Global Warming Link to Diabetes Mediated by Less Brown Fat?

Veronica Hackethal, MD

March 20, 2017

Rising outdoor temperatures are associated with increased incidence of type 2 diabetes in the United States and higher glucose intolerance worldwide, according to a new study published online March 20 in BMJ Open Diabetes Research & Care.

The research is the first to investigate the link between rising outdoor temperatures and impaired glucose metabolism on a global scale, and the investigators estimate that every 1°C increase in outdoor temperature contributes to over 100,000 new cases of diabetes each year in the United States, based on population data from 2015.

"This emphasizes the importance of future research into the effects of environmental temperature on glucose metabolism and the onset of diabetes, especially in view of the global rise in temperatures with a new record set for the warmest winter in the US last year," write Lisanne L Blauw, a PhD student at Leiden University Medical Center, the Netherlands, and colleagues.

Is Less Brown Fat to Blame?

The authors hypothesize that their observations are related to brown fat, which becomes metabolically active and generates heat in response to cold.

Studies have suggested that cold activation of brown fat may improve insulin sensitivity. Higher outdoor temperatures, though, may decrease brown-fat activity and have a negative impact on glucose metabolism.

In their study, the researchers evaluated the link between outdoor temperature and type 2 diabetes incidence in the United States and prevalence of glucose intolerance worldwide.

As of 2015, type 2 diabetes affected 415 million people worldwide, according to background information in the article.

The researchers used the National Diabetes Surveillance System of the Centers for Disease Control and Prevention (CDC) to obtain data on diabetes incidence between 1996 and 2009 for 50 US states and three US territories (Guam, Puerto Rico, and the US Virgin Islands). Data on mean annual temperature per state came from the National Centers for Environmental Information.

They also used the World Health Organization's (WHO) Global Health Observatory to obtain data on prevalence rates of raised fasting blood glucose and obesity in 190 countries for the year 2014. Data on countrywise mean annual temperature for 2014 came from the Climatic Research Unit, University of East Anglia, United Kingdom.

Age-adjusted diabetes incidence — defined as the rate of adults who reported that they had been told they had diabetes by a healthcare professional in the past year — increased by 0.314 per 1000, on average, for every 1°C increase in temperature.

Adjusting for obesity only slightly weakened the association (incidence rate increased by 0.290 per 1000 for every 1°C increase in temperature).

Analyses adjusting for the passage of time and geographical location suggested that the results were probably not due to between-state differences in these variables.

Global results closely paralleled those from the United States.

Worldwide, the prevalence of raised fasting glucose increased by 0.170% for every 1°C rise in temperature. Likewise, adjusting for obesity only slightly weakened the association (0.106% increase in fasting glucose for each 1°C rise in temperature).

Further adjustment for sex, age, and income suggested that these results were probably not due to international differences in these factors.

The authors report no relevant financial relationships.

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BMJ Open Diabetes Res Care. 2017;5:e000317. Article

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