Sodium, Potassium, and Magnesium for Endurance Athletes

Endurance training raises sweat losses and metabolic demand, which puts extra pressure on three key minerals: sodium, potassium, and magnesium. Understanding what the evidence actually supports, and where the gaps are, helps athletes make informed choices rather than guesses.

Sodium

What it does

Sodium maintains the electrochemical gradient across cell membranes through the sodium-potassium (Na+/K+) ATPase pump, a process the NIH Office of Dietary Supplements (ODS) estimates accounts for 20 to 40 percent of resting energy expenditure. It also controls extracellular fluid volume and supports normal nerve impulse transmission and muscle contraction.

Daily target and safety

The NIH ODS Adequate Intake (AI) for adults is 1,500 mg per day. There is no traditional Tolerable Upper Intake Level. Instead, the 2019 National Academies (NASEM) Dietary Reference Intake report established a Chronic Disease Risk Reduction Intake (CDRR) of 2,300 mg per day, meaning adults consuming above that level are advised to reduce intake to lower long-term cardiovascular risk. Deficiency from low dietary intake alone is rare in healthy adults.

What the research shows

The evidence that sodium is essential for membrane potential, nerve transmission, muscle contraction, and fluid balance is graded Strong by the ODS. The evidence that higher sodium intake raises blood pressure, and that reducing it lowers blood pressure in people with elevated readings, is also Strong. Evidence that sodium supplementation benefits athletes beyond simply replacing sweat losses during prolonged intense exercise is graded Insufficient, meaning well-controlled trials are lacking.

Drug interactions to know

Loop and thiazide diuretics increase urinary sodium loss and can contribute to low sodium levels. Low sodium intake or heavy sodium loss reduces lithium clearance, potentially raising lithium blood levels toward toxicity. Corticosteroids and mineralocorticoid-active drugs promote sodium and fluid retention. High sodium intake can blunt the blood-pressure-lowering effects of NSAIDs, ACE inhibitors, and ARBs. Talk to a doctor or pharmacist before combining sodium supplements with any of these medications.

Potassium

What it does

Potassium maintains intracellular fluid volume and normal cell tonicity, working in balance with sodium, which governs extracellular fluid volume. It also helps maintain the transmembrane electrochemical gradient via the Na+/K+ ATPase pump and supports proper nerve transmission and signaling.

Daily target and safety

The NIH ODS AI is 3,400 mg per day for adult men aged 19 to 50 and 2,600 mg per day for adult women in the same age range. No Tolerable Upper Intake Level has been set for potassium from food, because healthy kidneys excrete excess efficiently. Potassium supplements may cause minor gastrointestinal side effects. People with chronic kidney disease, impaired potassium excretion, or those taking ACE inhibitors, ARBs, or potassium-sparing diuretics can develop dangerous high potassium levels (hyperkalemia) even at intakes that would be safe for others.

What the research shows

The evidence that potassium is required for intracellular fluid volume, electrochemical gradients, nerve transmission, muscle contraction, and normal kidney function is graded Strong. Evidence that potassium supplementation lowers blood pressure, most clearly in adults with hypertension, is graded Moderate. Evidence linking higher potassium intake to lower stroke risk is graded Limited. Evidence that potassium improves blood glucose control or reduces type 2 diabetes risk is graded Insufficient.

Drug interactions to know

ACE inhibitors (such as benazepril) and ARBs (such as losartan) reduce urinary potassium excretion and can cause hyperkalemia, especially when kidney function is impaired. Potassium-sparing diuretics such as amiloride and spironolactone carry the same risk. Loop and thiazide diuretics increase potassium excretion and can cause low potassium levels (hypokalemia), so monitoring is recommended. Potassium-containing salt substitutes can add 440 to 2,800 mg of potassium per teaspoon and raise hyperkalemia risk in vulnerable individuals. Consult a clinician or pharmacist for personal dosing guidance.

Magnesium

What it does

Magnesium serves as a cofactor in more than 300 enzyme systems that regulate diverse biochemical reactions, including protein synthesis. It supports normal muscle and nerve function and contributes to blood glucose control and blood pressure regulation.

Daily target and safety

The NIH ODS Recommended Dietary Allowance (RDA) is 400 to 420 mg per day for adult men aged 19 to 50 and 310 to 320 mg per day for adult women in the same range. Magnesium from food is safe for healthy people because the kidneys excrete excess. The Tolerable Upper Intake Level for supplemental magnesium is 350 mg per day for anyone aged 9 and older, set to avoid diarrhea and other gastrointestinal effects. This limit applies only to magnesium from dietary supplements and medications, not from food and beverages. Doses used in some research exceed this limit and should be taken only under a clinician's direction.

What the research shows

The evidence that magnesium is a cofactor for more than 300 enzyme systems and is required for normal muscle and nerve function, energy production, DNA and RNA synthesis, and bone structure is graded Strong. Evidence that magnesium may help prevent migraine headaches is graded Moderate. Evidence linking adequate dietary magnesium to lower blood pressure, better bone mineral density, lower stroke risk, lower cardiovascular disease risk, and lower type 2 diabetes risk is graded Limited across all of those outcomes.

Drug interactions to know

Magnesium can decrease absorption of oral bisphosphonates such as alendronate; separate dosing by at least 2 hours. Magnesium forms insoluble complexes with tetracycline and quinolone antibiotics; take those antibiotics at least 2 hours before or 4 to 6 hours after a magnesium supplement. Chronic use of loop and thiazide diuretics can increase urinary magnesium loss and lead to depletion. Potassium-sparing diuretics such as amiloride and spironolactone reduce magnesium excretion. Always speak with a doctor before combining magnesium supplements with any prescription medication.

Putting It Together for Endurance Training

Most athletes who eat a varied diet that includes fruits, vegetables, whole grains, dairy, and lean protein will come close to meeting the AI or RDA for all three minerals without supplements. Sweat losses during prolonged training do increase sodium and, to a lesser extent, potassium needs, but the ODS notes that evidence for sodium supplementation benefits beyond replacing those losses is Insufficient. If you are considering supplements, note that these products are not intended to diagnose, treat, cure, or prevent any disease, and these statements have not been evaluated by the FDA.

Individual needs depend on training volume, sweat rate, kidney function, and any medications you take. Please consult a clinician or registered dietitian for personal dosing recommendations, and always talk to a doctor or pharmacist before combining any supplement with prescription or over-the-counter medication.

Sources

• NIH Office of Dietary Supplements: Sodium, Potassium, and Magnesium fact sheets (ods.od.nih.gov)
• National Academies of Sciences, Engineering, and Medicine: Dietary Reference Intakes for Sodium and Potassium (2019)
• Linus Pauling Institute Micronutrient Information Center: Sodium
• FDA: Sodium in Your Diet