The warning signs that all active women need to be aware of.

Women and iron have a complicated relationship. This crucial mineral helps bring fresh oxygen to your system via your bloodstream, and it's a super important nutrient for women who exercise regularly and need that fresh O2 to power through workouts. New research from the Journal of Nutritionbacks this up: Researchers did a meta-analysis of previously published studies and determined that women who took oral iron supplements got a significant performance boost, allowing them to work out at a higher efficiency. 

But you probably know better than to pop supplements unless you really need them—so how do you know if you suffer from the kind of low iron levels that really justify taking iron pills? First, pay attention to the main symptom of iron deficiency: fatigue. "If you're chronically tired or if you've noticed that you don't have the energy to perform as well during your workouts and there's no clear reason why, you might have an iron deficiency such as anemia," says Boston-based sports nutritionist Nancy Clark, R.D., author of Nancy Clark's Sports Nutrition Guidebook. If your periods are heavy, that can be another tip-off since blood loss during menstruation can deplete your iron stores, says Clark. Irritability and frequently feeling cold are also clues. 

Sound familiar? Don't pop a supplement just yet. See your M.D. for a proper diagnosis by asking for a blood test, says Clark. Depending on the diagnosis, your doc may want you to take iron supplements or a multivitamin that contains iron. Or you might be advised to shore up your iron levels via food sources, which are better absorbed by your system. Women need 18 milligrams of iron daily; you can score it by consuming more red meat, dried beans, egg yolks, dark-meat chicken, whole grains, and iron-fortified cereals, says Clark. 

Oh, and even if you aren't fatigued and don't have any other symptoms that could point to deficiency, it's a good idea to get your iron levels checked at your annual exam, especially if you're an athlete, says Clark. Not all iron-deficient women have clear signs, so you'll want to play it safe.