Minerals: iron, magnesium, zinc and company
Iron with its ferritin thresholds, magnesium and what it actually does about cramps, zinc, calcium, and the three almost nobody checks: iodine, selenium and potassium.
Minerals resemble the fat-soluble vitamins in what matters: they accumulate, they compete with each other for the same intestinal transporters, and they have ceilings. They differ in something that changes practice: here deficiency really is common, and one of them —iron— has one of the largest effects on aerobic performance there is.
None of them is supplemented blind
Iron, zinc and selenium are toxic by accumulation, and they compete: high-dose zinc blocks copper, iron and zinc get in each other’s way on the same transporter, and calcium slows iron absorption. A "just in case" tub of one can create a deficiency of another. Here more than anywhere, the blood test comes first.
Iron: the one that costs the most performance when low
It carries oxygen in haemoglobin and stores it in muscle as myoglobin. When it drops, maximal oxygen uptake drops with it, and the symptoms —disproportionate fatigue, easy paces that are no longer easy, a high heart rate for the same effort— get mistaken for being unfit or overtrained.
Distance runners lose iron through routes other athletes do not have
Sim’s review describes the whole set: foot-strike haemolysis, sweat and urine losses, subclinical gastrointestinal bleeding and, above all, the rise in hepcidin after intense exercise, the hormone that shuts down intestinal absorption. Peeling’s work measured the window: hepcidin peaks between 3 and 6 hours after a hard session, which is exactly the worst moment to take the supplement. Added to menstruation, it explains why low ferritin is so common in female runners.
Serum ferritin
ng/ml (= µg/l)
- Iron deficiency< 15
Stores exhausted, with or without anaemia yet. Needs treatment, not a catalogue tub.
- Low stores15-35
Deficiency without anaemia. Where most female runners with unexplained fatigue sit.
- Target range35-150
Above 35-40 there is no evidence that going higher improves performance.
- High: find the cause150-300
Usually inflammation, not excess iron. Which is why CRP belongs alongside it.
- Suspected overload> 300
Haemochromatosis among other causes. A doctor’s job, and supplementing here does harm.
- When to take it
- In the morning and away from hard sessions
- How often
- Alternate days beat every day
- With what
- With vitamin C; without coffee, tea, calcium or fibre
- Form
- Ferrous sulfate, or bisglycinate if it upsets you
- When to retest
- At 8-12 weeks
Hepcidin peaks 3-6 h after effort. Before training or on waking, not after.
A high dose raises hepcidin for ~24 h and blocks the next one. Alternating absorbs more in total.
50-100 mg of vitamin C is enough. Tea and coffee polyphenols are the worst offenders.
Sulfate is the most studied and the cheapest; bisglycinate is better tolerated at the same elemental iron.
Earlier than that, stores have not had time to move.
Magnesium: a lot of marketing, one concrete effect
For cramps, the evidence does not support it
Garrison’s Cochrane review of randomised trials concludes it is unlikely that magnesium produces a clinically meaningful reduction in muscle cramps in adults, and that in older people it almost certainly does nothing. What Volpe’s review does describe is the mineral’s real role —over 300 enzymatic reactions, neuromuscular function, bone metabolism— and that correcting a low intake does matter. They are two different things: magnesium is necessary, and it is not the treatment for cramps.
- Pumpkin seeds (30 g)~150 mg
- Almonds (30 g)~80 mg
- Cooked spinach (100 g)~79 mg
- Cooked black beans (100 g)~70 mg
- Rolled oats (60 g)~70 mg
- 85% dark chocolate (30 g)~65 mg
- One avocado~58 mg
| Form | Elemental magnesium | Digestive tolerance | When to pick it |
|---|---|---|---|
| Oxide | ~60% | Poor: it is a laxative | Almost never. The cheap-tub form: a lot on the label, little absorbed |
| Citrate | ~16% | Good, mildly laxative | The default choice: cheap and well absorbed |
| Bisglycinate | ~14% | Very good | If citrate loosens you, or if you take it at night |
| Malate | ~15% | Good | A reasonable alternative to citrate, with no demonstrated advantage |
| Threonate | ~8% | Good | Sold on brain claims with preliminary data and a very high price |
Zinc: useful when low, counterproductive when high
Deficiency impairs immune function; so does excess
Wessels’ review describes zinc as a regulator of innate and adaptive immunity, and why deficiency —common on vegetarian diets high in phytates and on low intakes— increases susceptibility to infection. The nuance marketing skips is that the curve is not monotonic: sustained high doses block copper absorption and end up depressing immune function themselves. EFSA puts the upper level at 25 mg/day.
- Reference intake: 9-14 mg/day in adults, higher when the diet is rich in phytates (legumes and wholegrains), which bind it.
- Who is most at risk: vegetarian and vegan diets, low calorie intake, very heavy sweating.
- Zinc lozenges for colds are a different thing: high doses for days, not a background supplement.
