The Supplement Stack for Runners

Endurance athletes have a genuinely different nutritional profile than the average person. High training volumes raise iron and energy needs, sweat loss can move micronutrient balances, and repeated impact and energy deficit can stress bone, immune function, and hormones. That can pull some nutrients down faster than ordinary food intake restores them. Here is what the evidence actually supports.

Iron: The Most Important Micronutrient for Runners

Iron deficiency — with or without anemia — is the most common nutritional gap in endurance runners, especially in female athletes. Even iron deficiency without anemia (low ferritin, normal hemoglobin) can impair VO₂ max and running economy. Female runners, vegetarians, and anyone training high mileage should test ferritin and hemoglobin at least annually. Sports-medicine consensus statements now treat ferritin below roughly 30 ng/mL with associated symptoms or low transferrin saturation as warranting evaluation, and many clinicians aim for ferritin above 35 to 50 ng/mL in symptomatic endurance athletes.

Newer evidence on absorption: oral iron transiently raises hepcidin and blunts absorption from a second dose taken within 24 hours. Trials by Stoffel and colleagues in iron-depleted women showed that giving the same total iron as a single morning dose every other day produced higher fractional and cumulative absorption than splitting the same dose across two consecutive days. Practically, this supports an alternate-day morning regimen for many people, with vitamin C on board to improve uptake; calcium-rich foods, tea, and coffee should be separated from the iron dose by an hour or two.

Vitamin D, Calcium, and Bone Stress

Stress fractures are the most feared overuse injury in distance running. Low vitamin D status is common in athletes living at higher latitudes, and observational and prospective studies link low 25-hydroxyvitamin D to higher stress-fracture risk. The IOC consensus statement on dietary supplements suggests targeting serum 25(OH)D in the sufficient range (broadly ≥30 ng/mL or 75 nmol/L) and individualizing supplementation. Doses of 1,000 to 2,000 IU/day of vitamin D3 are usually enough to maintain that level in deficient adults, with higher short-term doses sometimes used to correct frank deficiency under clinical supervision. Adequate dietary calcium (and energy availability overall) protects bone better than calcium pills used in isolation.

Beetroot Juice and Caffeine: The Best-Supported Ergogenic Aids

Among performance aids, two have the strongest evidence base for endurance work. Dietary nitrate from beetroot juice (about 6 to 13 mmol nitrate, roughly 400 to 800 mg, in concentrated shot or juice form) taken 2 to 3 hours before effort improves running economy and time-trial performance by a small but real amount in many trials. The mechanism is improved nitric oxide availability and a small reduction in the oxygen cost of exercise.

Caffeine at 3 to 6 mg per kilogram of body weight, taken about 60 minutes before exercise, is one of the most consistently effective legal ergogenic aids in endurance sport, with typical performance gains of around 2 to 4% in time-trial settings. For a 70 kg runner that is roughly 200 to 420 mg of caffeine, similar to 2 to 4 espressos. People react differently to caffeine; trial it in training before using it in a race.

What to Skip

Branched-chain amino acids (BCAAs) do not consistently outperform adequate total dietary protein for endurance athletes. Glutamine supplements are popular but have not shown reliable immune or recovery benefits in well-nourished runners. HMB (beta-hydroxy-beta-methylbutyrate) has limited support in endurance settings. For most runners eating enough calories and protein, the highest-yield supplement decisions are an annual iron panel, a vitamin D check, and judicious use of caffeine and dietary nitrate around hard sessions.