Yes, endurance athletes are at risk of an iron deficiency. And if you are iron-deficient, a supplement may indeed improve your performance and health. But if your iron levels are normal, or close to normal, taking a pill every day won't affect much besides your wallet.
Michael Zourdos, assistant professor of exercise science at Florida Atlantic University's Muscle Physiology Lab and lead author of a new study on this topic, says he’s skeptical of supplements. "But iron deficiency is one of the most common micronutrient deficiencies worldwide,” he says, “and we know that endurance athletes—and runners specifically—are likely candidates for it.”
That's because the act of running—that repetitive pounding for miles at a time—causes muscle damage and hemolysis, or the destruction of red blood cells. That's perfectly normal, and necessary to build new muscle. But in extreme cases, like when someone is training for a marathon or an ultra, it can trigger iron loss because most of the body's iron is stored in red blood cell proteins, called hemoglobin, that are being destroyed.
And runners often don't get enough iron in their diets, despite the fact that their total caloric intake tends to be higher than the average person's. "Because they're so focused on carbs, they eat less total protein and less high-quality protein," says Zourdos, "which for most people is their main source of iron."
Other endurance athletes may also experience hemolysis, Zourdos says, but sports like cycling and swimming aren't as muscle-lengthening (and muscle-damaging), as running. Athletes who focus on strength training are also somewhat protected from iron loss, thanks to their typically protein-heavy diets and workouts designed to promote overall muscle gain. In other words, they're building back and repairing more of that damaged muscle tissue during recovery than many endurance athletes.
But even if you are upping your weekly mileage, that's not a go-ahead to buy an iron supplement. If you already take a multivitamin, you're likely getting enough iron (most contain 100 percent of the daily value)—and even if you're not, it's not smart to assume your iron deficient, says Kristen Kizer, RD, a clinical dietitian at Houston Methodist Hospital and a USA Track and Field coach.
"I would never recommend taking an iron supplement without a doctor's prescription," she says. "With the exception of what's in multivitamins, most places won't even sell iron supplements over-the-counter since there is a high risk of toxicity in taking too much."
So talk to your doctor. Zourdos recommends having your iron levels tested twice a year, and adding a supplement if—and only if—you have a significant deficiency.
If your levels are just slightly below normal and you don't have dietary restrictions, you may be able to bring them up by adding more meat, fish, or poultry to your plate. "I promote plant products over animal products almost all the time, but in this case red meat is definitely going to be your best source of iron," says Kizer. To keep saturated fat low, she says, choose options like sirloin or super-lean beef tips.
Vegetarians can get enough iron from food too. They may just have to work harder at it. Nuts, seeds, dried fruit, and dark leafy vegetables are all good sources, and Kizer suggests pairing them with a little vitamin C, which can boost absorption. "Eating a strawberry spinach salad or kale with mandarin oranges will help unlock the iron inside those greens," she says.
If you doctor tells you that you have an iron deficiency, a supplement may make a noticeable difference in your fitness. One of iron's main jobs in the body is to assist with oxygen transport; when it's lacking, your muscles are less able to replenish themselves during aerobic exercise. In fact, unusual feelings of fatigue, burning in the legs, or the inability to recover after a workout may be signs that you're deficient, says Kizer. (Or, you know, just out of shape.)
"If iron levels are low enough, certainly endurance performance will be inhibited," says Zourdos. "It would have an effect on the total time you're able to run and the total time you can sustain a certain percentage of VO2 max or a certain heart rate zone, for example."
Bottom line: Don't start taking an iron supplement without having your iron levels tested by a doctor first. If you’re not iron deficient, supplementing may be a waste of money and could even be dangerous. But if you're a woman, a vegetarian, or a long-distance runner (or all of the above), it may be worth getting screened for a deficiency twice a year.