Vitamins, minerals and health
Vitamin D, iron, omega-3, magnesium and company: where there are real deficiencies worth fixing, where a multivitamin adds nothing, and why high-dose antioxidants can subtract.
Micronutrients behave the opposite way to ergogenic aids: fixing a deficiency produces large improvements, and supplementing without one produces none. The useful question is not "would vitamin D be good for me?" but "am I low?".
Supplementing without a deficiency does not improve performance
Larson-Meyer’s review of nutrient status assessment in athletes sums it up: micronutrients are necessary for performance, and adding them above requirement does not improve it. The joint ACSM and Academy of Nutrition and Dietetics position stand is equally explicit: a multivitamin is not routinely indicated in athletes who eat enough and vary their diet.
The condition almost nobody checks first
The commonest cause of multiple deficiencies in people who train is not bad luck: it is not eating enough. In an aggressive, sustained calorie deficit, every micronutrient drops at once. If that is you, the fix is eating more (at what rate), not a tub of multivitamin.
Vitamin D
Deficiency is common and is worth correcting
Farrokhyar’s meta-analysis found inadequate vitamin D levels in around 56% of the athletes studied, with higher prevalence in indoor sports, high latitudes and winter. Owens and Close’s review describes its role in muscle, bone and immune function. Correcting a real deficiency improves strength and bone health; supplementing someone who is already sufficient has no demonstrated benefit.
- How to know
- A 25-OH vitamin D blood test
- Usual maintenance dose
- 1,000-2,000 IU/day
- Who is most at risk
- Winter, darker skin, indoor sport, high latitude
- What to take it with
- A meal containing fat
It is cheap and it is the only way. Symptoms do not diagnose it.
Correcting an established deficiency needs a medical protocol, not a catalogue dose.
In northern Europe, almost everyone between October and March.
It is fat-soluble: absorption is worse on an empty stomach.
Iron
It is the deficiency with the most direct impact on aerobic performance, and the one most concentrated in one specific group: women who run.
Distance runners lose iron through routes other athletes do not have
Sim’s review lays out the set: foot-strike haemolysis, losses through sweat and urine, subclinical gastrointestinal bleeding, and — the main route — the rise in hepcidin after intense exercise, which blocks intestinal absorption for hours. Added to menstruation, it explains why low ferritin is so common in female runners. The effect of correcting it, when the deficiency is real, is large.
- Do not supplement blind. Excess iron is toxic and accumulates. You need blood work with ferritin, not just haemoglobin.
- When to take it: in the morning and away from hard sessions, because hepcidin stays elevated for hours after exercise.
- With what: vitamin C improves absorption; coffee, tea, calcium and fibre worsen it.
- Signs worth checking: disproportionate fatigue, paces that used to be easy now feeling hard, high heart rate at easy paces (when the problem is not the plan).
Omega-3
EPA and DHA have well-established cardiovascular and anti-inflammatory effects at population level. In sports performance, the evidence is more modest than the price suggests.
Small effect on recovery and the anabolic response; solid on general health
Philpott’s review describes signals in reduced muscle damage and delayed soreness, and in sensitising the response to protein, with inconsistent results between studies. What is not in dispute is the other part: if you do not eat oily fish twice a week, an EPA+DHA supplement is a sensible purchase for health, not for performance.
- Dose
- 1-2 g/day of EPA + DHA combined
- Vegan
- Algae oil
- Quality
- Declared heavy metal and oxidation testing
Look at the sum of EPA and DHA, not the milligrams of "fish oil".
It is the original source: fish get it from there.
A rancid oil smells and tastes rancid; send it back.
