Exposure to extreme heat events is linked to molecular changes that could reflect accelerated ageing, according to a preliminary analysis of DNA markers in more than 3,000 people.
The US-based work, presented at the Gerontological Society of America’s Annual Scientific Meeting in Seattle, Washington, last month, joins a host of other efforts to understand the effects of rising temperatures on human health, as heatwaves strike countries around the world with increasing frequency.
Heat is known to strain the heart and kidneys and to slow cognition. But extreme heat could also have effects that are invisible — at least at first. “The physical toll might not immediately manifest as an observable health outcome, but rather could affect our body at the cellular and molecular level,” says co-author Eun Young Choi, a gerontologist at the University of Southern California in Los Angeles. “And that biological deterioration could later develop into disability.”
Speeding up the clock
To find a sensitive measure of heat on the body, Choi and her colleagues turned to the ‘epigenetic clock’, a collection of chemical modifications to DNA that tend to change as people age. Although there is some debate as to how well such measures serve as a stand-in for ageing, previous studies have linked changes in these markers to environmental and social stress, as well as pregnancy and some health conditions.
The researchers analysed 2016–17 data on such markers from about 3,800 people aged 56 or older. They then cross-referenced this data with temperature maps of the United States and looked for correlations between the status of molecular markers and the number of days over various time periods in which the heat index — a measure of perceived temperature that factors in both heat and humidity — exceeded either 26.7 °C or 32.2 °C at the participant’s location.
The researchers also accounted for various factors including race and ethnicity, smoking status, location and income. They found that people who lived in areas with more days of hot weather over the course of either one or six years seemed ‘older’ than those who had experienced cooler weather, on the basis of their molecular markers.
By one measure, every 10% increase in the proportion of hot days added another 0.12 years to participants’ molecular age. Analysis of another set of molecular markers showed that such hot-weather dwellers aged up to 0.6% faster. But shorter heat exposures, on the order of days of months, did not correlate with differences in these markers.
“It’s a remarkable outcome,” says Rina So, an environmental epidemiologist at the University of Copenhagen. The project is unusual, she says, because it looked at biological markers in the blood rather than at death or disease, and evaluated the effect of both long-term and short-term exposure.
Feeling the heat
The study did not consider whether people had air conditioning, or how long they spent outside. It also was not able to track individual responses to heat — something that Choi hopes to do when she can access data from an analysis of the molecular markers based on blood samples taken in 2022.
Even so, it will be difficult for researchers to determine whether such heat is directly causing accelerated ageing, or whether another factor is at play, says Linda Enroth, who studies public health and gerontology at Tampere University in Finland. But, she says, it’s important to explore such possible links. “It’s a new approach and way of thinking that definitely is needed,” she says. “It’s important to understand how this heat is affecting us.”
Nature 636, 534 (2024)
doi: https://doi.org/10.1038/d41586-024-04007-8
