This article is written by Amy from The Running Dietitian.

Most runners know the drill. Consume carbohydrates before a run and consume carbohydrates and protein after. But ask those same runners about their iron levels, vitamin D status, or whether they're getting enough magnesium and you'll often get a blank stare.

Micronutrient deficiencies are one of the most common and most underdiagnosed causes of fatigue, recurring illness, and niggling injuries in runners. Sometimes, these feelings are easy to explain away with training load or life stress, but the answer might instead be sitting in a blood test result. This guide covers the key micronutrients that matter most for runners, how to recognise potential shortfalls, and how to build a diet that incorporates them.

Macronutrients (carbohydrates, protein, and fat) provide the fuel and building blocks your body runs on.

Training depletes certain minerals through sweat. The inflammatory response triggered by repeated hard efforts increases the body's need for antioxidant vitamins. Bone and soft tissue are under constant stress, and red blood cell turnover is higher in runners who regularly pound the ground. Eating enough calories is an important step in supporting your body through training, but it does not automatically translate to adequate micronutrient intake, particularly when the diet lacks variety.

Iron is central to oxygen transport in the blood and muscles. Deficiency presents as fatigue and breathlessness that can seem disproportionate to training load.

Best sources: red meat, poultry, fish, and fortified foods.

Vitamin D supports bone health, muscle function, and immunity. Deficiency is widespread in northern climates where sunlight is limited for much of the year.

Best sources: oily fish, egg yolks, and fortified foods — and a winter supplement is often a smart move.

Calcium is the foundation of bone density and stress fracture prevention. It leaves the body through sweat and works most effectively when vitamin D intake is also adequate.

Best sources: dairy, fortified plant milks, canned fish with bones, and leafy greens.

Magnesium supports energy metabolism and muscle relaxation, and is easily lost through sweat — making heavy training a common route to depletion.

Best sources: nuts, seeds, dark chocolate, legumes, and whole grains.

Zinc supports immune function and tissue repair, both of which runners place under strain. It is also lost through sweat.

Best sources: red meat, shellfish, pumpkin seeds, and lentils.

B12 is essential for energy metabolism and red blood cell formation. It is found almost exclusively in animal products, which can put plant-based runners at particular risk of deficiency.

Best sources: meat, fish, eggs, and dairy.

Folate supports cell repair and red blood cell production, and is especially important for female runners and those who are pregnant or planning to conceive.

Best sources: dark leafy greens, legumes, avocado, and fortified cereals.

Many of the symptoms of micronutrient deficiency overlap with overtraining or general fatigue — which is exactly why they can go unrecognised for so long. Below are common symptoms and the micronutrient deficiency they could be linked to.

These are indicators, not diagnoses. Multiple nutrient deficiencies can produce overlapping symptoms, and there are other possible causes for all of them. The only reliable way to know for certain is a blood test by a professional.

A few practical principles make a meaningful difference.

At very high training volumes, calorie restriction, whether intentional or not, reduces the total quantity of food consumed and makes it harder to meet micronutrient targets even with a well-rounded diet.

Supplements serve a purpose, but they work best as targeted solutions to identified shortfalls. Taking high doses of certain micronutrients without knowing your baseline can create problems of its own. Fat-soluble vitamins such as vitamin D accumulate in the body and can reach toxic levels with excessive supplementation.

If you recognise several of the symptoms listed above — or if your training and performance have stalled without an obvious explanation — the most sensible next step is to speak to your GP and request a blood test. A standard panel covering iron and ferritin, vitamin D, B12, and folate will give you a clear picture of where you stand. From there, any supplementation can be targeted, dosed appropriately, and reviewed over time.

Some groups have clearer supplementation needs than others. Vegan and vegetarian runners should consider supplementing with B12. Most people in the UK and northern United States benefit from a vitamin D supplement through autumn and winter. Female runners with heavy periods may need iron support under medical guidance.

Micronutrient deficiencies can be easily overlooked because the symptoms often resemble overtraining or general life fatigue. Runners have higher requirements than the general population due to losses through sweat, increased inflammatory load, and the demands of tissue repair and bone remodelling.

A varied, whole-food diet that meets calorie needs covers most requirements for most runners. Where diet alone may not be sufficient, particularly for B12 in plant-based runners and vitamin D during winter, targeted supplementation is a sensible approach.

If you suspect a deficiency, get tested before supplementing. A blood test and medical professional gives you the information needed to act appropriately.