Magnesium, zinc and the rest
| Nutrient | What it is credited with | What the evidence supports |
|---|---|---|
| Magnesium | Cramps, sleep, recovery | Correcting a deficiency helps; in people with normal levels the effect on cramps is not demonstrated. Bisglycinate or citrate are better tolerated than oxide |
| Zinc | Testosterone, immunity | Only relevant if deficient. In excess it interferes with copper absorption |
| Calcium | Bone health | Important, and better from food. A priority in amenorrhoea and low energy availability |
| Vitamin C | Colds, immunity | The Cochrane review finds no prevention in the general population; it does find a small reduction in duration, and somewhat more in people under extreme physical stress |
| Multivitamin | Filling gaps | A cheap insurance policy on restrictive or very low-calorie diets. No effect on performance in people who eat well |
| Probiotics | Digestion, immunity | The ISSN position stand describes strain-specific benefits for gut and respiratory health. The specific strain matters more than the brand |
Antioxidants: the case where supplementing subtracts
High doses of vitamin C and E can blunt training adaptations
Paulsen’s double-blind controlled trial found that 1,000 mg of vitamin C and 235 mg of vitamin E a day attenuated cellular markers of adaptation to endurance training. Merry and Ristow review the whole picture and reach a coherent conclusion: the oxidative stress of exercise is part of the signal that drives adaptation, and switching it off with pharmacological doses can switch off the response too. This does not apply to antioxidants from food.
The reasonable exception
In a race week, where recovering fast is worth more than adapting, it makes sense to use anti-inflammatory tools — tart cherry, for instance — and drop them when the training block resumes. The general logic: in a building phase, do not interfere with the signal.
Sleep
It is the biggest recovery lever and the one that needs the fewest supplements. Halson’s review of sleep in elite athletes summarises the nutritional interventions with some backing, and all of them are modest next to going to bed earlier.
- Melatonin: useful mainly for shifting the clock — jet lag, shift work — not as a sleeping pill. Low doses (0.5-3 mg) work as well as or better than high ones.
- Magnesium: if you are deficient, it helps; if not, little.
- Ashwagandha: Bonilla’s Bayesian meta-analysis finds small effects on performance and stress markers, from heterogeneous studies of variable quality.
- Caffeine: the supplement that most damages sleep. Watch the hour, not just the dose (half-life).
Takeaways
- Micronutrients only improve performance when they correct a real deficiency.
- Vitamin D and iron are the two worth a blood test, especially in female runners.
- Omega-3 is a good buy for health and a modest one for performance.
- High doses of vitamin C and E can attenuate adaptations: do not take them in building blocks.
- A multivitamin is insurance on a restrictive diet, not an ergogenic aid.
Sources for this chapter
- [1]Larson-Meyer DE, Woolf K, Burke L (2018). Assessment of Nutrient Status in Athletes and the Need for Supplementation. International Journal of Sport Nutrition and Exercise Metabolism. review
- [2]Thomas DT, Erdman KA, Burke LM (2016). Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise. position stand
- [3]Farrokhyar F, Tabasinejad R, Dao D, et al. (2015). Prevalence of vitamin D inadequacy in athletes: a systematic review and meta-analysis. Sports Medicine. meta-analysis
- [4]Owens DJ, Allison R, Close GL (2018). Vitamin D and the Athlete: Current Perspectives and New Challenges. Sports Medicine. review
- [5]Sim M, Garvican-Lewis LA, Cox GR, et al. (2019). Iron considerations for the athlete: a narrative review. European Journal of Applied Physiology. review
- [6]Philpott JD, Witard OC, Galloway SDR (2019). Applications of omega-3 polyunsaturated fatty acid supplementation for sport performance. Research in Sports Medicine. review
- [7]Hemilä H, Chalker E (2013). Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews. meta-analysis
- [8]Jäger R, Mohr AE, Carpenter KC, et al. (2019). International Society of Sports Nutrition Position Stand: Probiotics. Journal of the International Society of Sports Nutrition. position stand
- [9]Paulsen G, Cumming KT, Holden G, et al. (2014). Vitamin C and E supplementation hampers cellular adaptation to endurance training in humans: a double-blind, randomised, controlled trial. Journal of Physiology. trial
- [10]Merry TL, Ristow M (2016). Do antioxidant supplements interfere with skeletal muscle adaptation to exercise training?. Journal of Physiology. review
- [11]Halson SL (2014). Sleep in Elite Athletes and Nutritional Interventions to Enhance Sleep. Sports Medicine. review
- [12]Bonilla DA, Moreno Y, Gho C, Petro JL, Odriozola-Martínez A, Kreider RB (2021). Effects of Ashwagandha (Withania somnifera) on Physical Performance: Systematic Review and Bayesian Meta-Analysis. Journal of Functional Morphology and Kinesiology. meta-analysis
- [13]Mountjoy M, Ackerman KE, Bailey DM, et al. (2023). 2023 International Olympic Committee’s (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine. position stand