Nutrition Hotline
By Reed Mangels, PhD, RD
GOOD SOURCES OF CALCIUM
QUESTION: I thought that green vegetables, and grains such as quinoa and teff, were good sources of calcium. However, I recently read some materials... stating that grains contain phytates, and spinach contains oxalic acid, both of which inhibit the body's absorption of calcium. Should I be concerned about this, or avoid certain foods?
A.B., MD
ANSWER: Spinach, and a few other vegetables like rhubarb, Swiss chard, and beet greens do contain a substance called oxalic acid. Although these foods do contain calcium, it is poorly absorbed. Apparently the oxalic acid forms a complex with calcium in the intestines and both are excreted. Recently, careful studies done by Dr. Connie Weaver at Purdue University showed that only about 5% of the calcium in spinach was absorbed by humans while about 28% of the calcium in cow's milk was absorbed. This reduced absorption is seen only in vegetables which contain high amounts of oxalic acid. The calcium in kale, a low oxalic acid vegetable, was absorbed as well as, or slightly better than, the calcium in cow's milk. Presumably, the reduced absorption of calcium after a high oxalic acid vegetable is eaten, affects only the meal which contains that vegetable. High oxalic acid vegetables should not be avoided, as they contain vitamins and minerals which are absorbed. They should not, however, be relied on as a good source of calcium.
Phytate or phytic acid, is the form in which seeds store phosphorus. It is found in grains and legumes. Phytate does appear to reduce the amount of calcium absorbed. For example, the amount of calcium absorbed from high-phytate soybeans was about 25% less than the amount of calcium absorbed from cow's milk. Other grains and legumes have not been studied. This does not mean that one should stop eating soybeans. Instead, it supports the idea of eating a variety of foods rather than relying on only one or two foods to supply a specific nutrient.
For further information see:
Heaney RP, Weaver CM. Calcium absorption from kale. Am J Clin Nutr 1990; 51:656-7.
Heaney RP, Weaver CM, Recker RR. Calcium absorbability from spinach. Am J Clin Nutr 1988; 47:707-9.
Heaney RP, Weaver CM, Fitzsimmons ML. Soybean phytate content: effect on calcium absorption. Am J Clin Nutr 1991; 53:745-7.
