A new study suggests eating later in the day can directly impact our biological weight regulation in three key ways: through the number of calories that we burn; our hunger levels; and the way our bodies store fat.
With obesity now affecting hundreds of millions of people worldwide, this is a valuable insight into how the risk of becoming obese could be lowered in a relatively simple way – just by eating our meals a few hours earlier.
Earlier studies had already identified a link between the timing of meals and weight gain, but here the researchers wanted to look at that link more closely, as well as teasing out the biological reasons behind it.
"We wanted to test the mechanisms that may explain why late eating increases obesity risk," says neuroscientist Frank Scheer, from Brigham and Women's Hospital in Boston.
"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."
The research was tightly controlled, and involved 16 participants with a body mass index (BMI) in the overweight or obese range.
Each volunteer went through two different experiments lasting six days, with their sleeping and eating tightly controlled beforehand, and several weeks between each test.
In one experiment, the participants kept to a strict schedule of three meals a day around the normal times – breakfast at 9am, lunch at 1pm and dinner around 6pm.
In the other, the three meals were shifted back (the first around 1pm and the last around 9pm) – so lunch, dinner and supper.
Through blood samples, survey questions and other measurements, the team was able to make a number of observations.
When eating later, levels of the hormone leptin – which tells us when we're full – were lower across 24 hours, indicating participants may have felt hungrier. What's more, calories were being burned at a slower rate.
The tests also showed that adipose tissue gene expression – which affects how the body stores fat – increased the adipogenesis process that builds fat tissues, and decreased the lipolysis process that breaks fat down.
Here, we're looking at a combination of physiological and molecular mechanisms pushing up the obesity risk.
"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," says Scheer.
Of course obesity can lead to other health issues, including diabetes and cancer, and so finding ways to stop it from developing in the first place would make a huge difference to the health of the global population.
What this study shows is that eating earlier in the day can impact three key drivers of the way our bodies balance energy and the subsequent obesity risk – and it's a change that's perhaps simpler for some people to manage than sticking to a diet or exercise regime.
In the future, the team wants to see research involving more women (just 5 of the 16 volunteers were women in this case), as well as research that analyzes how changes in bedtime in relation to eating time might also factor into these processes.
"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," says Scheer.
The research has been published in Cell Metabolism.