While global warming is often linked to rising obesity rates, a new study from the University of Massachusetts Amherst and American University challenges that assumption, revealing that colder days—not hotter ones—are more likely to hinder Americans’ efforts to lose weight.
Published in the Journal of Environmental Economics and Management, the study found that U.S. adults are significantly less inclined to diet or exercise when temperatures dip below 68°F (20°C). In contrast, warmer days, especially those exceeding 77°F (25°C), saw an uptick in weight loss attempts, healthier eating habits, and increased physical activity.
“What surprised us is the degree to which the weather was a factor in shifting behavior related to weight loss,” said co-author Brandyn F. Churchill, assistant professor at American University. “From the coldest to the hottest day, we see about a 7% change in behavior—driven purely by the weather.”
The study, led by Sparshi Srivastava, a doctoral candidate in UMass Amherst’s Department of Resource Economics, analyzed data from the Behavioral Risk Factor Surveillance System. Researchers matched millions of survey responses collected between 1991 and 2010 with daily maximum temperatures by U.S. county to examine how short-term weather conditions influenced respondents’ health behaviors.
The findings were consistent across various geographic regions and demographic groups. Notably, individuals classified as overweight or obese showed the most significant behavioral fluctuations—being the least likely to engage in weight loss efforts during cold spells and the most motivated during warm periods.
However, the researchers found no evidence that these short-term behavioral shifts translated into actual changes in body mass index (BMI). “For BMI to change, you need consistent habits,” Srivastava explained. “We’re capturing temporary deviations in behavior. Once the weather returns to normal, so do people’s routines.”
With nearly 75% of U.S. adults now overweight or obese, and obesity rates increasing by more than a third in the last two decades, the issue remains a pressing public health concern. Yet the study found no causal link between rising global temperatures and the obesity epidemic, contrary to some earlier speculation.
“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 stated. “While rising temperatures may pose other public health risks, obesity doesn’t appear to be one of them.”
Srivastava suggested that public health campaigns may benefit from targeting behavior during colder months, encouraging consumption of fruits, vegetables, and unprocessed foods when motivation tends to wane. She added that the findings may not apply globally, noting that in developing countries, extreme heat can harm nutrition by limiting access to food through its impact on agriculture.
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