Hydration products typically contain anywhere from 250mg to well over 1,000mg of sodium per serving. Every product sells itself as the ideal amount, but with such variation in the market, it can be confusing to know which is right.

The question is: how much sodium do you actually need?

The answer depends on how much sodium you are losing, and therefore how much sodium you are required to replace. Like most things, it really does vary athlete-to-athlete. Some will lose relatively small amounts through sweat, while others can lose several times as much during exactly the same length of training session.

That means two athletes running side-by-side for two hours could finish with very different sodium losses, even if they complete the same session and drink the same amount of fluid. In order to work out how much you are losing, we need to start from the start.

Why do we lose sodium when we sweat?

Sweating is one of the body's main ways of controlling temperature during exercise. As sweat evaporates, it absorbs heat from your skin and releases it into the air. The reason that drinking just water to rehydrate is not always enough is because sweat contains also contains electrolytes [a large portion of which includes sodium and chloride].

A study of more than 1,300 athletes found significant variation in both sweat rate and sodium loss across different sports. Endurance athletes averaged a sweat rate of around 1.28L per hour, but there was considerable variation around that average. This is why looking at an 'average athlete' can only get you so far.

How to work out your sodium loss.

To estimate how much sodium you are losing per hour, you need to know two things:

1. Your sweat rate.

How many litres of sweat do you lose per hour? You can do this fairly easily because we know that 1 kilogram of body weight equals 1 litre of fluid, so comparing your weight before and after a session will tell you how much fluid you have lost. You also need to measure any fluids you consume within the session and add that to your calculation.

2. Your sweat sodium concentration.

How many milligrams of sodium are contained in each litre of your sweat? This is a little harder to measure. There are professional tests for this and sensors that track your sweat. Alternatively, you can look for signs that you may be a particularly salty sweater. Heavy salt residue left on your skin or clothing, very salty-tasting sweat or sweat frequently stinging your eyes can all suggest higher sodium losses. Research has found that these self-reported signs can correlate reasonably well with measured sweat sodium concentration, but they can't tell you precisely how many milligrams of sodium you're losing per litre. 

Unfortunately whether you do this test at home or professionally, there is bound to be some variation depending on a variety of factors, so results are best treated as information to help guide your hydration plan, rather than an exact number that needs to be replaced milligram-for-milligram in every session.

If you've managed to get this far, the rest is an easy calculation.

3. Sweat rate (L/hr) × sweat sodium concentration (mg/L) = sodium loss (mg/hr).

For example, imagine two athletes who both sweat at 1L per hour:

	Athlete A	Athlete B
Sweat rate	1L/hr	1L/hr
Sweat sodium	400mg/L	1,000mg/L
Sodium lost	400mg/hr	1,000mg/hr

Their fluid losses are identical, but Athlete B is losing two-and-a-half times as much sodium. That is why simply knowing that you're a “heavy sweater” isn't enough to tell you exactly how much sodium you lose.

How salty can sweat actually be?

In a study of experienced marathon runners, average sweat sodium concentration was around 43mmol/L, equivalent to roughly 990mg of sodium per litre. But individual measurements ranged from approximately 7 to 96mmol/L, showing just how different athletes can be.

Research across a large athlete population has repeatedly shown considerable differences in sweat sodium concentration between individuals. Exercise intensity, sweat rate, environmental temperature and heat acclimatisation can all influence these numbers, but are only responsible for one part of the variation.

So unfortunately, you can't reliably predict your sweat sodium concentration from your body size, fitness level or sport alone.

Plus, sweat rate changes depending on the conditions.

It gets a little more complicated, because doing one sweat rate test doesn't necessarily give you a number that applies all year round. A 90-minute run on a cold winter morning will likely produce a different result to the same run in hot, humid weather. Intensity, clothing, body size, environmental conditions and heat acclimatisation can all impact your sweat rate.

If you're using sweat rate to help build a hydration strategy, you should be testing it across the types of conditions you are actually going to be racing in. This is particularly relevant for Aussie athletes, where the same event or training block can involve significantly different temperatures throughout the year.

Do you need to replace every milligram you lose?

Your body already has sodium stores when you start your session, and sodium is also consumed through your normal diet. For shorter sessions or athletes with relatively low sweat losses, consuming a large amount of sodium during training may not offer any real benefit. Sodium replacement becomes more relevant when exercise is prolonged, sweat rates are high, you're in a hot environment, or you're someone who has particularly salty sweat. There is also a separate risk at the other end of the hydration spectrum: drinking significantly more fluid than you're losing. Excessive fluid intake during prolonged exercise can dilute blood sodium levels and contribute to exercise-associated hyponatraemia. Both under-drinking and over-drinking can therefore cause problems, which is another reason an individualised hydration strategy is useful.

So how much sodium should be in your hydration?

If an athlete loses 500mg of sodium per hour, a hydration strategy providing 400–600mg might cover a meaningful portion of that loss. An athlete losing 1,500mg per hour has a very different situation. It also depends on how much fluid they are drinking. A product containing 1,000mg of sodium per litre only supplies 500mg if the athlete drinks half a litre.

When you're comparing hydration products, it helps to consider sodium in the context of both milligrams per litre and how much fluid you will realistically consume, rather than looking at sodium per serving alone.

Work out your losses before chasing a bigger number.

There isn't one golden number for sodium intake. Start by understanding your sweat rate and how it changes across different sessions, temperatures, etc. If you're training or racing for several hours, consistently finishing covered in salt, experiencing very high sweat losses or trying to fine-tune an endurance hydration strategy, sweat sodium testing may provide another useful piece of information. From there, you can build a hydration plan around what you're actually losing rather than assuming that more sodium automatically means better hydration.

References.

1. Baker LB. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Medicine. 2017;47(Suppl 1):111–128.
2. Barnes KA, Anderson ML, Stofan JR, et al. Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: An update and analysis by sport. Journal of Sports Sciences. 2019;37(20):2356–2366.
3. Baker LB, De Chavez PJD, Nuccio RP, et al. Explaining variation in sweat sodium concentration: effect of individual characteristics and exercise, environmental, and dietary factors. Journal of Applied Physiology. 2022;133(6):1250–1259.
4. Lara B, Gallo-Salazar C, Puente C, et al. Interindividual variability in sweat electrolyte concentration in marathoners. Journal of the International Society of Sports Nutrition. 2016;13:31.
5. American College of Sports Medicine, Sawka MN, Burke LM, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine & Science in Sports & Exercise. 2007;39(2):377–390.
6. McDermott BP, Anderson SA, Armstrong LE, et al. National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active. Journal of Athletic Training. 2017;52(9):877–895.
7. Ranchordas MK, Tiller NB, Ramchandani G, et al. Normative data on regional sweat-sodium concentrations of professional male team-sport athletes. Journal of the International Society of Sports Nutrition. 2017;14:40.