Salt and Hydration: How Much Water and Sodium You Need

Salt and hydration are linked because sodium is the main mineral dissolved in the fluid outside your cells, including your blood. The total amount of sodium in your body largely sets how much of that fluid there is. The concentration of sodium in it largely sets the concentration of your body fluids, called osmolality. Thirst and antidiuretic hormone (ADH) hold osmolality in a narrow range by adjusting how much water you drink and keep. So hydration depends on both water balance and sodium balance.

For healthy adults, the National Academies set an Adequate Intake of 1,500 milligrams of sodium a day. For water, the Adequate Intakes are about 3.7 liters a day for men and 2.7 liters a day for women, counting the water in food and in every drink. The average adult in the United States already eats about 3,400 milligrams of sodium a day, so most people do not need to add salt to their water.

Heavy sweating is the exception, because sweat carries sodium out of the body. Long or hot exercise can mean losing several liters of sweat and several grams of sodium with it, and some of that sodium needs to be replaced during or after exercise.

I explain this in the hydration lecture of my human physiology course. The video below starts where the lecture turns to sweat and sodium.

Sodium Loss in Sweat

How Much Do You Sweat During Exercise?

Sweat is the loss that matters most for exercise. At rest in a comfortable environment, active sweating is minimal, and most of the water leaving the skin is insensible loss, water that diffuses out through the skin without sweating. That comes to about 450 milliliters a day for the average adult, and the amount varies with temperature, humidity and the condition of the skin.

During exercise, sweat rate typically runs about 0.5 to 2 liters per hour. Rates above 3 liters per hour have been documented, but they are rare, at about 2% of athletes in one large dataset, and they usually come with extreme heat, very hard work or a large body.

Sweat Sodium Concentration

Sweat contains sodium, and that matters because sodium and the chloride and bicarbonate that accompany it are the main dissolved particles outside the cells, so they largely set the osmotic gradient. How much sodium a person loses in sweat varies widely from one person to the next.

Measured at a patch of skin, sweat sodium in athletes commonly runs from about 10 to 90 millimoles per liter, and whole body sweat runs somewhat lower. In one study of 157 marathon runners it ranged from 7 to about 95, with an average near 43. People at the top of that range, sometimes called salty sweaters, can reach or exceed about 80 to 100 millimoles per liter.

To convert to grams, each millimole of sodium weighs 23 milligrams. So 20 millimoles per liter is about 0.46 grams of sodium per liter of sweat, 40 is about 0.92 grams, 80 is about 1.84 grams, and 100 is about 2.3 grams.

Heat acclimation generally lowers sweat sodium, because the sweat glands reabsorb more sodium from the sweat before it reaches the skin, and greater sweat gland responsiveness to aldosterone appears to be part of that. In one 10 day heat acclimation study of eight people, sweat sodium began to fall by the third day and had dropped to about 60% of its starting concentration by the tenth.

A well acclimated person might sit near 20 millimoles per liter and an unacclimated person somewhere in the 40 to 80 range. Diet, sweat rate, genetics and even where on the body the sweat is collected also change the number, so acclimation status alone cannot predict any one person’s concentration.

Why Sweat Is Hypotonic

Plasma sodium is around 140 millimoles per liter, about 3.2 grams per liter, and sweat sodium usually runs about 0.5 to 2 grams per liter. So sweat is hypotonic to plasma, meaning its effective osmolar concentration is lower, and its much lower sodium and chloride concentrations account for most of that difference.

Other Electrolytes in Sweat

Sweat carries other electrolytes too, and sodium and chloride account for by far the largest losses. The table below uses average concentrations from a whole body sweat collection study by Shirreffs and Maughan in 1997, in healthy young adults exercising in hot, humid conditions. The last column shows what a heavy sweat rate of 2 liters an hour would mean, as an illustration rather than a typical value.

