Nutrition Hotline

Question: Here's a puzzling question about calcium in broccoli. According to Food Values of Portions Commonly Used by Pennington, cooked broccoli is said to have more than four times the amount of calcium per serving than raw. Could this big a difference in calcium content really be the result of volume loss with cooking? D.B., Toronto

Answer: The nutrient composition guide that you cite shows that 1/2 cup of raw, chopped broccoli contains 21 milligrams of calcium, and 1/2 cup of boiled broccoli contains 89 milligrams of calcium--more than a four-fold increase. Similarly, 1/2 cup of raw, chopped spinach contains 28 milligrams of calcium, while cup of boiled spinach contains a whopping 122 milligrams of calcium. (Remember: the calcium in spinach is largely not available to the body because of the exceptionally high oxalate content of spinach, which binds the calcium and prevents absorption.) On the other hand, 1/2 cup of raw, chopped turnip greens contains 53 milligrams of calcium, while 1/2 cup of boiled, chopped turnip greens contains 99 milligrams of calcium a little less than double the amount.

The differences may be due in part to variations in the vegetable samples tested by the lab. It is not necessarily true that one stalk of broccoli is picked, half of it cooked, and both samples analyzed. In many cases different vegetables, grown in different soils, of different ages and picked at different times are analyzed. Some samples are analyzed after cooking and some are analyzed raw. There may also be variations in the portions of the plant that are used, such as the proportion of stems or leaves. All of these factors may affect the values given for calcium content of certain vegetables. However, some vegetables do "shrink" considerably during cooking. Foods lose water as they cook, which causes them to become more compact, cup for cup. Some foods shrink more than others.

For instance, spinach leaves have the ability to collapse more with cooking than do broccoli florets and stems, so cooked spinach contains proportionately more calcium compared to its raw state than does broccoli. The degree to which a food compacts with cooking also depends upon the length of time the food is cooked. When I cook broccoli, I steam it lightly, rather than cooking it down to mush. Consequently, broccoli cooked at my house may not have quite as much calcium as the same-sized serving of broccoli cooked at someone else's house where a longer cooking time has resulted in a more compact vegetable. For all of these reasons, you should think of the nutrient compositions listed in food composition tables as approximations--with some wide variations possible--and not as precise figures.

Incidentally, a long cooking time can destroy some heat-sensitive vitamins, such as vitamin C. Other water-soluble vitamins will be leached out if vegetables are cooked in large amounts of water for a long period of time. The best way to cook most vegetables is any method that limits the amount of cooking time as well as the amount of cooking water used. Steaming greens or broccoli, for instance, in a few tablespoons of water or vegetable broth for five to ten minutes (stovetop, microwave oven, or steamer) is ideal.