How Much Sodium Per Hour Do You Need for Ultra Running?

There is no one number, and that is exactly why you see advice run anywhere from 200 to over 1,500 mg per hour. For most runners a good planning band is roughly 300 to 1,000 mg of sodium per hour. The bottom of that on a cool easy day, the top on a hot exposed effort if you are a salty sweater. The honest reason it is such a wide spread is that your sodium needs follow your fluid needs, which follow your sweat rate, and your sweat rate is your own. So anchor your number to how much you sweat and how salty your sweat is. Do not just copy a figure off a label.

For sweat rate, weigh yourself nude before a hard 60 to 90 minute run, track every ounce of fluid you drink, then weigh yourself nude again after you towel off. Each pound of body weight you lost is about 16 oz (roughly 0.5 L) of sweat. Add back what you drank, divide by the hours you ran, and there is your sweat rate in liters per hour. Most runners land between about 0.5 and 1.5 L/hr, with heavy sweaters above 2 L/hr. For how salty your sweat is, a lab sweat test is the gold standard, but you can read it yourself pretty well. White salt crust on your skin and clothing, sweat that stings your eyes, a salty taste, strong salt cravings. The more of those you have, the saltier you run and the more sodium per liter you need.

Thinking in mg of sodium per liter of fluid is the cleanest way to plan, because it ties the two decisions together for you. Drink more in the heat and your sodium goes up right along with it. A good starting point is about 500 mg/L if you are a light sweater, 800 to 1,000 mg/L for an average sweater, and 1,000 to 1,500 mg/L if you are salty. How salty your sweat is swings a lot from person to person, from under 200 mg/L to over 2,300 mg/L, with the average athlete around 950 mg/L. So treat these as starting points you test and adjust in training, not numbers carved in stone.

Fluid needs swing a lot, from roughly 300 ml/hr in cool conditions to 1,000 ml/hr or more in heat and humidity. Drinking to thirst is the safest default, and the Wilderness Medical Society backs it as your best guard against over-drinking, because thirst tracks what you need well in most settings. The catch for long ultras is that thirst can lag in the heat or go quiet late in a race. So pair drinking to thirst with a little structure: a target hourly range and a habit of sipping at aid stations. You want to finish neither bloated and waterlogged nor several percent dehydrated.

Exercise-associated hyponatremia (EAH) is dangerously low blood-sodium, and the main cause is over-drinking. You take in more fluid (water or sports drink) than you sweat out, and it dilutes the sodium in your blood. When they screen ultra finishers who feel fine, up to half can show some degree of hyponatremia. Here is the part people get wrong: sodium pills alone will not save you if you are drinking too much. You cannot out-salt too much water. What actually protects you is behavior. Drink to thirst instead of on a rigid schedule, do not gain weight during a race, and do not force fluids. Watch for nausea, headache, bloating, puffy hands, and confusion. If you think you have it, stop drinking and get medical help, because it can kill you.

Mostly no, and this is where what everyone believes gets ahead of what the evidence shows. The main cause of exercise-associated muscle cramps is neuromuscular fatigue (overworked muscles and scrambled spinal reflexes), not a sodium deficit. Controlled studies found runners cramp at about the same rate no matter how much sodium they took in, and one ultramarathon study found sodium supplements did not prevent cramping, dehydration, hyponatremia, or nausea. That does not make salt tablets useless. They are a handy way to replace sweat sodium and keep your gut and fluid balance happy on long hot days, and some heavy salty sweaters really do notice a difference. Just treat them as one piece of your sodium plan, not a cramp cure. Better training, conditioning, and pacing stop more cramps than any tablet will.

Heat is the single biggest variable. As temperature and humidity rise, your sweat rate climbs, which raises both your fluid loss and your total sodium loss per hour. A runner who needs 400 mg/hr on a cool morning can easily need 1,000 mg/hr or more on a hot exposed afternoon, simply because they are sweating two to three times as much. This is the case for planning in mg of sodium per liter of fluid: when the heat pushes you to drink more, your sodium intake scales up automatically. Practice fueling in race-like heat, because a hot stomach also tolerates less, and start hydrating well before the temperature peaks.

Under-doing sodium on a long hot effort can show up as a sloshy stomach that will not empty, persistent thirst that water alone does not fix, a wrung-out flat feeling, and in some people muscle cramping, though cramps have other causes too. Over-doing sodium relative to your fluid is less common but real: it can cause nausea and GI upset and make you thirstier, prompting more drinking. The more dangerous error is over-drinking fluid (not over-eating sodium), which dilutes blood sodium toward hyponatremia and shows up as bloating, puffy hands, headache, and weight gain during the race. If you are sipping to thirst, getting sodium roughly matched to your sweat, and your weight is stable, you are in the right zone.