That the world is heating up is beyond doubt, but the subtler effects of rising temperatures on human health are largely unknown.
A paper published in BMJ Open Diabetes Research & Care is one of the first to explore the potential impact of climate change on chronic disease.
The observational study shows a relationship between type 2 diabetes and climate change using US data on diabetes incidence over 14 years and WHO data on glucose intolerance around the world.
Every time the global climate ratcheted up 1°C , the diabetes prevalence increased by 100,000 people in the US each year, the paper reported. Similarly, the worldwide prevalence of glucose intolerance increased by 0.2% per 1°C rise in temperature.
Causality could not be established in the study, but the authors argued that the associations between diabetes and global temperature could not be explained by other factors, such as obesity, age, sex or income.
“It is tempting to speculate that the mechanism underlying our present findings is related to interplay between BAT (brown adipose tissue) activity and glucose clearance from the circulation by metabolically active tissues,” the authors wrote.
Previous studies have suggested that exposure to cold temperatures can increase the uptake of glucose by skeletal muscle in patients with type 2 diabetes.
“[This] is probably explained by an increased overall flux of fatty acids towards BAT, reducing fatty acid flux to other tissues,” the study said.
Dr David Shearman, a gastroenterologist and Emeritus Professor of Medicine at the University of Adelaide, said the link between climate change and chronic disease was not unexpected.
Hot climates put humans under increased physiological stress, altering their metabolism, he said.
“It’s all logical. There is only one paper showing that so far, but it’s a promising hypothesis.”
Climate change was already having health impacts through heatwaves, bushfires, floods and the spread of infectious diseases, Professor Shearman said.
The effects of increased global temperatures on chronic disease were only beginning to become apparent, he said.
“We are moving into unknown territory.”
Other experts have taken a more sceptical approach to the study.
Associate Professor Barbora de Courten, an epidemiologist at Monash University, said the increased rates of diabetes were more likely to be due to lifestyle and diet than climate change.
“If one looks closely at the plot for the worldwide association [in the paper], you find … the correlation is driven by western Pacific region, which recently underwent transition to westernised lifestyle,” she said.
Paul Zimmet, a Professor of Diabetes at Monash University, also said it was unlikely that temperature played a major role in diabetes, which had many potential risk factors.