Those With Obesity Burn Less Energy During The Day

"It was surprising to learn how dramatically the timing of when our bodies burn energy differed in those with obesity," said the study's first author, Andrew McHill, Ph.D., an assistant professor in the OHSU School of Nursing and the Oregon Institute of Occupational Health Sciences at OHSU. "However, we're not sure why. Burning less energy during the day could contribute to being obese, or it could be the result of obesity."

A mountain of evidence shows that being overweight or obese increases the risk for a variety of health conditions like type 2 diabetes and high blood pressure. Body mass index (BMI) is a person’s weight in kilograms divided by the square of height in meters, and a high BMI can indicate high body fatness. The CDC suggests that an adult BMI of less than 18.5 falls within the underweight range, if it falls within 18.5 to 25 it is within the healthy weight range, if it falls within 25 to 30 it is in the overweight range, and if your BMI is 30 or higher it falls within the obesity range. Obesity is frequently subdivided into categories with Class 1 having a BMI of 30 to 35, Class 2 ranging between 35 to 40, and Class 3 is having a BMI of 40 or higher, and this range is also sometimes categorized as severe obesity. 

Every 24 hours people experience numerous changes that are triggered by the body’s internal clock, these changes typically occur at certain times of the day to best serve the body’s needs at any given time. However, personal schedules, when people sleep, eat and exercise can also affect our health by either complementing or going against these natural rhythms. Previous research suggests that circadian rhythm misalignment affects energy metabolism and glucose regulation, this study focussed on how circadian rhythms and sleep impact the human body. 

For this study, 30 volunteers stayed in a specially designed circadian research lab for 6 days following a rigorous protocol involving a schedule sequence design to have participants be awake and sleep at different times throughout each day. After each sleep period, the participants ate and underwent a variety of tests for the remainder of the day such as measuring exhaled carbon dioxide to estimate energy usage, and blood tests to measure glucose levels in response to meals, among others. 

Moving forward, the team plans to investigate eating habits and hunger among those who are obese and their healthy weight counterparts to follow up on a 2013 study that found circadian clocks naturally increase food cravings during the night.