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Pumping iron into your body

[This article originally appeared in the June 2000 issue of Northwest Runner magazine.]

Iron. The word conjures up images of barbells, steam-engine locomotives, and . . . red blood cells? It should. After all, 60 to 70 percent of your body's iron is contained in hemoglobin, the oxygen-binding protein in red blood cells. But that's not iron's only biological role; it also is a necessary component of myoglobin (an oxygen storage protein), antioxidant enzymes, and electron-carrying enzymes which harness the energy of the food you eat, among other things (Nielsen & Nachtigall, Sports Medicine 26: 207-16, 1998).

What happens when you don't get enough?

In extreme circumstances, iron depletion leads to anemia, i.e., a shortage of red blood cells and thus a reduced oxygen-carrying capacity. Full-blown anemia occurs in about 0.2% of U.S. men and in 2 to 3% of U.S. women; however, as many as one in five U.S. women shows signs of being iron-depleted (Cook et al., Blood 68: 726-31, 1986). Although these women (and some men) have a red blood cell count that is within the normal range, their blood contains below-normal amounts of iron bound to ferritin and transferrin. (Ferritin and transferrin are proteins which store iron and transport it through the blood.)

In some studies, iron-depleted but nonanemic subjects have improved their exercise capacity by taking iron supplements (Rowland et al., American Journal of Diseases of Children 142: 165-9, 1988; LaManca & Haymes, Medicine and Science in Sports and Exercise 25: 1386-92, 1993; Zhu & Haas, American Journal of Clinical Nutrition 66: 334-41, 1997; Hinton et al., Journal of Applied Physiology 88: 1103-11, 2000). These results suggest that a lack of iron can hinder endurance performance even if it doesn't cause anemia -- and, conversely, that adding more iron into one's diet can be beneficial to nonanemic people as well as anemic ones.

How much do runners need?

The U.S. Recommended Daily Allowance of iron is 10 milligrams (mg) for adult men. Interestingly, only about 10% of the iron you ingest gets absorbed by the small intestine, so if you consume the recommended 10 mg per day, only 1 mg or so actually gets incorporated into your body. Nevertheless, that's enough to replace a male's daily iron losses, which are likewise quite small.

Premenopausal women, in contrast, have greater iron losses and thus a greater need for iron in their diet. These women "donate" about 30 milliliters (ml) of blood per month to the process of menstruation (Hallberg et al., Acta Obstetricia et Gynecologica Scandinavica 45: 320-51, 1966). Assuming that each ml contains 0.33 to 0.5 mg of iron (Green et al., American Journal of Medicine 45: 336-53, 1968), that's an additional iron loss of 10 to 15 mg per month, or 0.33 to 0.5 mg per day. For this reason, the U.S.R.D.A. for adult women has been set at 15 mg -- 50% above the standard for adult men.

Just as women need more iron than men, serious runners need more iron than sedentary people, according to a 1996 study of elite German distance runners. Nachtigall et al. (International Journal of Sports Medicine 17: 473-9, 1996) fed radioactive iron to eight runners -- it's not nearly as dangerous as it sounds -- and monitored how this iron was absorbed and released by the body. They found that, during periods of intense training and racing, the amount of iron lost in the feces rose by 300 to 500%. (Iron losses in sweat and urine were minimal.)

While this study hints that runners have an increased risk of developing an iron deficit, another indicates that gentle, moderate endurance training programs do not appear to compromise the body's iron reserves (Bourque et al., Journal of the American Dietetic Association 97: 116-21, 1997). Therefore, runners who avoid gut-wrenching intervals and competitions may only need about as much iron as nonathletes -- not more. Further research will hopefully lead to a more precise definition of runners' dietary iron needs.

Are you getting enough?

Whether or not you're consuming the U.S.R.D.A. of iron, your intake may or may not be sufficient to cover your losses. For those who are concerned about a possible imbalance, a good first step is to get a blood test. As mentioned above, reduced levels of iron-containing ferritin and transferrin in the blood indicate that your iron intake may not be adequate.

Obviously, having one's blood analyzed takes time and money. Wouldn't it be easier just to eat lots of iron-rich foods? Perhaps. However, as with most things in life, taking in too much iron can be as dangerous as taking in too little; for example, 2% of people of Northern European descent have a condition known as hemochromatosis, in which excessive quantities of iron are deposited in the body's organs (Olynyk, Liver 19: 73-80, 1999). In other words, some people should ingest less iron rather than more, and blood tests can help identify who needs less, as well as who needs more.

For those who do need to keep their iron intake high, it is important to note that iron is much easier to absorb from some foods and dietary supplements than from others. For instance, although the liver boasts an impressive concentration of iron, those who dine on foie gras absorb only about 6% of that iron (Heinrich et al., Klinische Wochenschrift 49: 819-25, 1971). Similarly, supplements which contain ferric iron (i.e., iron atoms with a net charge of +3) are much less efficacious than supplements of iron in the ferrous (+2) form (Nielsen et al., Arzneimittelforschung 44: 743-8, 1994). A complete discussion of this topic is beyond the scope of this article, so interested readers should consult a nutritionist or a physician (or a textbook, or another website...) for further details. As a general rule, though, when it comes to dietary sources of iron, red meat is hard to beat.


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