This article is written by Amy from The Running Dietitian.
You feel you're doing everything right. You're consistent with training, but something feels off. Runs that used to feel comfortable now feel like hard work. Your heart rate is higher than it should be, your legs feel heavy, and no matter how much you rest, you never quite feel recovered. Before you start questioning your fitness or your training plan, it's worth considering something far simpler: your iron levels.
Iron deficiency is the most common micronutrient deficiency in endurance athletes, and one of the most overlooked. It can mimic general fatigue, low motivation, and a dip in performance that's easy to write off as overtraining or an off training cycle. Female runners, vegetarians, and high-mileage athletes are particularly at risk. Here's everything you need to know.
Why Iron Matters for Runners
For runners, performance is not just a product of training. It depends on the body's ability to deliver oxygen efficiently to working muscles, and that process hinges on one nutrient most athletes underestimate: iron.
Iron is the cornerstone of haemoglobin, which is essential for transferring oxygen in your blood from the lungs to the tissues and organs throughout the body. It also plays a central role in myoglobin, which stores oxygen directly within muscle tissue and releases it during intense effort.
Beyond oxygen transport, iron is essential to mitochondrial function, meaning it plays a vital role in how cells produce energy. Fatigue, laboured breathing, and a plateau in fitness are often associated with overtraining, when in reality could be trying to tell you that your iron levels need support.
Why Runners Are At Higher Risk
Research has found iron deficiency in as many as 50% of female endurance athletes and 30% of male endurance athletes. The running population are at higher risk than the general population for a variety of reasons.
Foot-strike haemolysis: the mechanical destruction of red blood cells from the repeated impact of feet hitting the ground. Every footfall ruptures a small number of cells, and over the course of high mileage weeks, that small amount adds up.
Losses via sweat: sweat contains iron in small amounts. The more you train, the more you lose through the skin. This can be more concerning for heavy sweaters.
Gastrointestinal bleeding: intense training diverts blood away from the gut, and microscopic bleeding in the intestinal lining can occur regularly.
Menstruation: adds a significant additional drain for female athletes.
Increased red blood cell turnover: the body is constantly breaking down and rebuilding cells at a higher rate than in sedentary people, requiring more iron to support rebuilding cells.
Diet: runners who follow plant-based diets, restrict calories, or simply do not eat enough red meat take in less bioavailable iron than their training demands.
Who Is Most At Risk
Certain runners carry a higher burden of risk and should treat iron monitoring as a routine part of training management:
Female runners, due to menstrual losses compounding training losses
Runners following vegan or vegetarian diets
High-volume trainers, particularly those logging 50 or more miles per week
Athletes who have recently increased mileage significantly
Those who train at altitude
Runners with a history of anaemia or disordered eating
Regular blood donors
Iron deficiency tends to build gradually, disguised as overtraining, stress, or simply a rough training block. Early signs include:
Persistent tiredness that does not resolve with rest
Feeling flat or unmotivated before runs
Sense of heaviness in legs
Slightly elevated heart rate at rest and during exercise
Mood disturbances including irritability and low motivation
More significant signs include:
Dizziness, especially at the end of a hard effort or when standing up quickly
Shortness of breath
Slower recovery
Declining performance
More frequent injuries or illness
Understanding Your Iron
Ferritin and haemoglobin each capture a different snapshot of your body's iron status.
Ferritin is a storage protein that reflects how much iron your body has in reserve, essentially acting as the iron "warehouse." Haemoglobin, found in red blood cells, shows how much iron is actively being used to carry oxygen through the bloodstream, making it the functional measure of iron at work.
Together, the two paint a bigger picture: you can have depleted stores (low ferritin) before your haemoglobin drops. Iron deficiency progresses through three distinct stages, and by the time haemoglobin drops, you are already at stage three.
Stage one is depleted iron stores. Ferritin falls below 30 ng/mL while haemoglobin remains normal. Most runners will start to experience unexplained fatigue at this stage and training adaptation is already being negatively affected.
