Feeling Slow? The Answer May Be in Your Blood

For months, Tara Brooks’ training was going well. “I was increasing my mileage, feeling good,” she says. Then, suddenly, the Portland-based runner started feeling sluggish and pulled out of a speed session. But it wasn’t just one bad day, because the sluggishness continued. “You know those training videos of people doing workouts dragging a tire behind them?” she says. “That’s what it felt like.”

Brooks was fortunate because she immediately knew what was wrong. She’d had a routine blood test a week before, one that had revealed that her iron levels were low. She also happens to be a third-year medical student with a strong science background, so she knew a lot about iron, including how to scour the scientific literature to learn more. She’d felt OK when she’d had the test, but apparently her iron levels had now dropped low enough to significantly interfere with her training.

More Than Just Red Blood Cells 

Runners tend to think of iron in terms of hemoglobin, the iron-containing molecule red blood cells use to carry oxygen from the lungs to the muscles. Not that this is surprising: Having lots of red blood cells and hemoglobin is a big part of the formula that makes elite runners elite. A drop in iron will almost certainly ruin your next 5K.

Producing hemoglobin, however, isn’t the only role iron plays in the body. “It affects everything,” Brooks says. For example, she says, iron is involved in the formation of mood-affecting neurotransmitters such as dopamine, norepinephrine, and serotonin, as well as being related to sleep disorders. It’s even been implicated in depression or depression-like symptoms (particularly, according to one recent study, in men). Brooks’ own experience has been enlightening. “You read about these things in textbooks,” she says, “but it’s really different when you feel them in your body.”

Katie Lucernoni, an exercise physiology graduate student at the University of Oregon whose work focuses on iron, says that iron’s importance comes from the fact that it easily shifts between two oxidation states, chemically known as Fe+3 and Fe+2. (Fe is the chemical symbol for iron.) She says this ability to flip back and forth makes iron especially useful for enzymes. By moving electrons between molecules, iron can trigger proteins to change shape—allowing enzymes to perform vital jobs in the body, such as carrying oxygen, producing energy, and fighting infections.

In addition to the roles listed by Brooks, Lucernoni notes that iron-containing enzymes are also important in aerobic energy production—meaning that in addition to being crucial for building the hemoglobin that brings oxygen to the muscles, iron is crucial for helping these muscles harness that oxygen for energy production.

How Low Is Too Low? 

If you suspect low iron, the first step is to get a blood test. Don’t just start taking iron supplements because one in 250 people has a genetic condition called hemochromatosis, in which they can absorb excess iron, with serious medical consequences.

If you are male, don’t dismiss the possibility of low iron as something that only happens to women. Yes, it is more common in women, but a 2024 study found that 15 percent of men aged 18-49 experience some degree of low iron. And that was for non-runners. Runners might or might not be more susceptible to low iron, but are almost certainly more sensitive to its effects.

The tests most likely to be prescribed by your doctor are hematocrit (the fraction of red blood cells in your blood) and total hemoglobin. These are important, but make sure you are also tested for ferritin, a protein that helps your body store iron. Your hematocrit and hemoglobin can be perfectly normal, but your ferritin can be low, making it not only an indicator of how much iron you have to support the non-hemoglobin needs identified by Brooks and Lucernoni, but an early warning sign of a potential crash in hematocrit and hemoglobin.

Finally, don’t blindly accept a “normal” test result as “good.” Get the numbers, and realize that the low end of the normal range might not be all that good. This is particularly the case for ferritin. Doctors and testing labs have been known to classify a ferritin level of 15 as normal, especially for women. And while that might be OK for a non-runner, it almost certainly isn’t for you.

Most coaches and sports medicine doctors want to see at least 30. Ed Kornoelje, a sports medicine practitioner at University of Michigan Health-West in Wyoming, Michigan, recommends a target of 50 or more (60 for elites), though he tells people not to worry if they top out in the 40s.

How to Deal with Low Iron Levels

Happily, applying all of this to your training isn’t as complex as the physiology. Here are six tips that can help get you through.

1. Iron deficiency is insidious. 

Not only can it come on slowly, but it can masquerade as overtraining. Suspect low iron if you feel overtrained and a few days’ rest doesn’t produce improvement. Without her fortuitously timed blood draw, Brooks might have wasted weeks treating her problem as overtraining.

2. Check your numbers often.  

You should feel empowered to ask your doctor to do bloodwork to check your iron levels, particularly ferritin, not just when you are feeling sluggish, but when you are feeling good. This will let you know what iron levels are normal for you, and give you advance warning if they are starting to slip, even if they have not yet fallen below what your doctor might label as “normal.” How often you should do this is up to you, but if you have a history of iron deficiency, I’d suggest at least every six months. Outside RUN has a staffer who does it monthly. For maximum confidence in the results, Lucernoni suggests taking these tests under consistent conditions, such as time of day, time since exercise, and time since your last meal. Because, she says, “these markers do fluctuate.”

3. If your iron is low, start by addressing your diet. 

My initial response to runners who aren’t vegetarians is to tell them to “eat a cow.” I’ve also been known to joke about blood sausage. That’s because heme iron—the type contained in red meat—is the most easily absorbed. After that, spinach, beans, oatmeal, broccoli, tofu, and a number of other common foods are also good. Iron supplements work for many people, but not for all.

4. If your iron is really low, make sure you rule out medically serious causes. 

For runners, these aren’t likely, but they include such things as a gastrointestinal bleed, colon cancer, H. pylori (a bacterial infection that can interfere with iron absorption), and celiac disease. Some of these are medical emergencies. My father once had his iron drop so low that he needed two units of blood simply in order to stand.

5. Know that the causes of low iron levels in runners are not well understood. 

One theory is foot-strike hemolysis, in which miles and miles of running crush red blood cells in the capillaries in your heels. However, while studies of marathoners and ultramarathoners suggest that this might have transient effects, there’s not a lot of research indicating it’s a major problem. “The body really hangs onto iron,” Brooks says. That is to say just because a few red blood cells get mashed in your heels, it doesn’t mean their iron is lost. More likely, it’s recycled. Iron can also be lost through sweat. But again, the amount isn’t exactly huge. A landmark 1986 study found it to be 22.5 micrograms per liter of sweat, which is only about 1/800th of the recommended daily iron intake.

Lucernoni’s research focuses on the hormone hepcidin, which regulates iron uptake from the gut. “When hepcidin is high,” she says, “it will block iron absorption.” This is a potential problem for runners, she says, because hepcidin is elevated by exercise, peaking three hours afterwards, then slowly subsiding. This means that if you are going to eat a big juicy steak—or take an iron supplement—you might not want to do it immediately after your run. “Take it in the morning,” she says, “or if you exercise in the afternoon, maybe wait until late evening.”

6. Fixing really low levels may mean you need an infusion. 

If that’s you, don’t feel alone; I myself have seen at least five runners who needed such infusions. None were able to figure out why their iron kept dropping, but that doesn’t matter. “Any cause of low iron can benefit from an infusion if oral supplementation isn’t working,” Portland, Oregon, internal medicine specialist Kelly Scott says.

If you do this, however, realize that without medical excuses, infusions of any kind are banned under anti-doping rules as masking agents that can block other anti-doping tests. The easy way to avoid embarrassment is to get a pre-TUE (Preliminary Therapeutic Use Exemption)—a process that takes only 2-3 minutes. Most likely, you’ll never actually need the exemption, but it’s gratifying to know that in today’s drug-testing world, you’re one of the ones who actually follow the rules.