Electrolyte	Average concentration	Loss per liter of sweat	Loss at 2 liters per hour
Sodium	50.8 mmol/L	1.17 g	2.34 g
Chloride	46.6 mmol/L	1.65 g	3.30 g
Potassium	4.8 mmol/L	0.19 g	0.38 g
Calcium	1.3 mmol/L	52 mg	104 mg
Magnesium	0.5 mmol/L	12 mg	24 mg

Chloride loses more mass than sodium at a similar concentration because each chloride ion weighs more. Potassium losses are much smaller, and ordinary food generally replaces them during recovery.

What Sweating Does to Osmolality and Blood Volume

When sweat is not fully replaced by drinking, we lose proportionally more water than sodium, because sweat is hypotonic. The sodium left behind in the blood becomes more concentrated, so plasma osmolality rises, and at the same time blood volume falls.

The rise in osmolality is sensed in the hypothalamus, which triggers thirst and the release of ADH from the posterior pituitary. ADH then has the kidney insert aquaporin 2 water channels, so more water is reabsorbed. The fall in volume activates the renin angiotensin aldosterone system, and aldosterone increases sodium reabsorption in the kidney. The two systems respond to different signals, but they also work hand in hand. When aldosterone increases sodium retention, osmolality rises, and that in turn stimulates thirst and ADH release. With repeated heat exposure, aldosterone also helps lower the sodium lost in the sweat itself.

How Much Sodium Do You Need Per Day?

Maintaining hydration comes down to meeting sodium and electrolyte needs as well as water needs. Proper hydration requires a balance between water and electrolytes, particularly sodium. Sodium is the primary determinant of extracellular fluid tonicity and plays an important role in maintaining fluid volume, while thirst, hormones and the kidneys regulate water and sodium balance. So adequate sodium intake is important, particularly when sweating increases sodium losses, but consuming more sodium does not necessarily improve hydration.

The Adequate Intake and What People Eat

The National Academies set the adult Adequate Intake for sodium at 1,500 milligrams a day in 2019, which is about 65 millimoles.

The same report set a Chronic Disease Risk Reduction Intake of 2,300 milligrams a day. That is the intake above which lowering sodium is expected to reduce chronic disease risk in the apparently healthy population, based on evidence linking sodium intake to cardiovascular disease, hypertension and blood pressure.

The Adequate Intake can be this low because the kidneys conserve sodium so well. When intake is low and aldosterone is high, the kidneys can bring urinary sodium down to a small fraction of a gram a day, so the sodium the body has to lose outside of sweat is very little. The National Academies describe 1.5 grams a day as an intake expected to be adequate for healthy adults, not a measured minimum for every person. It is pretty amazing that the body can conserve sodium this well.

The National Academies also note that people in high temperatures or doing high intensity physical activity may need more than the Adequate Intake, although they could not estimate how much.

Most people eat far more than that. Average intake in the United States is about 3.4 grams of sodium a day, and a global analysis of 24 hour urine and dietary surveys estimated average adult intake worldwide at close to 4 grams a day in 2010. With intake that high, the body keeps aldosterone low and the kidneys excrete the excess, so in a steady state, when little is lost in sweat, the sodium in the urine roughly equals the sodium eaten.

Salt Sensitivity and Blood Pressure

A large dose of salt can briefly raise osmolality, which increases thirst and ADH release, so we drink more and the kidneys keep more water. Blood volume rises for a time as a result, and in some people a sustained higher intake contributes to higher blood pressure.

Not everyone responds that way. People differ in how quickly their kidneys excrete extra sodium, and in how their blood vessels and their nervous and hormonal systems respond to it. Blood pressure responses to sodium vary continuously from person to person, and people with a larger response are described as salt sensitive.

Sodium Needs for Heavy Sweaters

A little exercise usually does not change sodium needs much. The kidneys can often make up for modest sweat sodium losses by excreting less sodium, and the sodium already in the diet covers the rest. Whether extra intake is needed depends on total intake and on the cumulative loss, and no single threshold decides it. With harder or longer exercise, someone can sweat several liters and lose more than a couple of grams of sodium. Extra sodium is needed when the usual diet and the kidneys’ conservation are not enough to restore sodium balance. Sweat sodium does not have to be replaced gram for gram during the exercise itself. How much helps during exercise depends on sweat sodium and how much fluid is replaced, and the usual diet and the kidneys make up much of the rest during recovery.

