"Men who visit saunas may be damaging their sperm," the Mail Online website warns. This warning comes from the results of a tiny study involving just 10 men asked to follow a Scandinavian-style sauna programme.
Researchers wanted to look at the effect of long-term sauna use on sperm quality, as previous research has indicated that higher scrotal temperatures in men can lead to poorer sperm health.
The men, who all had healthy sperm at the start of the study, were found to have lower sperm counts and reduced sperm motility (the ability to 'swim' towards an egg) after the prescribed sauna programme, but other aspects of sperm health were unchanged.
However, these negative effects were reversed after the men avoided sauna use for three months, so any damage appears to have been temporary.
Given that only 10 men were involved, the results of this study need to be confirmed by much larger studies. The effects of sauna exposure in men with abnormal sperm also needs to be tested further.
Still, most fertility experts would advise that men having trouble conceiving should keep their testicles cool. Hot baths, showers and wearing tight-fitting underwear can all raise the temperature of the testicles. This advice was bolstered by research published earlier this month that suggests that sperm quality drops during the summer.
The study was carried out by researchers from the University of Padova, Italy. No external funding was reported for this study.
It was published in the peer-reviewed medical journal Human Reproduction.
While the Mail Online's reporting of the story was broadly accurate, it failed to mention the crucial fact that the study only recruited 10 men. This may have led many readers to wrongly assume that it was a much larger study and its findings were therefore conclusive.
This was a tiny cohort study exploring the effects of repeated sauna exposure on various aspects of sperm quality.
The researchers reported that many previous studies found higher testicular temperatures had negative effects on the ability of the testicles to produce sperm. Heat could also cause alternations in the sperm DNA and cause it to self-destruct.
This type of very small-scale study is generally useful for testing a scientific hypothesis at a low cost. In this case, the theory being tested was that regular sauna use may damage or reduce sperm quality.
Small studies like this are generally undertaken to prove an idea rather than prove something with any degree of certainty. Being so small, they are potentially unreliable, biased and unrepresentative of the wider population. This means they don't really provide any sizeable evidence to prove a theory. Much larger studies involving hundreds or thousands of people are required to give any weight to an idea.
The researchers recruited 10 healthy male volunteers (average age 33.2 years) to take part in a Finnish sauna programme consisting of two sauna sessions per week for three months at 80-90°C, each lasting 15 minutes. Very few details about the men were reported, so their nationality, demographics and lifestyle behaviours are unknown. Men who had used a sauna in the previous year were not included in the study, so the 10 men didn't have a history of regular sauna exposure.
Sex hormones, sperm parameters, sperm DNA structure, sperm death ('apoptosis', a process where the sperm self-destructs in response to external stimuli) and the expression of genes involved in sperm response to heat and lack of oxygen were all evaluated at the start (baseline). Further measurements were then taken:
Scrotal temperature was measured with an infrared thermometer before and immediately after each sauna session.
The statistical analysis was basic and compared sperm measures at baseline with the different time points so the effect of the sauna visits on sperm health could be measured.
The average scrotal temperature before the sauna sessions was 34.5°C, which increased to 37.5°C after the session, a statistically significant increase of 3°.
The study found a statistically significant impairment of sperm count and sperm motility both at the end of each sauna session and again at three months. No significant difference in sex hormones was found.
Decreases in the percentage of sperm with normal DNA structure and other internal biological sperm structures were also observed after each sauna session and three months after sauna exposure.
This corresponded with an increase in the activity of sperm genes associated with coping with heat stress and lack of oxygen, which the researchers believe was brought on by the sauna experience.
The major effects of the sauna were to reduce sperm count and motility. Semen volume, sperm structure, and how often the sperm self-destructed did not change throughout the study.
Crucially, the researchers reported that all the effects were reported to be reversed at the six-month time point. This suggests the adverse effects of sauna exposure appear to be temporary, at least in men with healthy sperm.
The researchers concluded that in men with normal sperm, "sauna exposure induces a significant but reversible impairment of spermatogenesis [making sperm], including alteration of sperm parameters, mitochondrial function and sperm DNA packaging."
They went on to suggest that, "the large use of Finnish sauna in Nordic countries and its growing use in other parts of the world makes it important to consider the impact of this lifestyle choice on men's fertility."
Past research has shown that higher scrotal temperatures in men can lead to poorer sperm health. This proof of concept study tells us that this may also apply to men who use saunas regularly.
The study found that in men with initially healthy sperm, repeated sauna exposure over a three-month period lowered sperm count and reduced sperm motility. Many other aspects of sperm health were unchanged.
Crucially, it also showed that these negative effects were completely reversed at the six-month time point – that is, following a further period of three months without using a sauna.
On its own, this research provides very weak evidence due its tiny sample size – just 10 men were recruited. A study that plucks 10 men from a general population of millions is prone to chance findings and significant bias that may not be applicable to the vast majority of men. For these findings to have more weight, more men would need to be recruited to participate in a larger study.
Other areas that need further investigation include the effects of sauna exposure in men who already have abnormal sperm. These findings may be different (for example, the negative effects may not be as reversible) and this issue was not addressed by this study.
It is also worth pointing out that the majority of British men may not use saunas twice a week for months at a time, so the results are only applicable to the minority that do. Even then, larger studies need to confirm this theory before it can be believed with any certainty.
Overall, this research is consistent with existing fertility advice for men hoping to conceive. Men are advised to keep their testicles cool at around 34.5°C, a few degrees colder than the rest of the body. This helps your body produce the best quality sperm.