If there’s one thing we can take away from decades of nutrition research, it’s that eating healthy and losing weight are about far more than simply how much we eat. Maintaining a healthy metabolism and supplying our bodies with necessary nutrients mean paying attention not only to what kinds of food we eat, but how it’s prepared.
And to add another layer of complexity to the equation, recent research shows that when we eat our meals also affects our metabolism.
Meal timing affects our circadian rhythm and metabolism, which means that what time we eat breakfast, lunch and dinner has an impact on what our bodies do with those nutrients and calories. If that seems like too much to think about when you’re planning your weekly eating, don’t fear. While more research into meal timing is needed, the science so far has a few simple, solid conclusions that everyone can use right now.
Staying true to your circadian clock
Our bodies run on an internal timekeeper often referred to as the “circadian clock.” The “clock” is mostly located in the suprachiasmatic nucleus (SCN), which is in a part of the brain called the hypothalamus. As the hours pass, the SCN sends signals to regulate activity throughout the brain and body.
It’s what tells your body to start getting sleepy at night, and wake up in the morning, among many other cyclical processes (Roenneberg and Merrow, 2016). But unlike a classic clock, which runs independently, a number of external factors help keep our SCN “ticking” as it should, sending out health-promoting signals that keep us aligned with the day-night cycle.
One of these is the presence or absence of light. This is one of the main reasons going outside often – especially first thing in the morning – is a good idea, and can help improve health.
Eating is another factor that influences our circadian clocks: When we eat meals, it sends a signal to our body to ramp up or down certain processes like digestion and metabolism (Wehrens et al., 2016).
A 2019 review study on meal timing, aging and metabolic health found a consensus across multiple studies suggesting that eating at the wrong time can upset our circadian rhythms (Kessler and Pivovarova-Ramich, 2019). This alters our internal state in a number of ways, but the outcome seems to be consistently worse health outcomes.
What’s the “wrong time?”
Many studies find a consistent relationship between eating later in the day and worse health. That seems to be especially true if we eat shortly before going to bed. But those nighttime snacks, unhealthy though they may be, are hard to resist.
A 2015 study that used a smartphone app to track eating habits found that participants ate 35 percent of their calories after 6:00 pm (Gill and Panda, 2015). But when the researchers had them restrict their eating to a shorter time window that ended earlier in the day, the participants reported losing weight and sleeping better.
A similar study provides more insight into how the body reacts to later meals. The researchers looked at two groups of people: one that ate lunch earlier in the day and one that lunched later, around 3:30 pm (Bandín et al., 2015). The group that ate later burned fewer calories at rest, a signal its members’ metabolisms weren’t as active. (Read more: Can Larger Breakfasts (or Lunches) and Smaller Dinners Keep Pounds Off?)
It’s a finding backed up by another study that found that participants who ate closer to bedtime were more likely to be overweight or obese (McHill et al., 2013).
Another study simply swapped the largest meal of the day for two groups of people. One group ate most of their calories for breakfast, the other ate most of their calories for dinner. The big breakfast eaters came out on top, with greater weight loss and better levels of insulin and triglycerides, a type of fat molecule that, when blood level is high, is associated with poorer health (Jakubowicz et al., 2013).
But not every study shows clear impacts from eating meals earlier in the day. One study from 2020 fed overweight participants with diabetes or prediabetes the same meal, but had half of them eat it early in the day and half of them eat it later. In preliminary research presented at the American Heart Association’s Scientific Sessions 2020, the researchers found that both groups lost weight and had lower blood pressures.
Still, many studies do indicate that eating meals earlier is better for you, even if scientists don’t know exactly why. It’s something further research will tease out, and likely reveal more nuances about what kinds of meal timings work and which don’t.
Keeping to a regular eating schedule – that is, one that’s consistent day after day – can also be beneficial. Meal timing likely influences a few important markers of cardiovascular and metabolic health, according to the American Heart Association. They note in a statement that irregular eating schedules are linked to changes in lipid profile, insulin resistance and blood pressure.
Studies in mice and rats back up the data from human trials, too. Eating at odd, inconsistent hours can lead to obesity, insulin resistance and inflammation in rodents, reports a 2018 review of meal timing and health (Allison and Namni, 2018). A separate study of mice genetically altered to upset their circadian rhythms found they developed metabolic syndrome, the name for a cluster of conditions including obesity, glucose intolerance and poor cardiovascular health (Turek et al., 2005).
How time-restricted feeding works
The simple takeaway from the research is that we should restrict our eating to shorter time periods during the day, and eat more of our daily calories earlier. In fact, there’s already a name for this: time-restricted eating. A number of studies shows that keeping meals to a 10- to 12-hour window during the day is associated with weight loss and improved markers of health.
People who restricted meals to no more than 12 hours in each 24-hour period consistently saw between a one and three percent reduction in weight, according to a 2014 review study (Rothschild et al., 2014). They also had lowered levels of triglycerides in their bloodstreams, better cholesterol profiles and healthier blood glucose levels.
In the case of time-restricted feeding, it likely works because we’re aligning our eating patterns more closely with natural circadian rhythms that affect how active our metabolism is and how our bodies process the food we eat.
Research consistently shows that eating heavy meals late in the day is associated with higher weight and poorer metabolic health. Eating more calories earlier in the day, whether that’s for breakfast, lunch or just an early dinner is a simple way to avoid the negative effects of late eating. While it’s tempting to give in to the urge for a snack before bed, remember that it will probably be worse for your health than eating that same snack the next morning.
Meal timing is just one piece of the puzzle. If you’re eating unhealthy food, it’s bad for your health no matter when you eat it. Look for whole foods, and go light on sugar, processed flour, seed oils (such as “The Hateful 8”) and other less-than-healthy options.
Include foods with protein and fat in your meals, especially, later in the day — these keep you feeling full much longer than carbohydrates do and cut down on the chances you’ll want a snack later that night. And if you’re really feeling tempted, remember those leftovers will always be there in the morning!
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Bandín, C., Scheer, F., Luque, A. et al. Meal timing affects glucose tolerance, substrate oxidation and circadian-related variables: A randomized, crossover trial. International Journal of Obesity 39, 828–833. 2015; https://doi.org/10.1038/ijo.2014.182
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