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Obesity Linked to Burning Less Energy During Daytime

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A new study published in the journal Obesity has shown that body weight significantly impacts the timing and manner in which the human body burns energy. This research could offer new insights into understanding and managing obesity.

Metabolism changes

The study, led by Dr. Andrew McHill, an assistant professor at the OHSU School of Nursing and the Oregon Institute of Occupational Health Sciences, discovered that individuals with a healthy weight tend to use more energy during the day. In contrast, those with obesity expend more energy at night. "It was surprising to learn how dramatically the timing of when our bodies burn energy differed in those with obesity," McHill stated.

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Additionally, the research found that people with obesity have elevated insulin levels during daytime. Higher insulin levels suggest the body is having to work harder to utilize the energy source glucose. These findings are particularly important as obesity, defined by a body mass index (BMI) of 30 or more, elevates the risk for various health conditions, including high blood pressure and Type 2 diabetes. The findings, said McHill, could not conclusively say whether reduced daytime energy metabolism was a cause or consequence of obesity.

Appetite at night

Our circadian rhythms have powerful effects on the body. Bodily processes like hormone shifts are carefully governed by triggers in our body clocks. Schedules that defy these rhythms, like night shift work, are linked to worse health outcomes.

Dr. Steven A. Shea, the study's senior author and director of the Oregon Institute of Occupational Health Sciences at OHSU, together with McHill, focuses on how circadian rhythms and sleep impact the human body. Shea published work in 2013 that showed circadian clocks lead to an increased appetite during nighttime.

To conduct this study, the team invited 30 volunteers to participate in a rigorous five-day experiment at a specialized circadian research lab. The participants were specifically chosen to reflect a range of body sizes, as previous circadian rhythm studies had focused on people of healthy weight.

Unique sleep schedules

The study tasked participants to follow unique sleep schedules that would challenge their circadian rhythms. After sleep, participants completed a series of tests, such as exercising while wearing a mask connected to an indirect calorimeter, a device which estimates energy usage by measuring exhaled carbon dioxide. The participants donated blood samples, which the researchers analyzed to measure glucose levels in response to a standardized meal provided each day.

This study not only contributes to our understanding of how obesity affects the body but could also lead to better-informed strategies for how to address this increasingly prevalent health issue.

Reference: McHill AW, Thosar SS, Bowles NP, et al. Obesity alters the circadian profiles of energy metabolism and glucose regulation in humans. Obesity. 2023. doi:10.1002/oby.23940

This article is a rework of a press release issued by Oregon Health & Science University. Material has been edited for length and content.