Magnesium and Swimming: What Pool and Open-Water Training Changes

01Why the "swimming doesn't make you sweat" idea is only partly true

Water conducts heat away from the body roughly 25 times faster than air at the same temperature, so core temperature stays lower during pool swimming than during equivalent land exercise. That does reduce visible sweat. What it does not do is eliminate sweating entirely: the eccrine glands remain active, especially during high-intensity effort, and in warm pools the effect is even less pronounced.

Open-water swimmers in cool conditions often sweat less than pool swimmers, but session length tends to be longer, and cumulative mineral loss from hours in the water still adds up. The relevant question is not whether you are dripping — it is what the total daily loss looks like across training, rest, and diet.

02What magnesium actually does in working muscle

Magnesium participates in more than 300 enzyme-dependent processes, but the ones most relevant to a swimmer are fairly specific. It is required for the regeneration of ATP, the molecule muscle contracts on. It competes with calcium at the level of muscle fibers, acting as the counter-signal that allows a contracted muscle to relax. And it supports the sodium-potassium pump that resets excitable membranes after each nerve or muscle firing.

None of this means that taking a supplement will make your muscles work better if your intake is already adequate. The mechanism is real; the performance claim built on top of it is not the same thing. As we cover in more detail in Magnesium and Exercise: What Actually Changes With Training, the evidence for supplementation improving performance is strongest in people who were genuinely short of the mineral to begin with.

03Where swimming differs from running or cycling in terms of magnesium loss

Running generates mechanical stress on the foot that accelerates red blood cell turnover — a factor relevant to iron rather than magnesium. Cycling over several hours produces sweat volumes that are well documented and measurable. Swimming sits somewhere in between: session sweat rates are lower than for cycling at comparable intensities, but magnesium losses via urine appear to rise with aerobic exercise intensity regardless of the mode.

One factor that distinguishes open-water swimmers is the mental and thermoregulatory stress of cold-water immersion. Stress hormones — cortisol and the catecholamines — can transiently shift magnesium from intracellular to extracellular compartments, which may register as a drop in serum magnesium even when total body magnesium is unchanged. Serum magnesium is a poor proxy for tissue stores, a point worth keeping in mind if you have ever had a blood test that came back "normal" while still wondering if something is off. An unexplained blood test result deserves a doctor rather than a web page.

04What the evidence does not show

The swimming-specific magnesium literature is sparse. Most studies that inform supplementation recommendations were conducted in endurance runners, cyclists, or general athletic populations. Extrapolating from those populations to a daily pool swimmer is reasonable as a rough guide, but it is not the same as direct evidence. We do not know with confidence whether the magnitude of loss differs meaningfully between pool and open-water training, nor how well swimmers' dietary habits typically compensate for exercise-related losses.

What some evidence does suggest — based on the broader exercise literature — is that habitually active individuals have somewhat higher magnesium requirements than sedentary people. How much higher, and whether food can meet that increase without a supplement, is not settled for most people at most training volumes.

05Who is most likely to have a genuinely low intake

Dietary surveys consistently show that a large proportion of adults in the US and UK do not reach the recommended dietary intake for magnesium, independent of exercise. The gap is not dramatic for most people, but it is real. Foods that supply meaningful amounts — leafy greens, legumes, whole grains, nuts and seeds — are often underrepresented in diets built around convenience foods.

A swimmer who trains twice a day and eats a processed-food-heavy diet is in a different position from one who trains three times a week and eats varied whole foods. The supplement question should follow an honest look at diet first. If green vegetables, beans, and whole grains are regular fixtures in your meals, you may well be covering your needs without any supplement at all.

06The form question, and where a cheaper option likely wins

We sell magnesium, and the honest answer is that the form matters mainly when you have a specific goal. Magnesium glycinate is well absorbed and gentle on the gut — a reasonable choice for someone who wants to add supplemental magnesium without laxative effects. Our Magnesium Glycinate uses that form for exactly those reasons.

For someone whose only goal is to close a dietary gap at the lowest cost, basic magnesium citrate from any supermarket is adequately absorbed and substantially cheaper. We discuss that comparison in Magnesium Citrate: Supplement or Laxative? It Depends Entirely on Dose. Magnesium oxide, the cheapest of all, has poor fractional absorption and is better suited to digestive use than to mineral repletion — see Magnesium Oxide: Why the Cheapest Form Has the Biggest Number for why.

