Young female at laptop computer eats instant ramen noodles late in the evening. Woman working or studying online overtime at night has fast dinner – concept of unhealthy junk food at workplace

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BOSTON — Time to switch to the “early bird special.” A new study finds eating dinner late at night increases a number of factors that can lead to obesity.

Researchers from Brigham and Women’s Hospital say late-night eating reduces the burning of calories, increases hunger, and causes changes in fat tissue — all of which can contribute to weight gain. Lab experiments found meals just before bedtime have profound effects on the appetite-regulating hormones leptin and ghrelin.

Health experts have warned against the dangers of late-night snacking for years. These new findings provide new insight on why it can lead to chronic diseases.

“We wanted to test the mechanisms that may explain why late eating increases obesity risk,” explains senior author Frank A. J. L. Scheer, PhD, Director of the Medical Chronobiology Program in the Brigham’s Division of Sleep and Circadian Disorders, in a media release. “Previous research by us and others had shown that late eating is associated with increased obesity risk, increased body fat, and impaired weight loss success. We wanted to understand why.”

More than two billion adults worldwide are overweight or obese, making diabetes, cancer, cardiovascular disease, and other life-threatening conditions more likely. The study found dining at 10 p.m. instead of 6 p.m. simultaneously affected the three main players in body weight regulation. They are energy expenditure, regulation of food consumption, and chemical changes in fat tissue.

“In this study, we asked, ‘Does the time that we eat matter when everything else is kept consistent?’” adds first author Nina Vujović, PhD, a researcher in the Medical Chronobiology Program in the Brigham’s Division of Sleep and Circadian Disorders. “And we found that eating four hours later makes a significant difference for our hunger levels, the way we burn calories after we eat, and the way we store fat.”

Working late at night can affect weight management

Obesity afflicts more than four in 10 adults in the United States. Evidence is growing that when we eat is as important as what we eat. The results published in the journal Cell Metabolism have particular implications for shift workers.

Specifically, levels of the hormone leptin, which signals satiety, were lower across the 24 hours in late eating condition compared to early eating conditions. When participants ate later, they also burned calories at a slower rate and stored more fat.

Importantly, the discoveries combine physiological and molecular mechanisms underlying the link between late night eating and obesity. They are consistent with a large body of evidence suggesting eating too late increases one’s risk of weight gain – and explains how this might occur.

“This study shows the impact of late versus early eating. Here, we isolated these effects by controlling for confounding variables like caloric intake, physical activity, sleep, and light exposure, but in real life, many of these factors may themselves be influenced by meal timing,” Scheer says. “In larger scale studies, where tight control of all these factors is not feasible, we must at least consider how other behavioral and environmental variables alter these biological pathways underlying obesity risk.”

Do mealtimes impact energy balance as well?

The team recruited 16 patients with a body mass index (BMI) in the overweight or obese range. They completed two protocols, one with an early meal and the other with the same about four hours later in the day. For two to three weeks prior to testing, the participants maintained the same sleep and wake schedules.

In the final three days before, the participants strictly followed identical diets and meal schedules at home. In the lab, they regularly documented their hunger and appetite, provided frequent small blood samples throughout the day, and had body temperature and energy expenditure measured.

Researchers collected biopsies of adipose tissue from a subset during both protocols to analyze how eating time affected molecular pathways involved in adipogenesis — or how the body stores fat. The randomized crossover study tightly controlled for behavioral and environmental factors such as physical activity, posture, sleep, and light exposure. It enabled the investigators to detect changes to the different control systems involved in energy balance, a marker of how our bodies use the food we eat.

In future studies, Prof. Scheer and the team aim to recruit more women. The study group included only five female participants. The team set up the study this way to control for menstrual phase, reducing confounding results but making recruiting women more difficult.

Going forward, they are also interested in better understanding the effects of the relationship between mealtime and bedtime on energy balance. Previous research has found eating late at night increases blood sugar levels, raising the risk of Type 2 diabetes, which has a strong link to obesity. It can cause symptoms like excessive thirst, needing to urinate often, and tiredness. It can also lead to serious eye, heart, and nerve problems.

South West News Service writer Mark Waghorn contributed to this report.

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