The nutrient most commonly deficient in people's diets worldwide is iron. This has been true for decades, and the situation does not appear to be improving, especially for women. Part of the problem stems from difficulty in identifying foods that are good sources of iron.

Question: Why is it difficult to identify good food sources of iron?

Answer: Even though a food can contain a significant amount of iron, it might not be a good source of iron. The primary reason for this is that the chemical form of iron in some foods is difficult to absorb. For example, the iron content of spinach makes it appear to be a good source of iron, but other naturally occurring components in spinach inhibit iron absorption. So, spinach is not a good source of iron, even though it contains what appears to be a reasonable amount of iron.

Iron is better absorbed from foods that contain heme iron. This form of iron is found only in animal foods. The iron in plant foods, referred to as non-heme iron, is less efficiently absorbed. Non-heme iron also is much more strongly affected by other food components that inhibit iron absorption. Not all iron in animal foods is heme iron. For example, about a third of the iron in cooked chicken is heme iron, and in cooked beef about half is heme iron.

Q: Are clams a good source of iron?

A: The Dietary Guidelines for Americans and the USDA nutrient database indicate that 3 ounces of clams contain 24 milligrams of iron. This is 6 mg more than the 18 mg per day recommended for adult women. However, it turns out that there are big problems with using a single value for the iron content of clams.

In an article recently published in the Journal of Food Composition and Analysis, Jennifer Lai and co-authors at the University of Hawaii found some complications in listing clams as a food source of iron. To evaluate the reliability of recommending clams as an iron source, Lai analyzed the iron content of 26 clam products purchased in Honolulu supermarkets. The results were unexpected. There was an extremely broad range of iron content in these products.

Chopped or minced clam products contained only about 1 mg of iron per 3-ounce serving, much lower than the USDA database figure of 24 mg. In contrast, some whole baby clam products were very high in iron, containing 10 to 50 mg per 3-ounce serving. Consequently, 3 ounces of some products exceeded the current “;tolerable upper intake level”; for iron of 45 mg per day.

Q: Were the nutrition labels a good indicator of the iron content of the clam products?

A: Unfortunately, the nutrition labels were mostly inaccurate. Nutrition label data indicated iron contents that ranged from 13 to 800 percent of what was analyzed in the product. Consequently, it is not currently possible for a consumer to use label information to reliably determine the iron content of these types of clam products.

Lai and her colleagues in the Human Nutrition, Food and Animal Sciences Department are now analyzing data from a human intestinal cell culture system to assess how well the iron in these clam products is absorbed.

Joannie Dobbs, Ph.D., C.N.S., and Alan Titchenal, Ph.D., C.N.S., are nutritionists in the Department of Human Nutrition, Food and Animal Sciences, College of Tropical Agriculture and Human Resources, University of Hawaii-Manoa. Dobbs also works with University Health Services.