Does 'Brown Fat' Explain a Link Between Temperature and Diabetes?

Are rising temperatures around the world also increasing the rates of diabetes? A new study from the Netherlands suggests that there may be a link between warming global temperatures and a higher prevalence of the disease, but not all experts are convinced.

When the researchers analyzed average global temperatures and the rates of type 2 diabetes, they found that a 1.8-degree Fahrenheit (1 degree Celsius) increase in temperature was associated with an increase of 0.3 cases of diabetes per 1,000 people. In the United States, that would be the equivalent of more than 100,000 new cases of type 2 diabetes each year, according to the study, published Monday (March 20) in the journal BMJ Open Diabetes Research & Care.

The new research may be interesting, but it shows only an association between rising temperatures and diabetes rates, said Dr. Christian Koch, a professor of endocrinology at the University of Mississippi Medical Center who was not involved in the new study. Although both temperatures and diabetes rates are rising, "there's no causality" between the two, Koch added. [5 Ways Climate Change Will Affect Your Health]

Importantly, the study did not include two key factors when looking at this association: physical-activity levels and indoor climate control — namely, air conditioning, Koch told Live Science.

A possible link?

The study looked at the rates of type 2 diabetes in all 50 states, along with Guam, Puerto Rico and the U.S. Virgin Islands from 1996 to 2013. In addition, the researchers looked at data on the average temperatures in each state and territory for the same years.

Overall, the rate of people being diagnosed with type 2 diabetes was higher in warmer years, the researchers found.

Data on worldwide type 2 diabetes rates was not available, the researchers noted. Instead, they used World Health Organization data on the rates of high blood sugar levels, a factor that is linked to diabetes.

For each 1.8-degree-Fahrenheit increase in average temperatures worldwide, the average rates of high blood sugar increased by 0.2 percent, the researchers found.

The findings suggest that, overall, the rates of diabetes in the United States and the rates of high blood sugar around the world increased with higher outdoor temperature worldwide, the researchers wrote in the study. 

Brown fat

The potential link between rising temperatures and diabetes lies in a type of fat called brown adipose tissue, or brown fat, according to the study. [What Is Brown Fat? 5 Fascinating Facts]

Brown fat is metabolically active; it can break down smaller fat molecules to generate heat, the researchers wrote. Previous research has found that colder temperatures can activate brown fat and may lead to modest weight loss, according to the study.

Additionally, in a small study published in the journal Nature Medicine in 2015, a group of researchers found that when patients with type 2 diabetes were exposed to moderately cold temperatures for 10 days, their insulin sensitivity improved. (Decreased sensitivity to insulin, or insulin resistance, can put people on the path to type 2 diabetes.)

The researchers hypothesized that, based on the possible effects of brown fat on insulin, combined with the findings that cold temperatures activate it, warmer temperatures could have the opposite effect — namely, they could be linked to a decrease in insulin sensitivity and an increase in type 2 diabetes.

Not so simple

Koch noted, however, that most people spend the majority of their days exposed to indoor temperatures, and therefore, outdoor temperatures would not have such a significant effect.

In addition, he said the researchers found that increased temperatures had different effects on diabetes rates in two neighboring states: In Louisiana, the rates decreased, but in Mississippi, they increased. If temperature played a role, the findings would be similar in both states, he said.

The study authors, led by Lisanne Blauw, an endocrinology researcher at the Leiden University Medical Center in the Netherlands, noted that their findings do not prove cause and effect. In other words, the study does not prove that rising temperatures directly cause rising diabetes rates but rather highlights an important area of future research.

"These findings emphasize 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 USA last year [2015]," they wrote. 

In addition, the researchers described several limitations of the study. They noted that because the study was observational, confounding factors might have affected the results. In other words, there might have been other factors that had an effect on the results. In addition, the researchers were not able to account for changes in body mass index, which is associated with type 2 diabetes, over time because of a lack of available data.

Originally published on Live Science.