Common Nutrient Deficiencies in Endurance Athletes
And Why Food Alone May Not Always Be Enough
Endurance athletes ask a tremendous amount of their bodies. Whether you’re training for a marathon, cycling long distances, competing in triathlons, skiing, or spending hours each week on the trails, your nutritional needs can look very different from those of the average person.
And yet, when an endurance athlete begins experiencing declining performance, the first assumption is often that they need to train harder, improve their recovery, or adjust their macros.
Sometimes the problem is much simpler:
They don't have enough of the nutrients their body needs to produce energy, transport oxygen, repair tissue, and recover from training.
Research has identified iron, vitamin D, magnesium, zinc, folate, and vitamin B12 among the micronutrients commonly found to be inadequate in athletic populations. Female endurance athletes can be particularly vulnerable to iron deficiency.
For an athlete who is significantly deficient, simply adding a multivitamin may not be enough. In certain clinically appropriate situations, we may consider a repletion phase using IM injections or IV therapy followed by an at-home maintenance program, but the specific route and dose should always be based on labs, symptoms, medical history, and the nutrient being replaced.
Why Endurance Athletes Are Different
Endurance training increases both energy expenditure and nutritional demand.
Athletes can lose or utilize nutrients through sweat, increased red blood cell turnover, repetitive impact, menstrual blood loss, gastrointestinal losses, and the metabolic demands associated with prolonged training. At the same time, athletes who aren't eating enough to match their training volume can develop low energy availability, further increasing their risk for nutritional inadequacy.
This creates an important distinction:
Eating a "healthy diet" doesn't necessarily mean an athlete is meeting their individual nutritional requirements.
Someone can eat extremely well and still develop a deficiency, particularly when training volume is high.
1. Iron + Ferritin
Iron is arguably one of the most important nutrients to evaluate in an endurance athlete experiencing unexplained fatigue or declining performance.
Iron is required to make hemoglobin, which carries oxygen through the bloodstream. It's also involved in mitochondrial energy production and normal muscle function.
Endurance athletes may experience increased iron demands because of:
Foot-strike hemolysis from repetitive running
Iron losses through sweat
Gastrointestinal and urinary losses
Menstrual blood loss
Inadequate dietary iron
Reduced iron absorption
High training volume
Low energy availability
The IOC specifically identifies endurance-related mechanisms, menstrual blood loss, inadequate intake, and other iron losses as reasons athletes can struggle to maintain iron status.
And importantly, an athlete can develop iron deficiency before they become anemic.
That means hemoglobin may still appear "normal" while iron stores, reflected in part by ferritin, are becoming depleted.
Symptoms can include:
Declining endurance
Elevated heart rate during familiar workouts
Breathlessness
Heavy legs • poor recovery
Fatigue
Brain fog
Feeling cold
Hair shedding
Restless legs
Reduced exercise tolerance
A more comprehensive iron evaluation may include:
CBC • Ferritin • Serum Iron • TIBC/Transferrin • Transferrin Saturation
The IOC recommends evaluating several markers together rather than relying on a single measurement.
Supporting Iron Repletion
Mild deficiencies may respond to dietary changes and oral iron.
However, oral iron isn't appropriate or effective for everyone. Gastrointestinal intolerance, poor absorption, ongoing blood loss, the magnitude of the deficiency, and training demands can all influence treatment.
For appropriately diagnosed iron deficiency, a provider may sometimes recommend IV iron when oral replacement has been ineffective, poorly tolerated, or when more rapid repletion is medically indicated, followed by oral nutritional support and repeat labs. IV iron should not be used simply as a performance-enhancing "loading dose" in an athlete who isn't deficient.
2. Vitamin D
Vitamin D affects far more than bone health.
It plays roles in:
Immune function • muscle function • calcium metabolism • bone remodeling • inflammation • recovery
Athletes living at northern latitudes or training primarily indoors may be especially susceptible to low vitamin D because UVB exposure can be limited for significant portions of the year.
