Scientists have long warned that rising global temperatures may impact public health in a devastating way because climate change is associated with deadly weather events, the spread of infectious diseases and even food shortages.
Now, researchers are looking at whether climate change might be linked to another public health concern: Type 2 diabetes.
Between 1996 and 2009, as outdoor temperatures rose across the United States, so did the prevalence of diabetes, according to a study published in the journal BMJ Open Diabetes Research & Care on Monday.
"We were surprised though by the magnitude of the effect size," said Lisanne Blauw, a researcher at Leiden University Medical Centre in The Netherlands and lead author of the study.
"We calculated that a 1-degree Celsius rise in environmental temperature could account for more than 100,000 new diabetes cases per year in the USA alone," she said. "Future research into the effects of global warming on our health status is therefore of great importance."
However, this observational study simply reveals an association between climate and diabetes, not a causation. Among the factors known to cause Type 2 diabetes are being overweight or obese and having a family history of the disease.
"I think calorie consumption and weight are probably the biggest by a country mile," said Dr. Adrian Vella, an endocrinologist who has researched Type 2 diabetes at the Mayo Clinic. He was not involved in the new study.
"I think the general message always should be that association studies do not actually imply causation," he said of linking climate change and diabetes.
Currently, about one out of every three Americans will develop Type 2 diabetes, according to the Centres for Disease Control and Prevention.
For the new study, researchers analysed data on the incidence of Type 2 diabetes in the United States, including the territories of Guam, Puerto Rico and the Virgin Islands, between 1996 and 2009. The data came from the CDC's Behavioural Risk Factor Surveillance System.
The researchers also analysed data on the mean annual temperature per state using data from the National Centres for Environmental Information.
Global data were analysed, too. Information on fasting blood glucose levels and obesity was gathered through the World Health Organization's Global Health Observatory online database.
The researchers found that for every 1-degree Celsius increase in outdoor temperature, there was about a 4% increase of the total diabetes incidence in the United States per year between 1996 and 2009, Blauw said. An increase of 1 degree Celsius is equivalent to an increase of 1.8 degrees Fahrenheit on the Fahrenheit temperature scale.
The worldwide prevalence of glucose intolerance rose by 0.17% per 1-degree Celsius increase in temperature, the researchers found.
"To the best of our knowledge, this study is the first to assess the association of outdoor temperature with diabetes incidence and the prevalence of raised fasting blood glucose on a national and global level," the researchers wrote.
Although more research is needed to determine why -- and how -- this correlation even exists, the study posed one hypothesis that involves the way cooler temperatures can activate a type of body fat called brown fat, or brown adipose tissue.
"The function of brown fat tissue is to burn fat to generate heat, which is important to prevent a decline in body temperature during cold exposure. Therefore, we hypothesize that brown fat plays a role in the mechanism underlying the association between outdoor temperature and diabetes," Blauw said. "In warmer climates, brown fat may be less activated, which may causally lead to insulin resistance and diabetes."
On the other hand, Vella said that brown fat doesn't have that much of an impact on metabolism in humans.
"In humans, brown adipose tissue likely explains about 1% or 2% of energy expenditure in cold situations, and shivering explains far more, so it's an exaggeration," he said.
"Between 1996 and 2009, I believe the environment has changed," he added. "Lots of things changed that might change diabetes incidence, right? The actual population composition changed a little bit. The caloric consumption of that population likely changed. We don't know about physical activity."
In 2012, an International Diabetes Federation report indicated that Type 2 diabetes and climate change may be interconnected public health concerns.
Climate change threatens the fresh food supply, which may negatively impact the diets of those with diabetes, according to the report. Also, deadly heat waves may weaken overall health due to dehydration.
"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," according to the report.
Meanwhile, an increased prevalence of diabetes may lead to more carbon emissions being generated by the health care systems treating those patients.
"Diabetes-related complications -- such as (cardiovascular disease), stroke and renal failure -- cost lives and money. Hospitalizations from such complications are also energy-intensive and increase (greenhouse gas) emissions," according to the report.
Last week, the Medical Society Consortium on Climate & Health released a "medical alert" report titled "Climate Change Is Harming Our Health".
The report outlined how, across the country, climate change may lead to deadly heat waves; worsened outdoor air quality; hazardous extreme weather events; food-related infections; water-related infections; mosquito- and tick-borne infections; wildfires; and higher levels of depression, anxiety and PTSD among flood victims and farmers.
In the executive summary, the authors wrote, "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."
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