Athletes, especially endurance runners and menstruating women, run a meaningfully higher risk of low iron than the general population, and low iron shows up as flat performance and constant tiredness long before it shows up as diagnosed anaemia. Iron contributes to normal oxygen transport in the body, to normal formation of red blood cells and haemoglobin, and to the reduction of tiredness and fatigue. The honest approach is food first, testing before supplementing, and caution around taking iron blindly, since too much is its own problem. Low iron is one of the more common, and more commonly missed, explanations for a training plateau that food and sleep changes alone do not fix.
Why athletes lose more iron than other people
Training hard increases iron losses through several separate routes at once: sweat contains small amounts of iron, endurance exercise can cause minor gut bleeding, and repeated foot impact on hard surfaces damages red blood cells in a process called footstrike hemolysis, particularly in runners. On top of that, intense training triggers a temporary rise in a hormone called hepcidin, which blocks iron absorption from the gut for several hours after a hard session. None of these individually is dramatic, but stacked together across weeks of training they add up to a real deficit for many athletes. Athletes in high-volume phases of training, or those who have recently increased mileage quickly, are at the highest risk simply because more of these mechanisms are active at once.
Why menstruating women face extra risk
Regular menstrual blood loss is, on its own, a significant iron drain, and combining that with the training-related losses above puts menstruating female athletes at clearly elevated risk of low iron status compared with male athletes or non-training women. This is one of the more consistent findings in sports nutrition research, and it is a large part of why iron deficiency is flagged so often in female endurance athletes specifically. Anyone in this group who trains seriously and feels persistently tired despite adequate sleep and food has good reason to ask a doctor about iron testing.
What iron actually does for performance
Iron is central to haemoglobin, the protein in red blood cells that carries oxygen from the lungs to working muscles. When iron status drops, oxygen delivery to muscle during exercise becomes less efficient, which is felt as heavier legs, a higher heart rate at the same pace, and general fatigue that does not improve with rest. Iron contributes to normal energy-yielding metabolism as well, which is part of why low iron status is associated with reduced training capacity even before haemoglobin drops low enough to be labelled anaemia.
Signs low iron is affecting training
- Performance that plateaus or drops despite training consistently and eating reasonably well.
- Unusual fatigue that persists across a full rest day rather than easing with recovery.
- Higher heart rate than usual for a given easy pace or effort level.
- Frequent lightheadedness or breathlessness during moderate effort that used to feel comfortable.
None of these signs are unique to iron, which is exactly why testing rather than guessing matters here.
Food sources versus supplements
Red meat, liver, and shellfish provide the most readily absorbed form of iron (haem iron); dark leafy greens, legumes, and fortified cereals provide a less well-absorbed form (non-haem iron) that benefits from being eaten alongside vitamin C to improve uptake. For most athletes without a diagnosed deficiency, focusing on these food sources is the sensible first step rather than reaching for an iron supplement by default. Supplementation makes the most sense once a blood test has actually confirmed low iron stores, not as a routine addition to every training block.
When and how to get tested
A ferritin blood test, usually alongside a full blood count, is the standard way to check iron stores, and it is worth requesting specifically if fatigue and flat performance persist for more than a couple of weeks without an obvious cause like illness or poor sleep. Endurance runners and menstruating athletes with heavy training loads are reasonable candidates for periodic testing even without obvious symptoms, since iron status can decline gradually. A doctor or sports dietitian is best placed to interpret results and decide whether supplementation is warranted.
The risk of over-supplementing
Iron is not a “more is better” nutrient. Taking iron supplements without a confirmed deficiency can push iron stores too high over time, and excess iron is stored in the body rather than excreted efficiently, which is a real concern with sustained unnecessary supplementation. Iron supplements can also cause uncomfortable digestive side effects, including constipation and nausea, particularly at higher doses. This is exactly why testing before supplementing, rather than after, is the responsible order of operations, and why iron is best treated as a targeted fix rather than a routine addition to everyone’s stack.
Putting it together
For most athletes, especially endurance runners and menstruating women carrying a heavy training load, the sensible sequence is: eat iron-rich foods consistently, watch for the fatigue and performance signs above, get a ferritin test if those signs persist, and only supplement with a confirmed deficiency and ideally under guidance. Alongside iron status, adequate overall nutrition, including enough protein and attention to vitamin D and magnesium, supports the same normal energy-yielding metabolism iron contributes to. CapyFuel ships iron and other core nutrition products across Malta and Cyprus for athletes who have been tested and know supplementation is the right call.
Source: Sports Medicine review on iron status in endurance athletes (PMC), EU Register of nutrition and health claims, NHS guidance on iron deficiency anaemia.
Frequently asked questions
Why are athletes more likely to have low iron?
Endurance training increases iron losses through sweat, minor gut bleeding, and footstrike hemolysis, where red blood cells are damaged by repeated impact, especially in runners. Hard training also temporarily blocks iron absorption through a hormone called hepcidin. Together these losses add up across weeks of training.
Why are female athletes at higher risk of iron deficiency?
Regular menstrual blood loss is already a significant iron drain, and combined with training-related losses it puts menstruating female athletes at clearly elevated risk compared with male athletes. This is one of the most consistent findings in sports nutrition research on iron status.
What does iron actually do for athletic performance?
Iron contributes to normal oxygen transport in the body, to normal formation of red blood cells and haemoglobin, and to the reduction of tiredness and fatigue. Low iron status reduces how efficiently oxygen reaches working muscles, which is felt as heavier legs and fatigue that does not improve with rest.
Should athletes take an iron supplement without testing first?
No. Iron supplements should generally follow a confirmed low ferritin result from a blood test, not be taken routinely. Excess iron is stored in the body rather than excreted efficiently, and unnecessary supplementation can cause digestive side effects and push iron stores too high over time.
When should an athlete get tested for low iron?
A ferritin blood test is worth requesting if fatigue and flat performance persist for more than a couple of weeks without an obvious cause. Endurance runners and menstruating athletes with heavy training loads are reasonable candidates for periodic testing even without obvious symptoms.