rene Ficek, a registered dietitian and lead nutrition expert at Seattle Sutton's Healthy Eating, a meal-planning and delivery service based in Illinois. Arrange quartered tomatoes on a sheet pan in one layer; drizzle them with olive oil and balsamic vinegar; sprinkle with garlic, salt, and pepper; then bake for about a half-hour at 200° F.
This article also appeared in the February/March 2015 issue of ShopSmart magazine.