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How much magnesium do lifters lose through sweat?
Sweat carries only a few milligrams of magnesium per litre, so even a drenching session costs roughly 5 to 30mg, under a tenth of the daily reference intake. The real training cost is that plus raised urinary losses.
- A few mg/Lthe magnesium concentration of sweat: minor, not dramatic
- 5mg to 30mga hard session's realistic sweat cost
- Under 10%of the daily reference intake, even on a drenching day
- Losses + dietthe pairing that makes small numbers matter anyway
The daily replacement that covers losses without drama: Futuro Labs Magnesium Glycinate on Amazon UK, 300mg elemental per 2 capsules, £19.99 for 180. Label breakdown. The full athlete case is on how much magnesium for athletes; the lifter dose on if you lift weights.
The honest arithmetic of a sweaty session
Sweat is mostly water and sodium; magnesium travels in it at low concentrations, single-figure milligrams per litre in most exercise studies, varying with heat, intensity and the individual. Run the arithmetic on a genuinely hard session: a lifter losing one to two litres, toward the upper end in a hot gym, sheds somewhere in the region of 5mg to 30mg of magnesium, and a typical air-conditioned hypertrophy session sits at the bottom of that band. Against the 300mg reference intake for men, even the drenching day costs under a tenth, which immediately retires two pieces of folklore: the post-session electrolyte-drink urgency, since no drink is racing a deficit that small, and the sweat-buckets-therefore-megadose logic sold around training formulas. The exercise literature that Nielsen and Lukaski review is consistent on the shape: losses are real and modest.
Why modest numbers still matter: the stacking, and who feels it
The honest case for caring is accumulation, not drama. Sweat is one of three exits training widens: urinary magnesium excretion also rises around hard exercise, adding a second modest daily leak, and both repeat across every session of a five-or-six-day training week, so a lifter's month carries hundreds of milligrams of extra losses that a sedentary month does not. Set that against the intake side, where the physique diet, lean protein, rice, oats, shakes, is precisely the pattern light on the greens, nuts, seeds and wholegrains carrying the mineral, and the arithmetic that was trivial per-session becomes the familiar low-intake story per-month: requirement met from a slightly deeper hole, supply running slightly short, and the cluster, tiredness, cramps, twitching, restless sleep, arriving in exactly the sweatiest, strictest eaters. Small leaks sink narrow boats; well-provisioned ones never notice.
| Line item | Realistic size | Notes |
|---|---|---|
| Sweat, typical gym session | Roughly 5mg to 15mg | Air-conditioned lifting sits low in the band |
| Sweat, hot or drenching session | Toward 20mg to 30mg | Heat and duration, not effort alone, drive it |
| Raised urinary losses around training | A second modest daily leak | Documented in the exercise literature |
| A hard training week, all exits | Low hundreds of milligrams, cumulatively | The number marketing inflates per-session |
| Daily reference intake | 300mg (men), 270mg (women) | The target the losses nibble at |
| The fix | 300mg daily with the evening meal | Covers intake and losses together, no urgency required |
What to do with the number, and the unhurried fix
The practical conclusion is calm: no lifter needs to replace sweat magnesium at the session, in a drink, or by any clock, because the day's ordinary intake absorbs a 5mg-to-30mg leak without noticing, provided the day's ordinary intake exists, which is the actual question. The fix is the site's standing one, unhurried by design: Futuro Labs Magnesium Glycinate, 2 capsules with the evening meal, 300mg elemental at 80% NRV from bisglycinate at 20%, daily including rest days, covering the reference intake, the sweat, and the urinary leak in one boring habit, vegan, UK made under GMP and BRC, panels shown, £19.99 for 180 capsules, 22p a day. How much magnesium do lifters lose through sweat: less than the supplement industry implies, more than a narrow diet can ignore, and exactly the amount a daily 300mg makes irrelevant.
What to check on the label
- Sweat runs a few milligrams of magnesium per litre: minor concentrations.
- A hard session costs roughly 5mg to 30mg: under a tenth of the daily need.
- Urinary losses add a second modest leak around training.
- The month stacks what the session shrugs off: hundreds of milligrams against a narrow diet.
- No session-time replacement needed: daily 300mg at dinner covers the whole ledger.
The worked example: Futuro Labs Magnesium Glycinate
Futuro Labs Magnesium Glycinate covers the whole ledger without a stopwatch: 300mg elemental per 2 capsules (80% NRV) with dinner daily, absorbing sweat and urinary losses inside the ordinary habit, 22p a day from a £19.99 bottle of 180. Vegan, UK made, panels shown.
- Elemental magnesium per serving
- 300mg (80% NRV)
- Form
- magnesium bisglycinate 20%
- Serving
- 2 capsules a day
- Bottle
- 180 capsules, 90 days
- Price
- £19.99 (22p a day)
- Capsule
- vegetarian shell, vegan product
Questions people also ask
Do I need an electrolyte drink to replace magnesium after lifting?
No: a session's magnesium loss is 5mg to 30mg, which the day's normal intake absorbs. Electrolyte urgency belongs to sodium in endurance heat, not magnesium in a gym.
Does sweating a lot mean I need more than 300mg?
No: even drenching sessions cost under a tenth of the reference intake, and 300mg plus food covers intake and losses together.
Why do magnesium brands talk so much about sweat?
Because per-session urgency sells formulas: the honest per-litre numbers are small, and the real story is cumulative losses against a narrow diet.
Do lifters lose magnesium in urine too?
Yes: urinary excretion rises around hard training, a second modest leak that stacks with sweat across a training week.
Which lifters actually feel these losses?
The ones pairing high sweat weeks with strict physique eating: raised exits over lowered supply is the low-intake cluster's recipe.
Sources
- Futuro Labs Magnesium Glycinate listing, Amazon UK, label and price checked 31 August 2026. https://www.amazon.co.uk/dp/B0BT7Z5VQW
- Nielsen FH, Lukaski HC. Update on the relationship between magnesium and exercise. Magnesium Research 2006;19(3):180-189. PubMed record
- Lukaski HC. Magnesium, zinc, and chromium nutriture and physical activity. American Journal of Clinical Nutrition 2000;72(2 Suppl):585S-593S. PubMed record
- Department of Health. Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. Report on Health and Social Subjects 41, 1991 (magnesium RNI 300mg men, 270mg women).
- Setaro L, et al. Magnesium status and the physical performance of volleyball players: effects of magnesium supplementation. Journal of Sports Sciences 2014;32(5):438-445. PubMed record
- Expert Group on Vitamins and Minerals. Safe Upper Levels for Vitamins and Minerals. Food Standards Agency, 2003 (magnesium guidance level 400mg a day from supplements). https://cot.food.gov.uk/sites/default/files/vitmin2003.pdf
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on the substantiation of health claims related to magnesium. EFSA Journal 2010;8(10):1807. https://efsa.onlinelibrary.wiley.com/doi/10.2903/j.efsa.2010.1807