Heavy endurance training can quietly deplete iron stores, leading to unexplained exhaustion, slow recovery, and stalled progress—especially in women and those with restrictive diets
For runners and triathletes pushing their limits, unexplained fatigue is often chalked up to stress, overtraining, or lack of sleep. But according to Top Santé, a growing number of sports medicine experts warn that relentless cardio can quietly drain the body’s iron reserves, undermining both performance and recovery. The warning comes from Dr. Isabel Viña, a specialist in metabolism and supplementation, who recently highlighted the risks for endurance athletes on social media.
Iron is essential for moving oxygen from the lungs to working muscles. When iron stores drop, the body’s ability to deliver oxygen falters. The result: heavy legs, shortness of breath, and a sense that every workout is harder than it should be. Many athletes ignore these signs, blaming work stress or a tough training block, but the underlying issue may be a true iron deficiency that standard rest won’t fix.
How Endurance Training Depletes Iron
Iron loss isn’t just about diet. The body sheds iron through sweat and the digestive tract, and these losses accelerate with frequent, long-duration exercise. Extended cardio sessions can cause tiny injuries to the gut lining, increasing iron loss through the intestines. Heavy sweating also removes small but significant amounts of iron. For runners, there’s an added twist: the repeated impact of feet striking the ground can break down red blood cells in the capillaries, a process called foot-strike hemolysis, further lowering iron stores.
After tough workouts, the liver releases hepcidin, a hormone that temporarily blocks iron absorption from food. This means that even a balanced diet may not be enough to keep up with losses, especially for women with menstrual cycles, athletes training in heat, or anyone following a restrictive or plant-based diet. According to Psytheater.com, these factors combine to make iron deficiency surprisingly common among endurance athletes.
Spotting the Signs and Getting Tested
Persistent tiredness, plateaued progress, and slow recovery—especially when training volume increases—should prompt a closer look at iron status. Sports medicine doctors recommend blood tests for hemoglobin and ferritin, the main marker of iron stores. It’s important to rule out other causes of fatigue, such as infection or hormonal imbalance, before starting any supplementation. Self-diagnosing or taking iron without medical guidance can be risky, as excess iron can cause its own health problems.
Diet remains the foundation for maintaining healthy iron levels. Heme iron from meat and fish is most easily absorbed, while plant-based sources like beans and whole grains are less efficient but can be boosted by pairing with vitamin C-rich foods. When diet alone isn’t enough, doctors may recommend iron bisglycinate supplements, which are generally easier on the stomach. These work best when combined with vitamin C, copper, and B vitamins, but should always be taken under medical supervision.
Numbers and Context
Research published in the Journal of the International Society of Sports Nutrition estimates that up to 50% of female endurance athletes experience low iron stores at some point in their training. The CDC reports that iron deficiency is the most common nutritional deficiency in the U.S., affecting roughly 10% of women of childbearing age. Regular screening and early intervention can help prevent the drop in performance and quality of life that often goes unnoticed until symptoms become severe.
Iron deficiency is not just a sports issue—it’s a public health concern that can affect mood, cognitive function, and immune health. In the context of athletic performance, even mild deficiency can blunt training gains and increase the risk of injury. For those who rely on endurance exercise for mental health or stress relief, recognizing the subtle signs of iron loss can be the difference between thriving and burning out. As research continues to evolve, athletes and their healthcare providers are urged to treat unexplained fatigue as a signal worth investigating, not just a normal part of training. For those interested in how different forms of exercise impact the body, a recent analysis of energy efficiency in cycling versus walking offers further insight at this link.
Ferritin, the protein that stores iron in the body, is a key marker for both diagnosis and monitoring of iron deficiency. Low ferritin levels can occur even when hemoglobin remains normal, meaning athletes may feel the effects of iron loss before standard anemia shows up on routine bloodwork. Sports medicine guidelines now recommend regular ferritin checks for high-volume endurance athletes, especially women, to catch problems early and tailor interventions to individual needs.
