The results from the thousands of Sweat Tests that Precision Fuel & Hydration have carried out over the years has emphasised how sweat sodium concentration (ie how salty your sweat is) can vary greatly from one individual to another.
Some people lose as little as 200 milligrams of sodium per litre of sweat whilst those at the top end of the scale can lose in excess of 2,000mg/l.
People with the genetic condition Cystic Fibrosis (CF) tend to be at the higher end of this scale - their sweat is particularly salty - but why is that and how does that impact them?
But first, what is Cystic Fibrosis?
Cystic Fibrosis is caused by a mutation in the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene. Genes are inherited as pairs, with one gene inherited from each parent. CF occurs when both CFTR genes in the pair have a mutation. There are over 2,000 known mutations of the CFTR gene and each person’s genotype (genetic make-up) will dictate the symptoms they experience.
Fundamentally, CF causes the body to produce a thick mucus which primarily affects the lungs and digestive system, causing poor lung function, frequent and persistent lung infections and the inability to effectively digest food, particularly fats.
Cystic fibrosis is a life-long challenge, involving a huge daily burden of care, financial pressures, emotional challenges, and isolation from others with the condition.
People with CF often have to endure a huge burden of care just to stay healthy - taking inhaled and injected drugs to clear mucus and fight infections, regular hospital appointments, maintaining a high calorie diet and doing regular exercise and physiotherapy morning and night.
Additional complications include CF-related diabetes, bone disease, infertility and shorter life expectancy.
Why does sweat sodium concentration vary between individuals?
Sweat sodium concentration is largely genetically determined.
In those with CF, the mutation in the CFTR gene results in a defective function of the CFTR protein in human sweat ducts and results in extremely high sweat sodium and chloride losses (typically 3-5 times higher compared with sweat from healthy individuals).
In non-CF individuals, CFTR function is normal but sweat sodium concentration can still vary up to 10-fold depending on the amount of the CFTR protein in the sweat gland.
Healthy individuals with very high sweat sodium levels approaching values similar to CF are labelled ‘salty sweaters’ and have a lower abundance of the protein in the reabsorptive duct.
Key takeaway: CF sweat glands don't reabsorb as much salt so more leaks out onto the skin.
Explaining CFTR function in the sweat gland
Each sweat gland has two components – a reabsorptive duct and a secretory coil (see figure below).
'Primary sweat' is produced in the secretory coil which then travels to the reabsorptive duct where chloride and sodium are (you can guess it...) reabsorbed.
Sodium is transported through an epithelial sodium channel (ENaC) regulated by the CFTR protein through an ill-understood mechanism. It's this movement of sodium that sets an ionic concentration gradient (difference in charge across a membrane; sodium has a positive charge and chloride a negative one) that then drives the passive transport of chloride via the CFTR protein.
The result is hypotonic (lower concentration than blood) sweat secreted onto the skin’s surface (having been drawn from the blood, 'primary sweat' is isotonic - for more on isotonic and hypotonic, check out this blog).
Take a look at this paper for a comprehensive explanation of CFTR function.
Key takeaway: Whilst moving through the sweat duct to the skin, sodium and chloride undergo reabsorption via the direct or indirect action of the CFTR protein.
The links between PH and CF
Precision Fuel & Hydration's founder, Andy Blow, has a background in Sports Science and in his younger days he was a regular podium finisher in elite short course triathlon racing.
Despite achieving some minor successes as an athlete, he initially struggled when racing IRONMAN distance events in hot environments as he suffered with severe muscle cramping and hydration-related issues, despite following widely recognised hydration guidelines at the time.
Through many years of trial and error, Andy began to recognise his substantial sodium and fluid losses, and started to understand how he could maintain his performance through electrolytes.
This process was greatly helped by the input of Dr Raj Jutley, a highly regarded heart surgeon, who introduced Andy to the concept of sweat testing and the huge variance in sweat sodium losses that can occur between athletes.
Andy himself falls into that ‘salty sweater’ category, losing close to 1,800 milligrams of sodium in each litre of sweat. This puts into perspective how much sodium he'd been losing in past races and this undoubtedly had an impact on how he's approached his hydration in future races.
