Fuel 101: Micronutrients for Athletes

Fuel 101: Micronutrients for Athletes

The Details That Derail Training.

When athletes talk about nutrition, the conversation almost always centres on macros. Carbohydrates, protein, fat — these are the headline numbers, and for good reason. But beneath the macro picture, there's a layer of micronutrients that quietly determine how well everything works: how efficiently you produce energy, how well you recover, how strong your bones stay and how robustly your immune system functions.

Most athletes do reasonably well on micronutrients when they're eating enough of a varied diet. The problem is that training increases demand for several key nutrients while calorie restriction or limited dietary variety can reduce supply. The result is that specific deficiencies are surprisingly common in endurance athletes, and several of them have a direct and measurable impact on performance.

Iron: the most common problem in endurance sport

Iron is required for the production of haemoglobin, the protein in red blood cells that carries oxygen, and myoglobin, which stores oxygen in muscle tissue. Without adequate iron, your blood simply can't carry as much oxygen, and aerobic performance suffers accordingly.

Iron deficiency is the most prevalent nutrient deficiency in endurance athletes, particularly in female athletes and runners. Runners face an additional challenge called foot-strike haemolysis, where the repeated impact of running literally destroys red blood cells in the soles of the feet. Distance and frequency of running are directly linked to this effect.

Symptoms of iron deficiency include persistent fatigue, difficulty sustaining training intensity, poor recovery and frequent illness. These are all easy to attribute to overtraining or under-recovery, which is why iron deficiency often goes unidentified for longer than it should.

Good dietary sources include red meat, liver, shellfish, legumes, tofu, fortified cereals and dark leafy greens. Iron from animal sources (haem iron) is absorbed much more readily than iron from plant sources (non-haem iron). Eating vitamin C-rich foods alongside non-haem iron sources improves absorption. Tea, coffee and calcium consumed at the same time reduce it.

If you're a high-volume runner, a female athlete or someone following a plant-based diet, getting your ferritin (stored iron) levels tested periodically is worthwhile.

Vitamin D: more than a bone nutrient

Vitamin D has a reputation as a bone nutrient, largely because its role in calcium absorption and bone mineralisation is well established. But it does considerably more than that. It's involved in immune function, muscle function, hormonal regulation and inflammatory response, all of which are relevant to athletes.

Vitamin D deficiency is extraordinarily common in the UK and northern European countries due to limited sun exposure for much of the year. Estimates suggest that a large proportion of the population has insufficient vitamin D levels during autumn and winter, and athletes training indoors or in northern latitudes for significant portions of the year are particularly susceptible.

Symptoms of vitamin D deficiency are often subtle: increased susceptibility to respiratory infections, unexplained muscle weakness, general fatigue. Stress fracture risk also increases with low vitamin D, which is particularly relevant for runners and triathletes.

Dietary sources of vitamin D are limited. Oily fish, egg yolks and fortified foods contribute something, but sunlight exposure is the primary natural source. Supplementation during autumn and winter months is recommended for most people in the UK regardless of athletic status, with 1,000 to 2,000 IU daily being a common and well-tolerated range.

Calcium: not just for non-athletes

Calcium is the most abundant mineral in the body and the primary structural component of bone. Athletes, particularly female athletes, who are in a state of energy deficiency or hormonal disruption are at increased risk of low bone density over time, even if they're training heavily.

High-impact sports and activities involving repeated loading are generally protective for bone density. But this protection depends on adequate calcium, vitamin D and overall energy availability working together. Any one of those three factors falling short can undermine the others.

Dairy products are the most concentrated dietary sources: milk, yogurt, cheese. Non-dairy sources include fortified plant milks, tofu made with calcium sulphate, almonds, tahini and canned fish with bones. Plant-based athletes need to be intentional about including these regularly.

Magnesium: the mineral that handles the load

Magnesium is involved in over 300 enzymatic reactions in the body, including energy production, protein synthesis, muscle contraction and nerve function. It's also involved in sleep quality, which matters considerably for recovery.

Athletes lose magnesium through sweat, and high-volume training increases demand. Deficiency is relatively common and often goes undetected. Signs include muscle cramps and twitching, poor sleep, fatigue and impaired recovery.

Good sources include wholegrains, nuts, seeds, legumes, dark chocolate and green vegetables. It's one of the easier deficiencies to address through diet, but it requires consistent intake of these foods rather than occasional large amounts.

B12 for plant-based athletes

Vitamin B12 is essential for red blood cell formation, nerve function and DNA synthesis. It's found almost exclusively in animal products, which means plant-based athletes are at genuine risk of deficiency without supplementation or regular consumption of fortified foods.

B12 deficiency develops slowly, as the body has substantial stores that take time to deplete. But when it does develop, the consequences include neurological damage and a form of anaemia that impairs exercise capacity. It's easy to prevent and very difficult to reverse once established.

Plant-based athletes should supplement B12 directly or ensure consistent intake of fortified plant milks, cereals and nutritional yeast. This isn't optional.

The practical takeaway

You don't need to track every micronutrient every day. The foundation is a varied diet with plenty of vegetables, wholegrains, legumes and good protein sources, which covers most of the ground for most athletes most of the time.

The specific areas worth monitoring are iron (especially for female and plant-based athletes), vitamin D (especially in northern climates from October to April), calcium (especially if dairy intake is low) and B12 (essential for plant-based athletes). Periodic blood tests are a more reliable indicator of status than dietary estimates alone.

Previous: Fuel 101: Hydration and Electrolytes | Next: Fuel 101: Timing