Replacing sodium that has been lost in sweat restores a deficit rather than adding sodium on top of a neutral balance. A habitually high intake is different. It can raise the body’s sodium and the volume of fluid outside the cells, by an amount that varies a lot from person to person, and in salt sensitive people it can raise blood pressure. Long-term evidence on sodium replacement in salt sensitive heavy sweaters is limited, though, so the optimal amount for those people has not been established.

As a rule of thumb, I start thinking about extra sodium when sweat rate goes above about 2 liters an hour, or when someone is a salty sweater. The loss itself is sweat rate multiplied by sweat sodium concentration, multiplied by the duration, so how long the exercise lasts matters as much as how fast we sweat.

A person sweating 1 liter an hour at 50 millimoles per liter loses about 1.15 grams of sodium an hour, which comes to 4.6 grams over four hours. Someone sweating 2 liters an hour at 80 millimoles per liter loses about 3.7 grams every hour.

When Do Electrolyte Drinks Help?

Electrolyte drinks help in specific situations, and they make the most sense in very long or hot exercise, for heavy or salty sweaters, for repeated sessions with little recovery, and when normal food is not available. For short to moderate exercise followed by normal meals, water plus food is usually enough.

During exercise, the goal is to replace enough fluid to avoid a problematic deficit without drinking so much that body weight goes up, because overdrinking is the main behavioral cause of exercise associated hyponatremia. The American College of Sports Medicine’s 2007 position stand recommends individualizing intake, since sweat rate and sweat electrolyte content vary so much.

For exercise lasting more than an hour, the college’s 1996 position stand recommended drinks with about 0.5 to 0.7 grams of sodium per liter, which is roughly 22 to 30 millimoles per liter. Modeling by Alan McCubbin in 2023 found that targeted sodium replacement was unnecessary in every realistic soccer and elite marathon scenario it modeled. In the model’s 160 kilometer ultramarathon, significant targeted sodium replacement to keep plasma sodium stable appeared necessary when more than 80% of fluid losses were replaced and whole body sweat sodium was at least 40 millimoles per liter.

Sodium and Potassium

When sodium intake is raised to replace heavy sweat losses, potassium intake should stay adequate as well, without needing to rise in step with sodium. People with kidney disease or taking medications that raise potassium need individual advice.

Higher sodium and lower potassium intake tend to push blood pressure and cardiovascular risk in the wrong direction, and potassium can blunt part of the blood pressure effect of sodium. Potassium is the main ion inside the cells, so the two are held in balance.

Urine Color and Body Weight as Practical Checks

Keeping a stable body weight and checking urine are practical ways to keep an eye on hydration, and each one measures something specific.

Urine Color

Urine color is a low cost indicator of how concentrated the urine is. A systematic review published in 2021 found moderate to strong correlations between urine color and laboratory measures of urine concentration. The relationship was weaker in adults over 60. Color is also changed by vitamins, some foods, medications and disease. Urine color tells us how hard the kidneys are conserving water, and it does not measure blood volume or cellular hydration.

Body Weight

Body weight is often the most practical measure around a training session. For a typical session, roughly 1 kilogram of weight lost during exercise corresponds to about 1 liter of fluid, once anything eaten, drunk or passed as urine is accounted for, and comparing weight before and after a session is how athletes estimate their sweat rate. For endurance exercise, a loss of about 2% of body mass is a useful marker for when performance losses become more likely, especially during prolonged work in warm to hot conditions.

