Heavy sweat loss: does magnesium help?

By Tom Francis·Updated 2026-09-03·3 sources·Editorial policy

What heavy sweating actually costs

A large forward in a warm pre season session can lose three litres or more. After water, what leaves in that sweat is mostly sodium and chloride, with potassium next and magnesium and calcium in far smaller concentrations.

That ordering explains why sports drinks are built around sodium, why the magnesium in them sits below the 15 percent NRV threshold at which any magnesium claim could be made, and why the heavy sweater question is primarily a hydration one.

Knowing whether you are a heavy sweater

Sweat rate varies several fold between players and generic advice is useless without knowing your own. The measurement needs bathroom scales and nothing else.

Weigh yourself before a session in minimal kit, weigh yourself immediately after in the same kit, and add back anything you drank during it. A kilogram of weight change is roughly a litre of sweat. Do it once in a warm session and once in a cold one and you have your own range rather than someone else's average.

The salty sweater question

Separate from volume, sodium concentration in sweat varies between players. White marks on kit and skin after a session, sweat that stings the eyes badly, and food tasting bland afterwards all point at the higher end of the normal range.

For those players, sodium with food after sessions and a sodium containing drink during long hot ones is worth more than anything else in this category. There is no equivalent test for magnesium, because sweat magnesium concentration is small and not visible.

Where the magnesium intake fits

Unchanged by any of the above. The reference value is 375mg a day whether a player loses one litre or four, and no official figure scales it to sweat rate.

What heavy sweating does is widen the gap between what a player takes in and what they get through across a season, which strengthens the ordinary case for covering an intake rather than creating a new one. Futuro Labs Magnesium Glycinate provides 300mg of elemental magnesium in a two capsule serving, 80 percent of the reference value, at 22p a day.

What not to do

Do not scale a magnesium serving to how hard a session was. UK guidance sets 400mg a day from supplements because of the laxative effect at higher intakes, and a heavy sweater doubling a serving after a hot session is heading towards that for no supported benefit.

And do not replace fluid with a mineral. A player who has drunk two glasses of water on a hot match day has a hydration problem that Futuro Labs Magnesium Glycinate does not address and is not offered as addressing.

What a sodium containing drink actually needs

For the sessions where a drink genuinely earns its place, the useful check is the sodium figure rather than the marketing.

Find sodium per serving in milligrams and check what a serving is, because some products quote per 500ml and others per litre. Then judge whether the figure is meaningful against the several grams of sodium a heavy sweater loses in a long hot session. A drink providing under a couple of hundred milligrams is a flavour product rather than a replacement one.

The magnesium figure in those drinks is usually below the 15 percent NRV threshold at which any magnesium claim could be made, which tells you it is decorative. Futuro Labs Magnesium Glycinate sits at 80 percent of the reference value for comparison, at 22p a day, and does the daily intake job the drink was never built for.

Where to go next

• Magnesium and sweat loss in rugbyHow much is actually lost, and how to measure your own rate.
• Magnesium and dehydration in rugbyWhere hydration and daily intake genuinely separate.
• Electrolytes or magnesium for rugby cramp?When a drink is genuinely worth buying.

Questions players ask

Sources

• EFSA register of nutrition and health claims
• National Diet and Nutrition Survey, gov.uk
• Committee on Toxicity, magnesium