Global warming has aggravated in the last century and has impacted the world in diversified aspects. However, one may not know it can also lead to a potential increase in the incidence of diabetes until a recent study was published online in BMJ Open Diabetes Research & Care, which investigated the relationships between the rise of outdoor temperatures and the incidence of type 2 diabetes and glucose intolerance in the United States and the globe respectively.1
This research is the first to globally examine the prevalence of glucose intolerance worldwide and the surge of outdoor temperature. “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 United States last year,” stated Lisanne L Blauw, the lead investigator of the study.2
A meta-regression model was used to analyze the association between mean annual temperature and incidence of diabetes during 1996-2009 for each state in the United States respectively. Results were then pooled in a meta-analysis and compared with another meta-regression analysis that assessed the relationship between mean annual temperature and the global prevalence of glucose intolerance.1 Data were obtained from the Centers for Disease Control and Prevention (CDC) in the United States and the Global Health Observatory online system from the World Health Organization (WHO).1
Surprisingly, it was shown that every 1oC increase in the annual temperature would prompt over 100,000 new diabetes cases in the United States alone with respect to the population data in 2015.1 Similarly, the worldwide prevalence of glucose intolerance would increase by 0.170% per 1oC rise in the annual temperature.1
The study investigators attempted to explain such observation with the activity of brown adipose tissue (BAT). Admittedly, BAT is activated by cold exposure3-5 and generates heat in response. Meanwhile, studies have suggested that this activation may also improve the insulin sensitivity.6-7 Therefore, the elevation of outdoor temperature may reduce the activity of BAT and negatively influence the glucose metabolism.
However, the associative design of this study cannot conclude the causality hypothesized by the investigators. “I think calorie consumption and weight are probably the biggest by a country mile,” mentioned Dr. Adrian Vella, an endocrinologist researching type 2 diabetes at the Mayo Clinic. “I think the general message always should be that association studies do not actually imply causation (of linking climate change and diabetes),” he explained.8
Dr. Christian Koch, the professor of endocrinology at the University of Mississippi Medical Center, agreed with the lack of causation shown in the study. “Although both temperatures and diabetes rates are rising, there’s no causality between the two.” He also pinpointed the defects of the study. “Importantly, the study did not include two key factors when looking at this association: physical-activity levels and indoor climate control – namely, air conditioning. As most people spend the majority of their days exposed to indoor temperatures, and therefore, outdoor temperatures would not have such a significant effect.”9
Nevertheless, the investigators believed despite the need for more research, this study posed a new hypothesis. “It is tempting to speculate that the mechanism underlying our present findings is related to an interplay between BAT activity and glucose clearance from the circulation by metabolically active tissues,” written the lead investigator.
Indeed, an International Diabetes Federation report suggested that type 2 diabetes and climate change may be correlated. “In hotter temperatures, dehydration and heatstroke increases morbidity and mortality in people with diabetes. People with diabetes are predisposed to cardiovascular events during heat waves and higher mortality from heart attack on days of high air pollution,” stated in the report.10
The influence of climate change on our health has been showcased in another report from the Medical Society Consortium on Climate & Health in the United States. Detrimental consequences on our health from climate change, such as heat waves, deteriorated air quality, hazardous weather events, food-related infections, to name but a few, were highlighted in the report. “We — physicians in medical societies representing over half of the nation’s doctors — see a need to share our growing understanding and concern about the health consequences of climate change with all Americans,” concluded by the authors of the report.