Stage two is iron deficiency without anaemia. Ferritin falls below 20 ng/mL while haemoglobin remains normal. This is where fatigue becomes more noticeable, workouts feel disproportionately hard, and recovery is delayed. A standard blood test will still return normal results here, as ferritin reference ranges are based on the general population and do not account for the higher demands of runners.
Stage three is iron deficiency anaemia. Ferritin falls below 12 ng/mL and haemoglobin finally drops below normal. This is the stage a standard blood test will catch, but it should never have been allowed to progress this far.
When you get blood work done, ask for a full iron panel, not just a standard blood count. This should include ferritin, serum iron, transferrin saturation, and haemoglobin.
Target ferritin ranges for runners are meaningfully higher than the reference ranges used for the general population. For female runners, the minimum is 35 to 40 ng/mL, with optimal levels between 50 and 70 ng/mL. For male runners, the minimum is 40 to 50 ng/mL, with optimal levels between 50 and 100 ng/mL.
Haem vs non-haem iron
Iron from food comes in two forms, and the body absorbs them very differently.
Haem iron, found in animal products, is absorbed efficiently. The body has a dedicated transport mechanism for it, and absorption is relatively unaffected by what else you eat at the same meal. Good sources include red meat, liver, mussels, sardines, chicken, turkey, and fish.
Non-haem iron, found in plant foods and fortified products, is absorbed far less efficiently. The body absorbs anywhere from 2 - 20% of the non-haem iron consumed, compared to 15 - 35% of haem iron. This does not make plant-based eating incompatible with healthy iron levels, but it can require more work to meet daily iron needs. Good sources include lentils, chickpeas, beans, tofu, spinach, fortified breakfast cereals, baked potatoes with skin, pumpkin seeds, apricots, prunes, and enriched pasta.
Maximising Iron Absorption
How you combine foods matters as much as which foods you choose, especially when eating a diet high in non-haem iron.
Pair non-haem iron with vitamin C: Consuming a vitamin C source alongside plant-based iron significantly increases absorption. Foods high in vitamin c include: orange juice, bell peppers, kiwi, strawberries, and tomatoes.
Be mindful of inhibitors: Calcium, coffee, and tea all reduce iron absorption when consumed at the same meal. Avoid coffee and tea for at least an hour before and after iron-rich meals, and it is best to not take iron supplements with dairy.
Cook in cast iron: Cooking acidic foods like tomato-based sauces in cast iron cookware can increase the iron content of a meal.
Time meals around training: The hormone hepcidin, which regulates iron absorption in the gut, rises significantly in the three to six hours after a workout. During that window, the body actively limits how much iron it absorbs from food and supplements. Eating iron-rich meals before training, or more than six hours after, gives your body the best chance of making use of them.
Iron Supplementation
Iron supplementation can correct a deficiency, but it should only happen under the guidance of a doctor who has confirmed the deficiency through blood work. Iron is not a nutrient to self-prescribe because excess iron accumulates in tissues and organs which could lead to iron overload. If supplementation is recommended, discuss the form with your doctor, as different iron compounds vary in absorption rates and side effect profiles.
Ferrous sulfate is the most common over-the-counter option, but its low absorption rate of 10 to 15% leaves most of the iron unabsorbed in the digestive tract, leading to nausea and constipation. Iron bisglycinate shields the iron through stomach acid for better delivery to the small intestine, offering superior absorption rates with fewer side effects.
Key Takeaways
Iron deficiency is particularly common in runners, making it worth going beyond a standard blood count to a full iron panel that includes ferritin, and knowing how to interpret those numbers through an athlete's lens, not a sedentary adult's.
On the nutrition side, eat iron-rich foods regularly, pair plant-based sources with vitamin C to boost absorption, and avoid common inhibitors like coffee and calcium around iron-rich meals. If your ferritin does come back low, work with a sports medicine doctor or registered dietitian to address it properly.