Eat Late, Gain Weight? Study Shows Eating Later Increases Appetite, Reduces Caloric Burn, and Changes Fat Tissue Activity
With 42% of the American population now classified as obese, having an unhealthy body weight could be considered one of our society’s most prevalent public health problems. Accounting for millions of premature deaths each year—and numerous coinciding chronic health conditions—it’s clear that simply advising to “eat less and move more” is not working.
Weight gain is an undeniably complex issue, with myriad factors influencing how we gain, lose, or maintain body weight and fat. Researchers out of Brigham and Women’s Hospital now show that when we eat can play a significant role in how we burn and store energy even if what we eat remains exactly the same. Published in the journal Cell Metabolism, Vujovic and colleagues demonstrate in a carefully controlled human study that delaying meals by about four hours significantly alters appetite, hunger, caloric burn, and fat tissue activity, suggesting that eating earlier may be a simple intervention to support healthier body weights in the general population.
Does Meal Timing Really Matter?
Although midnight snacking has never been thought of as a healthy habit, researchers haven’t quite pinned down the mechanisms behind why late eating is linked to increased weight—and experimental trials have been lacking on this subject. Now, Brigham and Women’s researchers show how later eating affects weight by analyzing energy intake and expenditure, hunger hormones, and molecular pathways that affect fat storage. As this study regulated nutrient intake, physical activity, sleep, and light exposure by having participants remain in a lab during the study, this is the most rigorously controlled trial to date on this topic.
“We wanted to test the mechanisms that may explain why late eating increases obesity risk,” explained senior author Frank A. J. L. Scheer, Ph.D., Director of the Medical Chronobiology Program in the Brigham's Division of Sleep and Circadian Disorders. “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.”
Later Eating Linked to Undesirable Fat, Energy, and Hormone Levels
In this study, the research team studied 16 overweight or obese people and had them complete two protocols in the laboratory—one “early eating” schedule and one “late eating” schedule, which had meals shifted by four hours (the equivalent of eating dinner at 9 pm versus 5 pm, for example). Both eating windows had exactly the same amount of calories and macronutrients, but each meal in the late eating schedule was pushed further into the day.
The researchers regularly documented the participants’ hunger and appetite levels—both subjectively with questionnaires and analytically with blood samples of hormone levels—and measured their body temperature and energy expenditure throughout the day.
They found that later eating increased feelings of hunger and appetite, with late eating doubling the odds of being hungry compared to early eating. This was also verified with blood measures of our main appetite hormones—leptin, which promotes feelings of fullness, and ghrelin, which increases hunger. During the waking period, late eating significantly decreased leptin levels by 16%, indicating that people would be less satiated. The ratio of ghrelin-to-leptin also increased by 34%, suggesting that people would be feeling more hungry during the day.
These results also correlated with lower energy expenditure—the number of calories burned. Late eating decreased waking energy expenditure by about 60 calories, or 5% fewer calories burned compared to the early eating window. While it doesn’t sound like a ton, this equates to 21,900 fewer calories burned per year, which can certainly influence weight over time.
Lastly, Vujovic and colleagues measured how eating time affected molecular or genetic pathways involved with adipogenesis—the storage of fat. In a subset of seven participants, late eating altered several pathways related to fat metabolism that were consistent with increased fat storage and reduced lipolysis (fat breakdown for energy). These changes may promote fat mass accumulation or weight gain.
As the authors state in their paper, “Our results show that late eating consistently altered physiological functions and biological processes involved in regulation of energy intake, expenditure, and storage—each of these three in a direction favoring weight gain.”
Rethink That Midnight Snack
Although many studies have linked later eating to increased weight gain, this is the first tightly controlled trial to detect changes in the different mechanisms involved in energy balance—such as appetite hormones, adipose tissue molecular pathways, and energy expenditure.
The researchers state that future research is needed to verify these results in greater populations, as this was a small study with only five females. However, unlike many other studies, the researchers controlled for the menstrual phase and the appetite and energy balance changes that menstruation can induce. Plus, the authors believe that the effects of late eating may be even more pronounced in the “real world,” where all the variables of day-to-day life are not as regulated.
Senior author Frank A. J. L. Scheer, Ph.D., Director of the Medical Chronobiology Program in Brigham's Division of Sleep and Circadian Disorders, concludes, “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. 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.” Overall, it seems like we should all aim to hit the early bird special and eat our dinners while the sun’s still out.
Vujović N, Piron MJ, Qian J, et al. Late isocaloric eating increases hunger, decreases energy expenditure, and modifies metabolic pathways in adults with overweight and obesity. Cell Metab. 2022;34(10):1486-1498.e7. doi:10.1016/j.cmet.2022.09.007