The supplement industry sells the idea that more is always better when it comes to vitamins and minerals. The reality is considerably more nuanced. Most athletes eating a varied, whole-food diet get adequate amounts of most micronutrients. But several specific nutrients are genuinely difficult to obtain in sufficient quantities through diet alone — particularly for people training at high volumes. Here are the six that actually move the needle.
Key Takeaways
- Vitamin D deficiency affects over 40% of adults globally and directly impairs muscle function
- Iron deficiency is the most common nutritional deficiency worldwide — especially in female athletes
- Magnesium is involved in over 300 enzymatic reactions and is depleted by exercise and stress
- Get blood tests before supplementing — guessing wastes money and can cause harm
- Whole food sources are always preferable to supplements when achievable
1. Vitamin D
Vitamin D is technically a hormone precursor, and virtually every cell in your body has receptors for it. Its roles relevant to athletes include: muscle protein synthesis regulation, bone mineral density maintenance, immune function, and testosterone production. Research has consistently linked vitamin D sufficiency with improved muscle strength, faster recovery, and reduced injury rates.
The problem: most people in northern latitudes are deficient for significant parts of the year due to limited sun exposure. Athletes who train indoors are at particular risk. The recommended supplemental dose for most adults is 1,500–2,000 IU of vitamin D3 daily, ideally taken with a fat-containing meal for optimal absorption. Get your 25-OH vitamin D blood level tested to establish your starting point.
2. Magnesium
Magnesium is involved in over 300 biochemical reactions, including ATP synthesis (the energy currency of exercise), protein synthesis, and muscle relaxation. Exercise increases magnesium excretion through sweat and urine — meaning athletes have higher requirements than sedentary individuals. Deficiency is associated with muscle cramps, poor sleep, increased fatigue, and impaired recovery.
Dietary sources include dark leafy greens, nuts (especially almonds and cashews), seeds, whole grains, and dark chocolate. Many people struggle to hit the RDA of 310–420mg through diet alone. Magnesium glycinate or malate are the best-tolerated supplemental forms (avoid magnesium oxide — it's poorly absorbed and causes digestive issues). 200–400mg before bed also has a mild sleep-improving effect.
3. Iron
Iron is essential for haemoglobin production — the protein in red blood cells that carries oxygen. Iron deficiency anaemia causes fatigue, reduced aerobic capacity, poor concentration, and impaired immune function. Female athletes who are menstruating are at substantially higher risk. Endurance athletes are also at risk due to "foot-strike haemolysis" — the destruction of red blood cells through repeated impact.
Sources: red meat, organ meat, shellfish (highest bioavailability), and plant sources including lentils, tofu, and fortified cereals (lower bioavailability — enhance absorption by consuming with vitamin C and avoiding concurrent coffee or tea). Do not supplement iron without a confirmed deficiency via blood test — excess iron is harmful.
4. Omega-3 Fatty Acids (EPA and DHA)
EPA and DHA — the active omega-3 fatty acids found in fatty fish and fish oil — reduce inflammation, support joint health, and have emerging evidence for supporting muscle protein synthesis and reducing exercise-induced muscle damage. Most Westerners consume a dramatically omega-6 dominant diet, creating a pro-inflammatory environment.
If you eat fatty fish (salmon, sardines, mackerel) 2–3 times per week, you likely meet your needs. If not, 2–3g of combined EPA+DHA from fish oil or algae-based supplements (for vegetarians/vegans) daily is a reasonable target.
5. Zinc
Zinc is involved in testosterone synthesis, immune function, wound healing, and protein metabolism. Athletes — particularly those in high-sweat sports — lose significant zinc through perspiration. Vegetarians and vegans are also at risk because plant-based zinc has lower bioavailability than animal-based zinc.
Good sources: oysters (by far the richest source), red meat, poultry, pumpkin seeds, and legumes. Supplemental dose if needed: 15–30mg elemental zinc per day. Note that high-dose zinc supplementation (50mg+) can impair copper absorption — another reason to test before supplementing.
6. Vitamin B12
B12 is essential for red blood cell formation, neurological function, and DNA synthesis. It is found almost exclusively in animal products, making vegans and strict vegetarians highly vulnerable to deficiency. Symptoms of deficiency include fatigue, weakness, neurological symptoms, and megaloblastic anaemia.
If you eat animal products regularly, B12 deficiency is unlikely. If you follow a plant-based diet, supplementation is non-optional — 250–500mcg of cyanocobalamin or methylcobalamin daily, or 2,000mcg weekly. This is one of the clearest cases in nutrition where supplementation is simply required.
Final Thoughts
The return on investment from micronutrient optimisation is genuine but often overstated. Address deficiencies — particularly vitamin D, magnesium, and iron if applicable — and eat a varied whole-food diet. Beyond that, additional supplementation provides diminishing returns. Test, don't guess.