07Dose, timing, and the 350 mg supplemental ceiling

The tolerable upper intake level for supplemental magnesium is 350 mg per day in adults. This refers only to supplemental magnesium — food magnesium is not counted against this figure and does not carry the same risk of excess. The concern above 350 mg from supplements is primarily osmotic diarrhea, not toxicity in healthy individuals; the body limits intestinal absorption and excretes excess renally. But renal excretion is the key phrase: anyone with reduced kidney function operates under different rules, and should speak with a doctor before supplementing.

Timing for a swimmer is a practical rather than a pharmacological question. Magnesium does not need to be taken post-workout. If you find that taking it in the evening supports your sleep quality, that is a reasonable slot. If it sits better with food in the morning, that works too. The form matters more than the hour, as we explain in The Best Time to Take Magnesium.

08Safety notes: two things swimmers specifically should know

First, magnesium can bind to certain antibiotics — tetracyclines and fluoroquinolones — reducing their absorption if taken within a few hours of each other. If you are prescribed either class, separate the timing and check with your pharmacist. Second, athletes who train heavily sometimes take anti-inflammatory medications for joint or overuse pain. There are no well-documented interactions between standard magnesium supplements and over-the-counter NSAIDs, but the combination of heavy training, NSAID use, and higher supplement doses is worth monitoring, since NSAIDs can affect kidney function over time and renal health changes how the body handles supplemental magnesium.

The broader safety context for magnesium supplementation is covered in Can You Take Too Much Magnesium? and in our full Supplement Upper Intake Limits reference.

09The practical summary for swimmers

If you swim recreationally two to three times a week and eat a varied diet with regular whole foods, you probably do not need a magnesium supplement. The case becomes more credible if you train daily at high intensity, sweat visibly even in the water, notice disrupted sleep or persistent muscle tightness, and honestly review your diet and find it magnesium-sparse. In that scenario, 100–200 mg of elemental magnesium from a well-absorbed form is a reasonable, low-risk addition. Start lower rather than higher, and stay within the 350 mg supplemental ceiling.

If you are also interested in how electrolyte losses stack up more broadly during training, Magnesium and Sweating: How Much Do You Actually Lose? goes into the numbers in more detail, as does Electrolyte Powders: It's Mostly Sodium, and That's the Point for the wider mineral picture.

10Frequently asked questions

does swimming deplete magnesium

Swimming can contribute to magnesium loss through sweat and increased urinary excretion during exercise, but the magnitude is generally lower than in high-sweat land sports. Whether it produces a meaningful deficit depends on training volume and dietary intake, not on swimming alone.

should swimmers take magnesium supplements

Most recreational swimmers do not need to. High-volume swimmers who train daily and have a low dietary magnesium intake are the more plausible candidates. Reviewing your diet before buying a supplement is a better first step than buying and then reviewing.

what is the best form of magnesium for athletes

Magnesium glycinate and magnesium malate are both well absorbed and unlikely to cause loose stools at standard doses, which makes them practical for athletes. Magnesium citrate is also adequately absorbed and costs less — a reasonable choice if budget matters more than dose precision.

how much magnesium should a swimmer take per day

The supplemental upper intake level is 350 mg of elemental magnesium per day. Most people who supplement at all do so in the 100–300 mg range from supplements, on top of dietary intake. There is no evidence that pushing to the ceiling produces better results than a moderate dose in people who are not severely deficient.

can you get enough magnesium from food without supplements

Yes, if you eat a varied diet that regularly includes leafy greens, legumes, whole grains, nuts, and seeds. Many people in practice do not reach recommended intakes from food, but the solution can be dietary adjustment before it becomes a supplement question.

does magnesium help with muscle cramps in swimmers

Some evidence suggests magnesium supplementation supports normal muscle function in people with low intake. Whether it resolves cramps in people who are already replete is less well established. The evidence for exercise-associated cramping specifically is mixed, and other factors — hydration, sodium, training load — are often more relevant than magnesium.

is magnesium safe to take every day

In healthy adults with normal kidney function, daily magnesium supplementation at doses at or below 350 mg of elemental magnesium is considered safe. Anyone with kidney concerns should speak to a doctor before taking supplemental magnesium, as impaired renal function changes the risk profile substantially.

when should I take magnesium if I swim in the morning

There is no evidence that timing magnesium around a morning swim session matters for mineral repletion. Taking it with a meal — whenever that fits your day — reduces the chance of any stomach discomfort and is as effective as any other timing strategy.

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Anyone with kidney concerns should speak to a doctor before taking supplemental magnesium. Persistent sleep difficulty deserves a doctor rather than a supplement.