The study focused on women who were chronically tired and had relatively low iron stores. They did not, however, have full-blown iron-deficiency anemia, in which the body has too few oxygen-carrying red blood cells.

It has not been clear whether iron supplements can help battle fatigue in non-anemic women.

To find out, Swiss researchers randomly assigned 200 women with unexplained fatigue to take either 80 milligrams of iron a day or a placebo (identical-looking pills with no active ingredient.)

Over 12 weeks, both groups improved. But women on iron supplements fared better, the researchers report in the Canadian Medical Association Journal.

On average, scores on a standard measure of fatigue fell by nearly half - from about 25 to 13, on a scale of zero to 40 - among women getting the extra iron.

That compared with a 29 percent decline in fatigue reported by the placebo group, whose average score fell from about 25 to just over 16.

The findings suggest that when a woman's persistent fatigue cannot be explained by any health condition, low iron should become a suspect, according to lead researcher Paul Vaucher, a doctoral candidate at the University of Geneva in Switzerland.

Physicians often test for iron deficiency by measuring blood levels of hemoglobin, a protein in blood cells that carries oxygen. But hemoglobin levels usually don't fall until the later stages of iron deficiency, when a person has full-blown anemia.

In their study, Vaucher's team measured women's blood levels of ferritin, which is a marker of the body's stored iron. Doctors may or may not order that test when looking for iron deficiency.

Vaucher said that for a woman with unexplained fatigue, measuring ferritin would be wise.

"This marker will then give a better idea of whether iron load is low or not, even if women are not anemic," Vaucher said in an email.

All of the women in this study had ferritin levels below 50 micrograms per liter, which would be considered low to borderline-low.

Since women in the placebo group also improved, the effects of the iron supplement were not huge. They amounted to an extra 3.5 points shaved off a woman's fatigue score.

"That might seem like a small difference," said Dr. Christine Gerbstadt, a spokesperson for the Academy of Nutrition and Dietetics who was not involved in the study.

But, she said in an interview, the iron-pill advantage was still significant in statistical terms, and that's something doctors should be aware of.

Fatigue is complex and subjective, and women in the placebo group might have improved for a number of reasons, Gerbstadt noted. Some, for example, may have started getting more sleep and generally taking better care of themselves.

So if you're habitually worn out, could taking iron be worth it?

Gerbstadt cautioned that you should first see a doctor to try to get at the cause of your fatigue. She said a range of problems, from depression to sleep disorders to physical health conditions, can lead to fatigue.

"It may or may not be iron," Gerbstadt said.

Vaucher, whose study was funded by Pierre Fabre Médicament, a French maker of pharmaceutical and health products, agreed.

"We do know (fatigue) is related to many different diseases that can be treated," he said. "Iron deficiency should therefore only be considered once other causes have been ruled out by a physician."

On top of that, iron pills can have side effects, like constipation, diarrhea and nausea. In this study, 35 percent of iron users reported at least one side effect - though 25 percent of placebo users did as well.

In general, experts recommend that women in their reproductive years get 18 milligrams of iron per day, and no more than 40 mg. (If you are actually iron deficient, your doctor may recommend supplements with a higher dose.)

Iron deficiency is much less common in men and in women who are past menopause. The current study included only women who were still menstruating (which increases the chances of low iron), so the findings do not apply to men or postmenopausal women.

To help prevent iron deficiency in the first place, experts advise getting a balanced diet that includes food rich in iron - like beef, poultry, fortified cereals, beans and spinach. Eating vitamin C-rich foods will also help your body absorb the iron in plant foods.

SOURCE: bit.ly/PFNK44 Canadian Medical Association Journal, online July 9, 2012.

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