The IOC recognizes vitamin D as one of the nutrients athletes may require supplementation for, but also emphasizes that dosing should be determined by actual vitamin D status rather than indiscriminate high-dose supplementation.
Testing is simple:
25-OH Vitamin D
When levels are significantly low, a clinician may use a short-term repletion strategy before transitioning the athlete to a maintenance dose.
Depending on the clinical situation and available preparations, this could involve clinician-directed oral repletion or IM vitamin D followed by an individualized at-home maintenance program.
The important part isn't simply taking vitamin D.
It's testing → repleting → retesting → maintaining.
3. Vitamin B12
Vitamin B12 is essential for:
Red blood cell production
Neurologic function
DNA synthesis
Normal energy metabolism
Athletes following vegan or predominantly plant-based diets deserve particular attention because natural dietary B12 is primarily obtained from animal-derived foods. The IOC specifically notes that B12 supplementation may be warranted for vegan and near-vegan athletes.
Potential symptoms of deficiency can include:
Fatigue
Weakness
Numbness or tingling
Cognitive changes
Poor recovery
Exercise intolerance
Testing may include:
Serum B12, with additional markers such as methylmalonic acid and homocysteine when clinically appropriate.
For a confirmed deficiency, providers may sometimes begin with IM B12 injections to restore levels, particularly when absorption is a concern, before transitioning to an appropriate oral or sublingual maintenance strategy.
4. Magnesium
Magnesium participates in hundreds of enzymatic reactions and is deeply involved in:
ATP production • muscle contraction and relaxation • electrolyte balance • nerve signaling • glucose metabolism
Athletes with high sweat losses or inadequate dietary intake may struggle to meet magnesium needs.
Possible signs of inadequate intake can include muscle cramping, twitching, poor sleep, fatigue, headaches, and difficulty recovering, although these symptoms are nonspecific and shouldn't automatically be attributed to magnesium.
Food sources include:
Pumpkin seeds • nuts • leafy greens • avocado • legumes • dark chocolate
When additional support is warranted, oral magnesium is generally the foundation of a maintenance program. Different forms may be selected based on tolerance and clinical goals.
5. Zinc
Zinc doesn't get nearly as much attention in endurance nutrition, but it plays an important role in:
Immune function • wound healing • protein synthesis • hormone function • tissue repair
Athletes experiencing frequent illness, prolonged recovery, restrictive diets, or inadequate overall caloric intake may be more vulnerable to insufficient zinc intake. Research in athletic populations has identified zinc among the micronutrients in which deficiencies can occur.
Long-term high-dose zinc supplementation shouldn't be undertaken casually because excessive zinc can interfere with copper status.
This is another reason we prefer targeted supplementation rather than simply adding more supplements.
6. Folate + Other B Vitamins
B vitamins participate extensively in energy metabolism and red blood cell production.
Folate is particularly important for rapidly dividing cells and healthy red blood cell formation.
Athletes with restrictive diets, poor overall caloric intake, gastrointestinal problems, or inadequate food variety may be at greater risk for inadequate intake.
Rather than assuming every athlete needs a high-dose B-complex, supplementation should complement dietary intake and identified needs.
7. Calcium
Endurance athletes need to think beyond today's race.
We also need to protect the athlete's bones for the next decade.
Calcium becomes particularly important when an athlete has:
Low energy availability
Menstrual irregularities
Limited dairy/calcium intake
Vitamin D deficiency
History of stress fractures
High-volume repetitive-impact training
Low energy availability combined with inadequate calcium and vitamin D can create a particularly concerning environment for bone health.
The IOC notes that athletes with low energy availability or menstrual dysfunction may require greater attention to calcium and vitamin D intake.
The Bigger Problem: Low Energy Availability
Sometimes the athlete doesn't have six separate nutrient deficiencies.
They simply aren't eating enough.
An athlete training 8–15+ hours per week may unintentionally create a substantial mismatch between energy expenditure and energy intake.
Over time, this can contribute to Relative Energy Deficiency in Sport (RED-S) and affect recovery, reproductive hormones, bone health, immunity, metabolic function, and athletic performance.
