We've conducted thousands of Sweat Tests over the years and one of the questions we're regularly asked is "will eating more salt result in a higher sweat sodium concentration?".
So, does more in equal more out?
There's been some interesting (and quite extreme) studies that have researched the effects of dietary salt intake upon the concentration of the sodium in your sweat and these are a great place to start looking for answers.
And the big takeaway from a recent paper by Dr. Alan McCubbin is that your dietary sodium intake needn't fundamentally influence your nutrition and hydration strategy. Here's why...
Does restricting sodium intake impact how salty your sweat is?
Early studies in this area were published either side of the Second World War. Rather than looking at sweat sodium concentration in response to an excess of dietary sodium, they focused heavily on restricted sodium intakes and the effect this had on sweat composition.
In general, significant reductions in sweat sodium concentration were reported when sodium was completely off the menu, which indicated that a ‘less in, less out’ rule might hold true.
Whilst fascinating, the limitation of these early studies is the lack of application to the real world - how often are you able to (or do you want to) totally eradicate all salt from your diet? Whilst it's somewhat easy to stop actively adding salt to foods with the majority of sodium coming from 'hidden' sources, actually reducing sodium intake to very low levels is probably harder than you might think.
Basically, the research groups used extreme sodium restrictions in order to create an extreme deficiency.
For example, one of the earliest (craziest?) trials involving salt deprivation was run by Robert McCance, a Professor at the University of Cambridge in 1938.
Forced sodium loss was induced by taking hot baths to instigate heavy sweating and the participants followed a very strict salt-free diet, even boiling and draining foods to reduce their sodium content.
It’s unclear precisely how long the participants followed this protocol for, though the phrase ‘for several days’ is used in the write-up.
On top of seeing a drastic decline in their sweat sodium output after a few days, two of the participants ('Robert and a friend', who probably wasn’t a friend after being roped into this!) suffered ‘considerably’ from muscle cramps throughout the experiment, as well as enduring breathlessness, an ongoing sense of fatigue, weight loss, nightmares, insatiable thirst, a loss of taste, and nausea following the consumption of food. Delightful!
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 athletes during typical training or competition. The participants were often from a sedentary or untrained background.
Simply put, whilst these experiments were brilliantly and diligently conducted, such conditions are clearly not reflective of the lifestyles of modern-day athletes and the levels of sodium deficit they are realistically likely to incur in and around exercise.
As a result, the relevance of these early findings to athletes today is questionable. But, they did give rise to the next stage of research investigating the relationship between excess salt and sweat sodium concentration...
Does taking in excessive amounts of sodium impact how salty your sweat is?
A researcher who is driving a lot of research in this area nowadays is Dr. Alan McCubbin, an accredited Sports Dietitian and researcher at Monash University in Australia.
In 2018, he and colleague Ricardo Costa published a systematic review which 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.
So, inconsistency was the main takeaway!
The change in mean sweat sodium concentration in these studies ranged from −5 mmol/L to +30 mmol/L.
One study found just a 4 mmol/L mean difference in whole-body sweat sodium concentration over six weeks when consuming either 3.4 or 5.6 grams of sodium per day (Na/day).
In contrast, a study using only two weeks of dietary sodium manipulation reported a difference of 12 mmol/L despite having a similar difference between controlled sodium intakes (1.15g Na/day or 3.45g Na/day).
The inconsistency across the studies could be attributed to many factors.
For example, the studies had varied levels of sodium intake (<196 to a staggering 9,177 mg/day), intervention times (e.g. 3 to 42 days), exercise modalities (e.g. cycling ergometry, treadmill running or walking), and sweat collection methods (e.g. whole body washdown, regional patch techniques).
In addition, the participants involved were a mixture of trained/untrained and acclimated/non-acclimated individuals.
The variation in results in the studies using different intervention times may be explained by the sweat glands’ response to variations in salt balance and associated changes in circulating aldosterone.
As noted in the early 1950s, the renal system (the primary system responsible for maintaining salt balance in the body) responds to a salt deficiency or excess within 1–3 hours, whereas the sweat glands typically require 1–4 days.
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 as the impact of dietary sodium intake on sweat sodium concentration during exercise remained uncertain.
Real-world applications for athletes
One of the biggest holes which exists in the literature at the moment is that sodium intake used in trials rarely reflect those of typical endurance athletes or even those used in “normal” dietary sodium trials.
For instance, one study’s high sodium diet consisted of >9 grams of sodium per day sustained over four days, which is arguably a pretty unrealistic amount to consume for the majority of people, even for high-level athletes who've been shown to require a greater intake in some circumstances.
In comparison, the low sodium diet in studies is sometimes as low as 0.5 to 2.3g Na/day, much lower than many athletes would reasonably be expected to consume with a normal diet.
Trials that don't use sodium intakes reflective of everyday consumption 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 and co. tried to tackle some of these limitations in a paper published a year later 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 kg of bodyweight 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 (a finding found previously by the research group).
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, we 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.
So, 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".
What does the research mean for the Advanced Sweat Test?
So, when we take into account the findings above, our takeaway is that, as long as you're not doing anything extreme - like completely starving yourself of any salt intake whatsoever (very difficult to do!), or consuming a crazy amount of salt like 9,000mg 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.
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 and when normal fluctuations in dietary sodium intake occur, the kidneys - not the sweat glands - carry out the vast majority of correcting for these changes.
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 another Sweat Test or a major change in the way you approach hydration and nutrition for your sport.
