While public health experts have raised concerns that warming global temperatures may be contributing to obesity, a new study by researchers at the University of Massachusetts Amherst and American University reveals a surprising twist: in the U.S., colder days actually pose a greater challenge to managing weight.

The research, co-authored by Sparshi Srivastava, a sixth-year doctoral candidate in the Department of Resource Economics at UMass Amherst, found that adults in the U.S. were less likely to report attempting to lose weight, including dieting and exercising, when temperatures fell below 68°F (20°C). When the daily maximum temperature rose above 77°F (25°C), people were more likely to report trying to lose weight, consuming the recommended servings of fruits and vegetables, and spending more time exercising.

“What surprised us is the degree to which the weather was a factor in shifting behavior related to weight loss,” says co-author Brandyn F. Churchill, assistant professor of public administration and policy at American University. “Going from the coldest day to the hottest day, we find a roughly 7% change in behavior — just based on whether it’s nice out.”

Despite wide temperature variations across the country, the results were consistent across different locations and demographic groups.

However, Srivastava and Churchill found one notable variation: Overweight and obese individuals were those least likely to pursue weight loss when it was cold and the most likely to do so when it was warm. Still, the study found no evidence that these behavioral shifts had any measurable impact on respondents’ body mass index (BMI).

“For BMI to change, you need consistent habits,” Srivastava explains. “We captured short-term deviations in behavior based on whether it’s really hot or really cold. When the temperature normalizes, people fall back into their normal patterns.”

The study analyzed survey responses from the Behavioral Risk Factor Surveillance System, matched with daily maximum temperatures for each county in the U.S. to pinpoint unseasonable local weather conditions when each respondent was interviewed. The dataset included millions of telephone interviews collected between 1991 and 2010, the years for which county identifiers were available.

Nearly 75% of adults in the U.S. are overweight or obese, according to the National Institutes of Health, with the prevalence of obesity jumping by over a third in the past two decades. Policymakers and public health officials have declared obesity a national epidemic, amid warnings that severe obesity can shorten life expectancy by up to 14 years.

While obesity rates have increased alongside a rise in global temperatures, the researchers found no causal link between the two, despite speculation by some public health researchers that the two may be connected.

“To the degree that people were previously concerned that rising temperatures might be contributing to the obesity epidemic within the United States, we find no evidence of that,” Churchill notes. “While there are many reasons why rising temperatures might be bad for public health, our results suggest that obesity is not one of them.”

Srivastava adds that the findings indicate public health campaigns encouraging Americans to eat more fruits, vegetables and unprocessed foods during periods of extreme cold may yield better results than in warmer periods. However, she cautions that the situation differs in developing countries, where extreme heat may reduce access to nutritious foods — largely due to its impact on subsistence agriculture.

The full study is published in the Journal of Environmental Economics and Management.