No supplement protocol can fully compensate for chronic under-fueling.
Food remains the foundation.
A Different Approach: Replete First, Then Maintain
One strategy we like for significant, documented deficiencies is thinking about supplementation in two phases.
Phase 1: Repletion
When a deficiency is substantial, the initial goal is to restore nutrient stores.
Depending on the nutrient, severity of deficiency, absorption, symptoms, medical history, and provider assessment, this may involve:
Oral therapeutic supplementation • IM injections • IV replacement when medically indicated
This can be especially relevant for nutrients such as iron or B12, where medically supervised parenteral replacement has established clinical uses.
Not every nutrient needs, or should receive, IV or IM therapy.
Phase 2: At-Home Maintenance
Once stores have improved, the goal changes.
Instead of continually "loading" nutrients, we want to determine what the athlete needs to maintain adequate levels despite their training demands.
That might include:
Daily or intermittent oral supplementation • nutrition changes • electrolyte replacement • adjusting supplementation around training cycles • addressing menstrual blood loss or GI issues • periodic lab monitoring
Think of it this way:
Repletion fills the tank. Maintenance helps keep it full.
Don't Guess—Test
One of the biggest mistakes we see in wellness and sports nutrition is treating symptoms with supplements without determining whether a deficiency actually exists.
Fatigue doesn't automatically mean low B12.
Muscle cramps don't automatically mean low magnesium.
Poor endurance doesn't automatically mean low iron.
The IOC specifically cautions against indiscriminate micronutrient supplementation and recommends identifying deficiencies through nutritional assessment and appropriate testing whenever possible.
For endurance athletes experiencing declining performance, fatigue, prolonged recovery, recurrent injury, or difficulty adapting to training, a provider may consider evaluating:
CBC • Ferritin • Iron/TIBC • Transferrin Saturation • Vitamin D • B12 • Folate • Magnesium • CMP
Additional testing can then be individualized based on symptoms, diet, sex, training volume, menstrual history, gastrointestinal health, and previous results.
Your Training Plan Shouldn't Be More Sophisticated Than Your Nutrition Plan
Endurance athletes meticulously track mileage, heart rate, pace, power output, HRV, sleep, recovery, and race times.
But sometimes the basic physiology underneath all of those metrics gets overlooked.
If your body doesn't have enough iron to adequately support oxygen transport, enough B vitamins to support normal cellular processes, enough vitamin D and calcium to support your bones, or enough overall energy to recover from training, another recovery gadget isn't going to solve the underlying problem.
Start with physiology.
Test. Identify deficiencies. Replete what's actually low. Then build a sustainable at-home maintenance strategy that supports the demands you're placing on your body.
Because better endurance isn't always about pushing harder.
Sometimes it's about finally giving your body what it needs to keep up.
References
Maughan RJ, Burke LM, Dvorak J, et al. IOC consensus statement: dietary supplements and the high-performance athlete. British Journal of Sports Medicine. 2018;52(7):439-455. doi:10.1136/bjsports-2018-099027.
Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics. 2016;116(3):501-528. doi:10.1016/j.jand.2015.12.006.
Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine Joint Position Statement: Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise. 2016;48(3):543-568. doi:10.1249/MSS.0000000000000852.
Maughan RJ, Burke LM, Dvorak J, et al. IOC Consensus Statement: Dietary Supplements and the High-Performance Athlete. International Journal of Sport Nutrition and Exercise Metabolism. 2018;28(2):104-125. doi:10.1123/ijsnem.2018-0020.
U.S. Anti-Doping Agency (USADA). IV Injection and Infusion Guidance for Athletes and Prescribing Physicians. USADA. Guidance includes specific considerations for IV iron used in the treatment of documented iron deficiency and iron-deficiency anemia.
This article is for educational purposes only and is not intended to diagnose or treat a medical condition. IV and IM nutrient therapy should be administered under appropriate medical supervision and based on individual clinical need. Competitive athletes should also verify anti-doping rules before receiving IV therapy; USADA notes restrictions related to IV infusion volume that can apply even when the infused substance itself is permitted.