Global warming, already linked to countless human health problems, may be increasing the incidence of Type 2 diabetes, including 100,000 or more new annual cases in the United States alone, according to a new study published in the journal BMJ Open Diabetes Research & Care.
The worrisome finding comes at a time when the world is living through the hottest years on record, and the incidence of diabetes has reached epidemic levels. Worldwide, the number of people with diabetes has roughly quadrupled since 1980.
More than 29 million Americans have diabetes, and another 86 million have pre-diabetes, putting them at risk for developing the disease. People with diabetes either don’t make enough insulin (Type 1) or can’t use insulin properly (Type 2), resulting in a buildup of blood sugar. High blood sugar can lead to heart disease, stroke, blindness, kidney failure and amputation of toes, feet or legs.
To be sure, heat exacerbates many chronic health conditions, including diabetes. “Hot weather can be more difficult for people with diabetes,” says Mona Sarfaty, director of the Consortium on Climate Change and Health, who was not involved in the study.
“The heat keeps people from being active, which means they expend less calories, which can lead to more weight gain,” a major risk factor for developing Type 2 diabetes, she said. “Also, people with diabetes often have kidney problems. Dehydration — which comes with heat — can worsen kidney problems when people are dehydrated.”
The development of Type 2 diabetes has long been linked to the growing problem of obesity, since the accumulation of fat makes body tissues less responsive to insulin.
But the study names another potentially important mechanism in the onset of diabetes: the harmful impact that higher ambient temperatures can have on metabolism. Higher temperatures reduce the activity of brown fat or “good fat,” which turns food into body heat.
Exposure to cold, on the other hand, stimulates brown fat, prompting modest weight loss and greater sensitivity to insulin — making a person less likely to develop diabetes, according to the study’s authors.
“Obesity only partially explains the association between outdoor temperature and diabetes,” says Patrick Rensen, professor of endocrinology at Leiden University Medical Center in the Netherlands and a co-author of the paper. “We showed that more people get diabetes when the outdoor temperature is higher, but this is not because these people get more obese when it is warmer outside.”
Cold weather activates brown fat, which produces heat to keep the body warm. Rensen explained that activated brown fat “reduces the amount of fat stored in organs, without necessarily reducing body weight. Vice versa, reduced brown fat activity may thus cause glucose intolerance and diabetes by increasing unwanted fat storage in organs without necessarily increasing body weight.”
Authors say this study is the first to demonstrate a link between global warming and glucose intolerance worldwide, and between global warming and the onset of diabetes in the United States.
The scientists theorized that rising temperatures were contributing to the onset of Type 2 diabetes by reducing brown fat activity. They used data from the Centers for Disease Control and Prevention on the incidence of diabetes in the United States. And they used data from the World Health Organization on the prevalence of glucose intolerance worldwide. They compared these numbers with country-level temperature data to see how they changed over time.
Controlling for factors like age, sex, income and obesity levels, researchers found that a 1.8º F increase in average temperature was associated with an increase in the incidence of diabetes and prevalence of glucose intolerance. An additional three in 10,000 Americans developed diabetes, and an additional 17 in 10,000 people worldwide developed glucose intolerance. The Earth has warmed by nearly 1.8º F since the pre-industrial era.
Eric Ravussin, associate executive director of the Pennington Biomedical Research Center in Baton Rouge, who was not involved in the study, said the findings regarding brown fat did not surprise him, but he was concerned as to whether the researchers adjusted for racial disparities in the different parts of the country. Hispanics, Asians and Native Americans, for example, he said, “have higher genetic susceptibility to develop insulin resistance and diabetes than the rest of the population.”
Lisanne Blauw, an epidemiologist at Leiden and a co-author of the study, said that “by first doing a per-state analysis, we minimized the effect of potential geographically determined factors — like more Hispanics and Mexican-Americans in the South.”
Researchers acknowledged that physical activity could have played a role in their findings, because exercise varies with outdoor temperature, but data on Americans’ physical activity were not available for the period studied.
However, while a decrease in physical activity when it is hot “may theoretically explain our findings, physical activity has been shown to be highest between daily temperatures of [around 60º F and 70º F] and decreases with higher, but also lower temperatures,” the study says. “Therefore, we do not expect that physical activity completely explains the positive association between outdoor temperature and diabetes incidence.”
The authors stressed that because their study was observational (as opposed to experimental), they could not draw firm conclusions about cause and effect.
“Obviously we cannot prove a causal effect of higher temperature on diabetes in this specific study,” Rensen says. “However, we did show that activation of brown fat by a cold environment has beneficial health effects. So feeling a little cold is not that bad at all.”
Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.