A. Alice H. Lichtenstein, DSc, executive editor of the Tufts Health & Nutrition Letter, answers: “Spinach is rich in many nutrients, including carotenoids such as beta-carotene and lutein, vitamin K, folate, minerals and dietary fiber, regardless of whether it is raw or cooked. The bioavailability of carotenoids in spinach tends to be higher after cooking; however, for most people the difference is minor. The most important point is for people to enjoy spinach; hence, to eat it in whatever form they prefer.”
In spinach as in other foods, cooking does cause a loss of heat-sensitive nutrients such as vitamin C; it does not result in the loss of other nutrients. On the other hand, boiling spinach for as little as a minute and discarding the cooking water reduces the amount of oxalic acid, which can inhibit the absorption of calcium from spinach and foods consumed around the same time.