Key takeaway: Non-CF individuals can have extremely high sweat sodium concentrations as well!
The Sweat Test
The Sweat Testing procedure used by Precision Fuel & Hydration is the same gold-standard test used by medical practitioners diagnosing CF.
Screening for CF is part of the UK national new-born bloodspot screening programme (the heel prick test) and the sweat test is subsequently performed on babies suspected of having CF.
The test is simple, painless and accurate, and Precision Fuel & Hydration has exclusive patented access to it outside of it's medical use cases. Watch this video to see an Advanced Sweat Test in action.
Precision Fuel & Hydration was launched in a bid to provide an accessible way for athletes to understand their own hydration needs. The development of our own range of electrolyte products came a little later after it became clear that existing products were not strong enough to meet serious athletes’ needs.
Key takeaway: The Advanced Sweat Test procedure is the same as those used by medical practitioners to diagnose CF
What do high sweat sodium losses mean for ‘salty sweaters’ and those with CF?
The higher an individual’s sweat sodium concentration, the easier it is to consume too much fluid without restoring at least some of the sodium lost through sweat, which can result in hydration-related problems (ranging in severity from a loss in power/performance, dizziness/nausea right up to severe hyponatremia (low blood sodium levels) which can be fatal).
One real-life example of the above was reported in a case study published in the British Medical Journal. A British Army soldier who was found to have undiagnosed CF after collapsing with hyponatremia whilst on deployment in Saudi Arabia and again in Cyprus.
On both occasions he presented at the hospital with nausea, dizziness and muscle cramping and was found to be suffering from hyponatremic heat exhaustion at the age of 24.
A sweat test revealed his sweat sodium losses were in the region of 2,000mg of sodium per litre of sweat and subsequent genetic testing confirmed that he carried two mutations of the CF gene.
Now that sounds like an extreme situation and a rare case. But is it?
We’ve conducted thousands of Sweat Tests on a wide variety of individuals and it's clear that the soldier described above wasn’t a complete outlier.
Our data shows that around 10% of all individuals we have tested to date have sodium loss rates ranked as “Very High”, which are right up there with the soldier. In fact, our most extreme sodium concentration measured to date was at 2,300mg sodium per litre of sweat.
It's important to point out there that just because an individual presents with high sodium concentration loss, it doesn't automatically mean that they have Cystic Fibrosis - it’s more complex than that. Take Andy as an example, he loses ~1,800 mg/l and doesn't have CF.
This case report of the soldier struck a chord with Andy though as it wasn't dissimilar from his own experiences and highlighted that this perhaps wasn’t just a problem in those with CF but was also applicable to ‘salty sweaters’ too.
As part of Andy’s search for a greater understanding in this area, he connected with Dr Douglas Lewis, a family medicine doctor and CF specialist (FYI, he’s recently written a book all about CF).
Dr. Lewis took a similar view to Andy and they published their own paper in the BMJ, which proposed that exercise-associated hyponatremia (EAH) should be considered on a continuum.
Whilst EAH will always be caused by some form of over-hydration relative to the available exchangeable sodium reserve, viewing the condition as a continuum takes into account the differences between EAH caused by over-hydration in those with normal sweat sodium losses and the increased susceptibility of individuals with excessive sodium losses.
Key takeaway: It's important to replace fluids lost through sweat with some form of sodium supplementation, particularly for those with CF or very salty sweaters.
Precision Fuel & Hydration and the Cystic Fibrosis Trust
Whilst the links between Precision Fuel & Hydration and Cystic Fibrosis may not at first glance be particularly obvious, we're closely aligned when it comes to the subject of salty sweat.
The Cystic Fibrosis Trust strongly encourages individuals with CF to keep active and undertake activities which causes them to get out of breath as it can clear mucus in their lungs and improve physical strength, bulk and overall health.
This physical exercise obviously increases sweat losses, so it’s important that those with CF have an awareness of their additional sodium needs and supplement with an appropriate, high-strength electrolyte drink to avoid detrimental health effects.