Over days, weight becomes much less specific, because glycogen, gut contents, energy balance and sodium balance all move it. A gradual fall in weight can reflect sodium and water depletion, since losing sodium shrinks the extracellular fluid along with the water that goes with it. Fat loss, muscle loss, eating less and a range of medical conditions produce the same observation, so unexplained weight loss is a reason to look further. Combining weight, thirst and urine gives a better picture than any single marker.

How Much Water Do You Need Per Day?

The goal is optimal hydration, with no excess in either direction, and the kidneys handle most of that work for us. Our job is to give the body what it needs.

The Adequate Intake for Total Water

Daily intake has to cover the daily water losses, through breath, skin, feces, urine and sweat. The National Academies’ Adequate Intakes for total water, published in 2005, are about 3.7 liters a day for adult men and 2.7 liters a day for adult women.

Those totals include plain water, every other beverage, and the water in food, including fruits and vegetables. In the United States dietary data they were based on, roughly 80% came from beverages and about 20% from food, though that share varies with diet.

The values were set largely from the median intake of apparently healthy people, because hydration can be maintained across a broad range of intakes. So they describe what healthy people typically consume rather than a measured minimum that every adult must reach. As long as intake keeps up with losses and sodium is sufficient, the kidneys do the rest and maintain sodium levels and blood volume.

When Drinking More Helps

There is no need to drink a lot of extra water. Extra water lowers plasma osmolality, which suppresses ADH, and the kidneys then put out a larger volume of dilute urine to clear it. With ADH suppressed, a healthy kidney can clear a lot of extra water, so for most people this is unnecessary work rather than harm. During long exercise, though, ADH can stay raised for reasons other than osmolality, which cuts how much water the kidneys can clear. Drinking more than is being lost on top of that is the main way hyponatremia develops during exercise, even in healthy people.

Drinking a bit more does have benefits for some people. For people prone to kidney stones, increased fluid intake is one of the better supported ways to prevent recurrence, although the trial evidence is limited.

Drinking too little all the time carries its own risk, and the kidney stone evidence above is one example of it. The healthy range of intake is wide, and the upper end depends less on a daily total than on drinking faster than the kidneys can clear water.

Thirst Becomes Less Reliable With Age

As we get older, thirst becomes a less reliable guide. Older adults generally feel less thirsty and drink less to replace a fluid deficit after water deprivation, exercise or heat, and they may restore the deficit more slowly. Some experiments find thirst relatively preserved in healthy older people, and many drink enough day to day, so the change shows up most after a deficit has already developed.

Some people describe older adults as mistaking thirst for hunger. The direct evidence for that is limited, but two related findings are well supported. With age, thirst becomes less responsive to dehydration, as Phillips and colleagues showed in healthy older men after water deprivation in 1984. And in a controlled study by Van Walleghen and colleagues in 2007, drinking water 30 minutes before a meal increased fullness in younger and older adults alike, and it modestly reduced how much the older adults ate, an effect not seen in the younger group. So rather than mistaking thirst for hunger, older adults may simply feel less thirst, while water before meals may independently help curb appetite and excess calories.

Maintaining Optimal Hydration

There is no need to overthink hydration. We do not need special waters or excessive water. We need enough water to replace what we lose, and when electrolyte losses are substantial, as with prolonged or heavy sweating, enough sodium to replace those too. The body does the rest through thirst, ADH and a healthy kidney.

Vasopressin, aldosterone and atrial natriuretic peptide all help regulate fluid balance, but they respond to different signals. Vasopressin responds mainly to osmolality and to volume and pressure signals, aldosterone mainly to angiotensin II and potassium, and atrial natriuretic peptide mainly to the stretch of the heart’s atria when volume rises. Avoiding substantial overdrinking and dehydration reduces how much these systems have to compensate. That is homeostasis. Thirst, ADH and the kidneys together hold osmolality in a tight range, much like pH, and a small rise in osmolality above the relevant threshold is enough to stimulate thirst and ADH release.

If you want to keep learning, more of my physiology lectures are written up as articles in my Advanced Human Physiology series. My podcast conversations and interviews are collected on my Learn page.