Is global warming driving diabetes?

Global warming could be fanning the worldwide diabetes epidemic, according to a study published this week in the journal BMJ Open Diabetes Research & Care. However, it’s a conclusion that has been questioned by others in the field.

The researchers, led by Lisanne Blauw of the Department of Medicine at Leiden University Medical Centre in Germany, found that a 1°C rise in ambient temperature could be responsible for 100,000 annual cases of diabetes in the US alone.

Diabetes affects 415 million people across the globe. The condition is a major cause of stroke, kidney disease and heart attack. The World Health Organisation (WHO) predicts it will be the seventh highest cause of death by 2030.

To arrive at their unsettling conclusion, Blauw’s team analysed data from the US Diabetes Surveillance System across all US states and territories between 1996 and 2000, and from 190 countries included in the WHO Global Health Observatory.

The authors used data on mean annual temperature from the US National Centre for Environmental Information and the Climatic Research Unit at the University of East Anglia. They found that for every 1°C rise in temperature the US incidence of diabetes rose by 0.314 cases per 1000 people and the global prevalence of glucose intolerance went up by 0.17%, or 1.7 per 1000 people. The results held after adjusting for the effects of obesity.

But how could a warming planet possibly make diabetes worse? The researchers speculate that it might all come down to a little known site of body fat storage called brown adipose tissue (BAT). In cold weather BAT gets active as fat reserves are burned to generate heat. In the process, increasing amounts of free fatty acids, a decent proportion of which we eat as animal fats and vegetable oils, are deposited in it.

The researchers hypothesise that hot weather reverses this. Lower BAT activity means more free fatty acids circulating elsewhere, where they can hamper insulin doing its job of clearing glucose from the blood, and have been implicated in diabetes.

The findings have been received cautiously by Australian experts. Monash University physician and epidemiologist Barbora De Courten stressed that poor diet, physical inactivity and socioeconomic disadvantage are likely to be more important determinants of diabetes than ambient temperature.

“The study shows no association between prevalence of diabetes and ambient temperature in Europe or Asia. In the South Pacific, which drives the association, transition to a Western diet is a more plausible cause,” she says. “Importantly, our environment is not always determined by outside temperatures. We’ve modified our environment to make it more liveable. In Ottawa you have central heating and in Arizona you have efficient air conditioning.”

Paul Zimmet, leading diabetes researcher at Monash and Honorary President of the International Diabetes Federation, said, “Some data on diabetes are self-reported, which is rather unreliable, and the use of fasting plasma glucose in both US and global studies to diagnose diabetes underestimates its prevalence by up to 30%.

“While temperature might contribute to the epidemic, I suspect it is pretty minor.  Type 2 diabetes is very heterogeneous with many potential risk factors.”