You might blame your job, kids, or stress for making you feel dull and listless. But the problem might be as simple—and fixable—as an iron deficiency. It's the No. 1 nutritional deficiency in the world, according to the World Health Organization. That's because lots of people are at risk, including adolescent girls and premenopausal women, who lose iron through menstrual bleeding; vegetarians, who don't always get enough in their diet; and seniors. Athletes are also at risk because they might use more iron than they consume.

Iron is an essential part of hemoglobin, the protein in red blood cells that ferries oxygen to the body's tissues. Most people get more than enough iron from food and the body's reserves to make up for the small amount lost each day. But when your supply is exhausted, iron deficiency progresses to anemia, which can cause all kinds of problems. Even a mild deficiency can be serious, especially in children and teenagers.

Large U.S. studies have shown that the average math scores of school-aged kids and adolescent girls who are iron-deficient, though not anemic, are lower than those of kids with normal iron levels. And long-term studies of Costa Rican children show that those who were iron-deficient as toddlers continued to lag behind their peers in cognitive test scores even into their late teens.

So what are the tip-offs that you've got an iron deficiency? Symptoms are often so vague that people tend to ignore them. "If you become anemic slowly, as many women do, your body tends to adjust to the symptoms. You may not even realize your hemoglobin is low," says Lodovico Balducci, M.D., chief of geriatric medicine at the H. Lee Moffitt Cancer Center in Tampa, Fla., and president of the National Anemia Action Council.

Here are some signs to be on the lookout for: You become pale, weak, tired, and irritable, have trouble concentrating, or feel chilled even in temperate conditions. Or you might get unusual cravings—for chewing ice, for example—or develop spoon-shaped nails or an inflamed tongue. Other symptoms can include an uncontrollable urge to move your legs when at rest and leg cramps at night. In severe cases, symptoms include headaches, ringing in the ears, spots before the eyes, shortness of breath, dizziness, and a rapid heartbeat. In rare cases, a lack of iron can bring on heart failure.

If you have any of these symptoms and suspect an iron deficiency, get tested. This is particularly true for women of childbearing age, women who have been pregnant, and women whose periods are heavy or last longer than a week. Testing begins with a simple CBC (complete blood count) that measures things like hemoglobin and hematocrit, the percentage of red blood cells in whole blood.

A doctor will often follow up with a test to check blood-iron levels. If the source of the deficiency isn't clear, the doctor may also test for undetected bleeding from an ulcer, for example, or polyps in the digestive tract.

If you're wondering if your baby is getting enough iron, don't worry. Iron deficiency is rare among U.S. infants because breast milk contains easily absorbed iron, and because formulas and cereals are iron-fortified. As your child is weaned to milk, though, keep in mind that milk is not iron-fortified. Toddlers between ages 1 and 2 need no more than three 8-ounce servings of milk daily, and for kids 2 to 8, that drops to two daily servings.

See "Boosting iron with diet" to learn how to get more iron in your diet.

This article first appeared in the August/September 2008 issue of ShopSmart.