Longevity Briefs: Hot Weather Might Accelerate Ageing

Longevity briefs provides a short summary of novel research in biology, medicine, or biotechnology that caught the attention of our researchers in Oxford, due to its potential to improve our health, wellbeing, and longevity.

The problem:

Heat is thought to be a hormetic, meaning that limited exposure is associated with health benefits while high exposure is dangerous. Numerous studies suggest that sauna use is associated with improved cardiovascular health, while long-term exposure to heat such as during heat waves places strain on the heart and increases the risk of cardiac events like heart attacks. There’s also some reason to believe that heat exposure might influence the fundamental biology of ageing. Temporary exposure to high temperatures activates the production of heat shock proteins, which protect cells from age-related damage and help other proteins to fold into the correct shapes. But what about long-term exposure to heat stress, such as from hot climates?

In this study, researchers investigate whether hot weather is associated with accelerated ageing according to DNA epigenetic clocks. These are algorithms that estimate how quickly someone is ageing by looking at the presence of methyl groups on the DNA molecule – small molecular ‘tags’ that are added or removed from the DNA to influence how the genetic code is read. These tags are added and removed from certain significant locations within the DNA throughout life and can disrupt gene function. By comparing patterns of methylation to the population average, it is possible to estimate whether someone is ageing more or less rapidly than expected.

The discovery:

In this study, researchers recruited a representative sample of 3686 adults in the US with an average age of 68.6 years. They collected blood samples and calculated age acceleration using three different epigenetic clocks. They then compared age acceleration to outdoor temperatures recorded in their neighbourhoods over various different time windows, extending up to six years before their blood was collected. After controlling for confounding factors such as socioeconomic status, there were statistically significant associations between age acceleration and the number of days meeting the criteria for caution (26.7° to 32.2°C) and extreme caution (32.2° to 39.4°C). These associations were significant for all methylation clocks when considered over a time window of at least one year. In the case of the PhenoAge clock, associations were statistically significant for all time windows.

Since these graphs can be a bit confusing at first glance, it’s worth spending some extra time to explain them. Each graph shows the epigenetic age acceleration for a different epigenetic clock. The orange/red dots represent, for each time window, how much epigenetic age accelerates if 100% of days within that time window were of the corresponding heat index, assuming a proportional increase. For example, for the PhenoAge clock at the 7 day window, 7 days of caution+ heat index correspond to a 1.15 year acceleration in epigenetic age compared to no heat days.

The implications:

This study suggests that people living in areas with higher exposure to high temperatures may undergo accelerated epigenetic ageing. The fact that only one epigenetic clock showed age acceleration for time windows under a year may be due to different epigenetic clocks emphasizing different methylation sites within the DNA. Some methylation changes may be short term responses to high temperature that subsequently reverse when conditions return to normal, while long-term exposure may lead to more permanent changes.

It’s important to note that even though researchers attempted to control for confounding factors, it’s impossible to rule out their influence entirely. The South-East of the US is generally the hottest and also contains many of the most unequal states in terms of income. It’s also noteworthy that just because someone lives in an area that is frequently exposed to high temperatures, this does not necessarily mean that they themselves are frequently exposed. Some people may spend more time outside or may have better access to temperature-controlled interiors, which opens up more confounding factors that are hard to control for. Spending more time outdoors because of good weather also results in other exposures like ultraviolet radiation and pollution, which are known to be associated with accelerated epigenetic ageing as well.