The Quiet Performance Limiter – First Endurance

By Jennifer Kurtz, PhD(c), CISSN, CSCS, EP-C

Photo: Warwick Purdy

Over the past several years, the conversation around fueling has focused on carbohydrates, fluids, and sodium, and with good reason: they are the building blocks of endurance performance. But beneath that revolution sits a quieter layer of performance physiology that rarely gets the same attention until it’s too late: micronutrients.

Micronutrients don’t give you an immediate boost like carbs. They don’t save a race the way sodium can. Instead, they work in the background supporting oxygen transport, mitochondrial function, immune resilience, bone health, and recovery. When they’re adequate, training feels normal. When they’re low, everything feels inexplicably harder. Crucially for endurance athletes, research consistently shows that they often miss dietary targets for key vitamins and minerals, even when overall energy intake appears sufficient. It’s something I see constantly: endurance athletes with strong aerobic engines held back by silent deficiencies. Micronutrients can’t win you a race tomorrow, but they can ensure that you can train hard without breaking down or repeatedly getting sick, and in endurance sports, durability is performance.

With that in mind, let’s break down what matters, what’s overhyped, and what to do about it.

The Most Common Micronutrient Deficiencies for Endurance Athletes 

A common question is “do endurance athletes need a multivitamin?” Short answer: if you’re deficient.

In general, my recommendation to endurance athletes is to maximize the amount of micronutrients they can get from real food by eating a balanced diet and sufficiently fueling the work they’re doing in training and racing. However, even the most well-fueled athletes can miss the recommended intake for several micronutrients that are hard to find in sufficient quantities in food. 

Iron

The first and most important of these micronutrients is iron, which is non-negotiable for endurance performance. It’s essential for oxygen transport (hemoglobin), oxygen storage (myoglobin), and enzymes involved in energy metabolism. Athletes with low iron might notice an elevated heart rate at easy paces, poor fatigue resistance, and a plateau in their fitness despite consistent training. Endurance athletes can be particularly susceptible to iron deficiency as a result of heavy training loads, sweat and GI losses, or foot-strike hemolysis. Athletes at altitude or on plant-based diets are often deficient and the risk is further elevated for women due to blood loss during menstruation. 

What to do: Despite the importance of iron, and how frequently endurance athletes find themselves with iron deficiency, the answer is not to simply supplement at random. Unlike many other micronutrients, an excess of iron can actually be more problematic than a deficiency, causing oxidative stress, GI issues, and mitochondrial damage. For that reason, it’s best to consult your physician and get blood work to understand ferritin and hemoglobin before adding supplementation. 

Vitamin D 

Vitamin D affects more than bones, playing a role in muscle function, immune health, and inflammation regulation. Low Vitamin D is linked with slower recovery, more frequent illness, and higher bone stress risk. Athletes with indoor jobs, low sun exposure, a heavy indoor training load, and/or winter schedules are particularly at risk. 

What to do: To combat a Vitamin D deficiency, athletes should consider testing serum 25(OH)D seasonally, and supplement periodically, with a particular focus on indoor/winter training, when sun exposure is at its lowest. 

Magnesium 

Magnesium is involved in over 300 enzymatic reactions, including muscle contraction and relaxation, ATP production, and nervous system regulation. Low intake–especially with high sweat losses and refined diets–is common among athletes and can increase perceived effort and reduce endurance. Potential signs of deficiencies can be muscle spasms, poor sleep quality, and increased stress response. 

What to do: Prioritize quality food sources of magnesium, as they will help with absorption regulation, and leverage supplementation to address magnesium deficiencies not addressed through diet alone. 

Zinc

Zinc supports immune function, wound healing, and protein synthesis. While deficiencies in athletes can be mixed, low intake and sweat loss can contribute to fatigue and immune challenges. 

What to do: Athletes struggling with frequent colds, poor recovery, low appetite, and fatigue should focus on consistent intake while avoiding chronic high-dose supplementation. 

What To Do About It 

Fuel Enough First 

Most micronutrient deficiencies stem from chronic underfueling. A balanced diet focused on high quality food and variety will go a long way toward assuring that your micronutrient needs are being met. With high levels of training or travel sometimes this can be a challenge which is where testing and supplementation come into play.

Test Strategically

Even the most carefully thought out diet cannot overcome periods of heavy training or environmental stressors when it comes to micronutrients. Therefore, it is a good idea to target strategic times of year to test blood levels, particularly with regards to iron and vitamin D. Endurance athletes are particularly prone to deficiencies in both, due to heavy training (both), sweat loss (iron), and lack of sunlight during winter training (vitamin D), so it’s worth checking.

Supplement Intentionally  

A multivitamin can act as nutritional insurance during periods of heavy training, limited food variety, high stress or energy restriction, but they are not a substitute for proper fueling. Studies show that supplements do not enhance performance in athletes who already meet their nutrient needs.

Using excessive supplementation as a band-aid for poor diet isn’t the solution either, because micronutrients do not scale linearly with performance, and can blunt training adaptations when chronically overconsumed. For best results use a high quality, sensibly-dosed multivitamin like MultiV or MultiV-PRO during high training loads or to address known dietary gaps. 

The Final Takeaway

Carbohydrates fuel the work. Electrolytes keep the system running. Micronutrients ensure the adaptations stick without breaking down your immune and musculoskeletal systems.

Micronutrients won’t win you a race tomorrow, but they might be the reason you can train hard, consistently, for years on end.

Durability is performance

Train hard. Fuel well. Stay topped up.

Key Micronutrient Summary 

*Key notes from this table: 

• All supplement ranges are conservative, low/modest doses intended to fill dietary gaps, not megadoses.

• Athletes should test lab values first before supplementing, particularly iron, vitamin D, and iodine.

• The table balances evidence-based recommendations with practical application for endurance athletes.

References

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Ghazzawi, H. A., Hussain, M. A., Raziq, K. M., Alsendi, K. K., Alaamer, R. O., Jaradat, M., … & Jahrami, H. (2023). Exploring the relationship between micronutrients and athletic performance: A comprehensive scientific systematic review of the literature in sports medicine. Sports, 11(6), 109.

Heffernan, S. M., Horner, K., De Vito, G., & Conway, G. E. (2019). The role of mineral and trace element supplementation in exercise and athletic performance: a systematic review. Nutrients, 11(3), 696.

Higgins, M. R., Izadi, A., & Kaviani, M. (2020). Antioxidants and exercise performance: with a focus on vitamin E and C supplementation. International journal of environmental research and public health, 17(22), 8452.

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Moss, K., Kreutzer, A., Graybeal, A. J., Zhang, Y., Braun-Trocchio, R., Porter, R. R., & Shah, M. (2023). Nutrient adequacy in endurance athletes. International journal of environmental research and public health, 20(8), 5469.

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