Iron & Vitamin D Deficiency In Athletes: The Hidden Performance Limiters

Iron deficiency and low vitamin D are two of the most common hidden performance killers in athletes, quietly draining energy, slowing recovery, and reducing endurance long before most athletes even realise something is off. The frustrating part? You can be training hard, eating well, and still feel flat because these two nutrients sit right at the centre of performance,  driving oxygen delivery, energy production, muscle function, and recovery. Even a mild dip can be enough to blunt training quality, slow adaptation, and leave you wondering why sessions suddenly feel harder than they should.

 

Table Of Content

• Why Iron & Vitamin D Matter For Performance

• Why Athletes Are At Higher Risk

• How Common Are Deficiencies?

• Signs & Symptoms

• Testing & Monitoring

• Food-First Approach

• Supplementation Considerations

• Key Takeaways

Why Iron & Vitamin D Matter For Performance

Iron and vitamin D are key nutrients for athletes. Iron helps carry and store oxygen in the body and supports energy production in muscles. Vitamin D acts like a hormone, helping muscles work efficiently, recover properly, and maintain bone health. Together, they influence oxygen delivery and energy availability, two crucial factors for endurance and high-intensity performance¹⁻⁴.

 

Why Athletes Are At Higher Risk

Athletes have higher nutrient needs and also lose more iron and vitamin D:

Iron:

• Endurance training boosts red blood cell production, increasing iron demand.

• Small gut bleeds and repeated impact, like runners’ foot strike, can cause iron loss.

• Exercise triggers a hormone called hepcidin that temporarily reduces iron absorption from food⁵.

Vitamin D:

• Indoor training or winter months limit sun exposure, which the body needs to make vitamin D.

• Wearing clothes and sunscreen blocks UVB rays needed for vitamin D production.

• Some athletes with higher body fat may store more vitamin D in fat tissue, making it less available for the body⁶.

Talk Nerdy To Me: Iron
Iron carries oxygen in blood and muscles and is essential for energy production in mitochondria³. The body regulates iron tightly using hepcidin, a hormone that can block absorption after intense exercise⁵. For athletes, timing matters, take iron in the morning or between sessions, on an empty stomach or with vitamin C, and avoid inhibitors like calcium, caffeine, or high-phytate foods.

Talk Nerdy To Me: Vitamin D
Vitamin D, produced in the skin via sunlight and activated in the liver and kidneys, acts like a hormone to support muscle function and energy production⁴˒⁶. Low vitamin D can weaken muscles, slow reflexes, and increase stress fracture risk⁷.

 

How Common Are Deficiencies?

Iron deficiency is particularly prevalent in endurance athletes and females, with studies reporting iron deficiency (with or without anaemia) affecting up to ~15-35% of athletes depending on population and sport¹.

Vitamin D insufficiency is even more widespread, especially in athletes training indoors or during winter months, with prevalence estimates often exceeding 40-60% in some cohorts⁶.

 

Signs & Symptoms

Deficiencies can present subtly; look for signs such as:

Iron deficiency:

• Reduced endurance capacity

• Increased perceived effort at submaximal workloads

• Prolonged recovery between sessions

• Fatigue disproportionate to training load

Vitamin D insufficiency:

• Increased susceptibility to illness

• Muscle weakness or reduced force output

• Bone stress injury risk

• Generalised fatigue or poor training resilience

These symptoms overlap significantly with overtraining, making biochemical testing essential.

 

Testing & Monitoring

Diagnosis requires blood testing rather than symptom tracking alone.

Iron markers:

• Serum ferritin (iron storage marker)

• Haemoglobin

• Transferrin saturation and serum iron

Vitamin D marker:

• Serum 25-hydroxyvitamin D [25(OH)D]8

Regular monitoring is especially important for endurance athletes, women, and those with high training loads or limited sun exposure

 

Food-First Approach

Iron-rich foods

• Red meat, poultry, seafood

• Lentils, chickpeas, beans

• Iron-fortified cereals

• Spinach and leafy greens (non-haem iron)

Haem iron (animal-based) has higher bioavailability than non-haem iron, with absorption rates typically 2-3x higher³. Vitamin C co-ingestion can enhance non-haem iron absorption via reduction of ferric to ferrous iron and formation of soluble complexes.

Vitamin D sources

• Oily fish (salmon, sardines, mackerel)

• Egg yolks

• Fortified dairy or plant alternatives

• Sun exposure (where appropriate)

 

Supplementation Considerations

Supplementation should be individualised and guided by blood testing. Iron supplementation is most effective when tailored to ferritin status and timing relative to training to avoid hepcidin-mediated absorption inhibition⁵.

Vitamin D supplementation may be required in athletes with confirmed insufficiency, particularly during winter or periods of limited sun exposure. Dosing should be conservative and monitored to avoid excessive serum levels.

 

Key Takeaways

Iron and vitamin D deficiencies are silent but significant performance limiters in athletes. Iron governs oxygen transport and mitochondrial energy production, while vitamin D regulates muscle function, bone health, and immune resilience. Because both systems are tightly regulated and sensitive to training load, deficiencies can develop even in well-nourished athletes.

Early detection through blood testing, supported by a food-first approach and targeted supplementation when required, remains the most effective strategy to protect performance consistency.

 

Ash Miller
Dietitian and Nutritionist (Masters)
Bachelor of Physical and Health Education
Instagram: @ashthomo_nutrition

 

References

1. 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. J Acad Nutr Diet. 2016;116(3):501-528.

2. Larson-Meyer DE, Willis KS. Vitamin D and athletes. Curr Sports Med Rep. 2010;9(4):220-226.

3. McArdle WD, Katch FI, Katch VL. Exercise physiology: nutrition, energy, and human performance. 8th ed. Philadelphia: Wolters Kluwer; 2015.

4. Girgis CM, Clifton-Bligh RJ, Hamrick MW, Holick MF, Gunton JE. The roles of vitamin D in skeletal muscle: form, function, and metabolism. Endocr Rev. 2013;34(1):33-83.

5. Peeling P, Dawson B, Goodman C, et al. Effects of iron injections on iron status and performance in iron-deficient athletes. Med Sci Sports Exerc. 2007;39(11):1888-1895.

6. Owens DJ, Fraser WD, Close GL. Vitamin D and the athlete: emerging insights. Eur J Sport Sci. 2015;15(1):73-80.

7. Cannell JJ, Hollis BW, Sorenson MB, Taft TN, Anderson JJ. Athletic performance and vitamin D. Med Sci Sports Exerc. 2009;41(5):1102-1110.

8. Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266-281.

 

Disclaimer:
The content in this blog is for general information only and is not a substitute for professional medical advice, diagnosis, or treatment. Always speak with your doctor or allied health team before changing your diet, exercise, or taking supplements, especially if you have a health condition or take medication. Please use this information as a guide only. Aid Station doesn't take responsibility for individual outcomes.