We all know the importance of applying sunscreen when the sun is shining and we’re exercising outdoors. But, does slapping on the sunscreen impede our ability to sweat?
It’s a great question we received from a curious athlete when they emailed firstname.lastname@example.org and it was genuinely the first time we’ve been asked about the effects of sunscreen on sweat, so I just had to dive in...
Why we use sunscreen
In hot, outdoor conditions, humans need protection from solar radiation (i.e. the UV rays emitted by the sun). Without stating the obvious, that’s where sun cream comes in as it helps protect our skin from the sun’s harmful UV rays.
Preferably, we need this protection to occur without impeding effective sweat production when we’re exercising because - if used over large skin areas - it could impair thermoregulation (i.e. the processes that help our body maintain its core internal temperature).
Sweat is the primary method that the body uses to offload metabolic heat to the environment.
Intrigued, I took a look at the scientific literature to see what the research into the relationship between sunscreen and sweat had found.
What the research says about sunscreen and sweat
Unfortunately, the application of sunscreen and its potential to interfere with sweating hadn’t received a great deal of attention.
But fear not, there’s one paper that came out of the Human Movement Science Research Centre at the University of Costa Rica in 2016 which proved very interesting.
The research team compared the effects of two different sunscreens and a no-lotion condition on localised sweat production (interestingly, they also included a fourth comparative treatment: anti-perspirant).
The tested sunscreens were both commercially available options, water-resistant and had a high sun protection factor (SPF) of 50. Sunscreen A - Banana Boat Sport - was an organic lotion (i.e. contains lots and lots of carbon-carbon bonds) and was a sport-specific brand, whereas Sunscreen B - Nivea Sun Solar Protection 50 - was an inorganic lotion (i.e. no carbon-carbon bonds).
The study design was simple - 20 participants cycled for 20-minutes on two occasions, each time with two conditions ‘applied’ to the shoulder blade. Lotion application was followed one hour later by placing an absorbent patch to collect the sweat. The patches were weighed pre- and post-exercise to determine the volume of sweat perspired.
The results were interesting. Localised sweat rate was lower when Sunscreen B (Nivea) was applied in comparison to the organic Sunscreen A or the no-lotion control. (For anyone interested, the antiperspirant was the lowest of all - see graph below).
So, what caused the difference in sweat rate?
The major active ingredients in the lotions were different: oxybenzone for Sunscreen A and titanium dioxide for Sunscreen B. Also, it was speculated that ‘phenylbenzimidazole sodium sulfonate’ - a secondary ingredient consisting of very small particles and used in combination with titanium dioxide in Sunscreen B - may penetrate the skin and physically interfere with sweating.
Ultimately, the paper highlighted a potential conflict between thermoregulation and the need for skin protection when you're out in the sun (i.e. high UV). It was clear that future research should aim to identify the ingredients that caused this interference with sweating.
What the paper didn’t investigate though was the physiological relevance of Sunscreen B and specifically its impact on core temperature. Whilst a reduction in sweat rate was measured in this case, it only makes a significant difference if it negatively impacts body temperature.
After all, a raised core body temperature is the limiting factor on performance.
A couple of earlier studies on sunscreen and exercise performance had fortunately looked into this and had measured core temperature changes (more here). After exercising for 45-minutes with and without sunscreen in hot conditions, no differences were found for rectal temperature or total body sweat loss.
Whilst this is encouraging to read and suggests that ‘real-world’ effects may be limited, only one type of sunscreen was used and there wasn’t any information provided about the sunscreen brand or ingredients. Therefore, it’s feasible that the one tested was similar in formulation to Sunscreen A from the study in Costa Rica and didn’t have an impeding effect on sweating.
To play the devil’s advocate, had the sunscreen been like B, would a difference have been seen?
Does sunscreen affect how much you sweat?
I think it’s pretty clear that when it comes to the effects of applying sunscreen and thermoregulation, we need much more research to make any solid conclusions.
One question worth asking is does the body upregulate sweating in other areas to compensate for an impediment in areas where sunscreen has been applied?
It does appear some sunscreens may impede our ability to sweat, but not all of them. (Note: Sunscreen A was marketed for use in sports and so whilst it can be easy to put that kind of thing down to ‘marketing’, maybe the manufacturers are onto something).
It’s also important not to get tunnel-vision with the research and disregard our own real-world experiences. A lot of us will know that wearing sunscreen hasn’t stopped us from sweating buckets when exercising (you’re definitely made aware of your sunscreen-tainted-sweat when it eventually runs into your eyes and stings like hell).
Therefore, it’s important to remember that what’s clinically significant (i.e. what matters in the ‘real-world’ and impacts on performance) should be given greater weight than statistical significance found in the laboratory.
Yes, some sunscreens may negatively impact our ability to sweat but enough to influence the body’s core temperature and impact on an athlete’s performance? It doesn’t seem likely.
So, keep slapping on the sunscreen when the sun is shining to make sure you're protecting yourself from those harmful UV rays. The health benefits of wearing sunscreen far outweigh the negatives of your sweat stinging your eyes when exercising.
And for now, we await further research that might help us understand whether we should be using different types of sunscreen for exercise.