- Do not take it with iron: they compete on the same transporter. Separate them by several hours.
Calcium and bone health
It matters more than it seems in impact sports and in low energy availability
Sale and Elliott-Sale’s review of bone health in athletes describes two useful things: that calcium and energy intake —not the isolated supplement— is what sustains bone, and that in very high-sweat sports there is calcium loss in sweat worth covering before the long session, not after. The serious scenario is the other one: in low energy availability and amenorrhoea, bone loss is fast and no supplement compensates while the underlying problem remains (REDs).
- Reference intake
- 950-1,000 mg/day in adults
- Without dairy
- Fortified drinks, calcium-set tofu, almonds, kale, sardines with bones
- With vitamin D
- Without it, calcium is poorly absorbed
- Ceiling
- 2,500 mg/day
A glass of milk or a yoghurt is around 300 mg each.
It is reachable, but you have to aim for it.
This is why they are sold together: here the combination does have a physiological rationale.
And at high doses it slows iron and zinc absorption.
Iodine, selenium, potassium and sodium
| Mineral | What for | Where the real risk is | What to do |
|---|---|---|---|
| Iodine | Thyroid hormones | Diets with no dairy, no fish and non-iodised salt. Zimmermann documents that deficiency is still common in parts of Europe | Use iodised salt at home. It is the public-health solution and costs the same. No supplement needed |
| Selenium | Antioxidant enzymes, thyroid | The window between enough and toxic is narrow: Rayman describes adverse effects from excess, including hair loss and nail changes | Two Brazil nuts cover the day. The supplement is unnecessary and the ceiling is 300 µg/day |
| Potassium | Muscle and nerve function, blood pressure | Low intake is common, but from eating little fruit and vegetables | Not supplemented freely: high doses are dangerous in kidney disease. Fix it with food |
| Sodium | Plasma volume, nerve transmission | The only one worth adding during long exercise, not in the daily diet | Covered in supplements for running |
All of them, at a glance
| Mineral | Reference intake | Ceiling | Blood test? |
|---|---|---|---|
| Iron | 11 mg (m) · 16 mg (f) | Not established; toxic by accumulation | Yes: ferritin + CRP + full blood count |
| Magnesium | 300-350 mg | 250 mg/day from supplements | Of little use: serum magnesium barely reflects stores |
| Zinc | 9-14 mg | 25 mg/day | Unreliable in plasma; assessed from the diet |
| Calcium | 950-1,000 mg | 2,500 mg/day | Not routinely; yes, a DXA scan after stress fractures |
| Iodine | 150 µg | 600 µg/day | Only on thyroid suspicion |
| Selenium | 70 µg | 300 µg/day | Not routinely |
Takeaways
- Iron has the biggest performance impact, and is the most badly measured: ferritin always with CRP.
- Take it in the morning, on alternate days and away from hard sessions: hepcidin peaks 3-6 h after.
- Magnesium is necessary, but it is not the treatment for cramps.
- Read the elemental percentage: 1,000 mg of citrate is ~160 mg of magnesium.
- Zinc above 25 mg/day blocks copper and stops helping immunity.
- Iodine is fixed with iodised salt and selenium with two Brazil nuts: neither needs a tub.
Carry on here
The vitamins on the other side of the same problem, and the page that turns it into a decision.
Sources for this chapter
- [1]Sim M, Garvican-Lewis LA, Cox GR, et al. (2019). Iron considerations for the athlete: a narrative review. European Journal of Applied Physiology. review
- [2]Peeling P, Sim M, Badenhorst CE, et al. (2014). Iron status and the acute post-exercise hepcidin response in athletes. PLoS ONE. trial
- [3]Clénin G, Cordes M, Huber A, et al. (2015). Iron deficiency in sports — definition, influence on performance and therapy. Swiss Medical Weekly. position stand
- [4]Garrison SR, Korownyk CS, Kolber MR, et al. (2020). Magnesium for skeletal muscle cramps. Cochrane Database of Systematic Reviews. meta-analysis
- [5]Volpe SL (2013). Magnesium in disease prevention and overall health. Advances in Nutrition. review
- [6]Wessels I, Maywald M, Rink L (2017). Zinc as a gatekeeper of immune function. Nutrients. review
- [7]EFSA Scientific Committee on Food / NDA Panel (2006). Tolerable upper intake levels for vitamins and minerals. European Food Safety Authority. official guideline
- [8]Sale C, Elliott-Sale KJ (2019). Nutrition and athlete bone health. Sports Medicine. review
- [9]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
- [10]Zimmermann MB (2009). Iodine deficiency. Endocrine Reviews. review
- [11]Rayman MP (2012). Selenium and human health. The Lancet. review
- [12]EFSA Panel on Dietetic Products, Nutrition and Allergies (2017). Dietary Reference Values for nutrients: summary report. EFSA Supporting Publications. official guideline