Q. Is it true tomatoes and spinach release more of their nutrients when they’ve been cooked?

A. It’s true for some nutrients, but not for others.

Water-soluble vitamins like vitamin C and the B vitamins, which include folate, leach out of vegetables if you boil them, but some will be retained in the stock, so “use it if you’re making soup,” said Helen Rasmussen, a senior research dietitian at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. Vitamin levels are reduced the longer you expose them to heat; “they’re just not stable,” she said.

Levels of other nutrients, however, may increase. One study found that while cooking decreased the amount of vitamin C in tomatoes, the cooking process increased levels of antioxidants that could be absorbed by the body, including levels of lycopene, the carotenoid plant pigment that helps protect the body from free radical damage. “You might be able to get more lycopene out because the heat starts to break down the cell matrix and that actually allows some of the tied-up carotenoids to be released from the cell walls,” Dr. Rasmussen said.

Raw spinach provides a lot of fiber, but cooked spinach may provide more beta carotene: One study found that three times as much beta carotene — an antioxidant that’s a form of vitamin A — was absorbed from cooked spinach compared with raw spinach.