Salt is a natural byproduct of sweat, though much to the chagrin of endurance athletes who are constantly battling the loss of electrolytes, including sodium, during racing events. Some dietitians recommend that you increase your sodium intake because of this loss — but what effect does that have on what you’re sweating out? There have been some interesting studies that have researched the effects of dietary salt intake upon the concentration of the sodium in your sweat, and here are the findings.
Low Sodium Diets
Some early studies focused heavily on restricted sodium intake and the effect this has on the composition of sweat. In general, significant reductions in sweat sodium concentration (i.e., how salty sweat is) were reported when sodium was completely off the menu, which indicated that a ‘less in, less out’ rule might hold true.
Whilst intriguing, the limitation of these early studies is the lack of application to the real world. How often are you able to totally eradicate salt from your diet? It might be easy enough to stop actively adding salt to foods; but with the majority of sodium coming from within the foods themselves, actually reducing sodium intake to very low levels is harder than you might think.
Another factor to take into account is that many of these early studies measured sweat sodium concentration at rest. Those which did measure sweat during exercise often used intensities lower than you’d expect from endurance athletes during training or competition. The participants were often from a sedentary or untrained background.
As a result, the relevance of these early findings to athletes is questionable. However, they did give rise to the next stage of research, which investigated the relationship between excess salt intake and sweat sodium concentration.
High Sodium Diets
Dr. Alan McCubbin, an accredited Sports Dietitian and researcher at Monash University in Australia, has been leading current research about the effects that high sodium intake may have on sodium loss via sweat. In 2018, he and colleague Ricardo Costa published a systematic review that sought to determine the impact of dietary sodium intake on sweat sodium concentration in response to endurance exercise.
Of the six studies which met the inclusion criteria, two found a statistically significant difference in sweat sodium concentration which could be attributed to changes in dietary sodium intake. Two didn’t show any statistical difference, and the other two were unfortunately not statistically analysed.
The inconsistency across the studies could be attributed to many factors. For example, the studies had varied levels of sodium intake (196 mg/day to a staggering 9,177 mg/day) in addition to varied intervention times (3 to 42 days), exercise modalities (cycling ergometry, treadmill running, or walking), and sweat collection methods (whole body washdown, regional patch techniques). In addition, the participants involved were a mixture of trained/untrained athletes and acclimated/non-acclimated individuals.
Ultimately, Dr. McCubbin and Costa found no relationship between the change in sodium intake and the change in sweat sodium concentration. It was concluded that more research was needed to enhance our understanding of the subject matter, as the impact of dietary sodium intake on sweat sodium concentration during exercise remains uncertain.
Average Sodium Intake and Sweat
One of the biggest holes which exists in the scientific literature is that the levels of sodium intake used in trials rarely reflect those of typical endurance athletes. Therefore, these fail to tell us the variation in sweat sodium concentration resulting from smaller deviations in dietary sodium intake (i.e., those that are more realistic for a ‘free-living’ person or athlete in the real world).
Dr. McCubbin et al. tried to tackle some of these limitations in a paper published in 2019. They included a ‘usual free-living diet’ trial in which participants were asked to eat their normal diet. When left to their own devices, the participants averaged a daily sodium intake of 0.046 grams per kilogram of body weight per day (g/kg/day). McCubbin felt it was important for participants to be provided with a dietary sodium intake proportional to their body mass.
In comparison, the ‘high’ trial meant ingesting 0.1 grams/kg/day, and 0.015 g/kg/day in the ‘low’ trial. The high sodium diet was chosen to reflect an intake that was realistically achievable through conscious sodium-loading by endurance athletes in the days preceding exercise. The study showed that after three days of doubling sodium intake (from a ‘usual’ intake to a ‘high’ intake), sweat sodium concentration increased by 10-12% (~6 mmol/L), which is a variation that will have little effect on an athlete’s hydration or nutrition strategy for sport.
In fact, I spoke with Dr. McCubbin about his paper and he highlighted something very important. He feels strongly that it’s important to discuss the literature on dietary sodium ingestion and sweat sodium concentration within a practical context. In the case of his own paper, he said that “anyone working with athletes knows that a change in sweat sodium concentration of 6 mmol/L isn’t a big enough difference to impact how they plan a race day’s nutrition”.
The Bottom Line
What does this mean for endurance athletes? When we take into account the findings above, my takeaway is that as long as you’re not doing anything extreme — like completely starving yourself of any salt intake whatsoever, or consuming a crazy amount of salt like 9,000 mg a day — then your sweat sodium concentration is highly likely to remain relatively stable, and it certainly won’t deviate far enough away from your baseline level to warrant any major changes in your hydration or nutrition strategy.
At Precision Hydration, we’ve conducted thousands of Sweat Tests over the years and we’ve tested dozens of individual athletes on multiple occasions, with virtually none of their sweat sodium concentration scores differing more than around 10% from their baseline.
Ultimately, the saltiness of your sweat is largely genetically determined. Furthermore, when normal fluctuations in dietary sodium intake occur, the kidneys — not the sweat glands — carry the vast majority of the regulatory burden. It’s therefore often safe to say that factors such as dietary intake or acclimation to heat won’t change the concentration enough to warrant a major change in the way you approach hydration and nutrition for your sport.