“Eat like an emperor in the morning, like a king at noon, and like a beggar at night”—this old dietary rule sounds more like something from Grandma’s kitchen than modern metabolic research. Yet it’s apparently not entirely without basis. That’s because the body processes a meal differently in the morning than it does in the evening.
When the Body’s Internal Clock Comes into Play
After eating, energy expenditure temporarily increases: digestion, absorption, transport, and storage of nutrients all require energy. This diet-induced thermogenesis—DIT for short—apparently follows its own circadian rhythm.
This is demonstrated by a recent study involving 16 adults who were overweight or obese. To isolate the influence of the body’s internal clock as clearly as possible from sleep, exercise, light, and usual mealtimes, the participants spent 36 hours under strictly controlled conditions and were given identical meals every six hours. DIT peaked in the biological morning and reached its lowest point in the evening. The difference between the peak and trough values was initially 44 percent; after accounting for resting energy expenditure—which also fluctuates circadianly—the difference was reduced to about 29 percent.
Eating in the morning: a faster way to lose weight?
However, this effect is not a new finding. Earlier studies had already shown higher thermogenesis after identical meals in the morning compared to the evening. A 2020 study, for example, found that thermogenesis was 2.5 times higher.
Nevertheless, this is not exactly good news for those trying to lose weight: If you factor out the body’s energy expenditure—which is already dependent on the time of day—only about 7 kcal remain over a four-hour period—an amount so small it’s practically negligible on the scale. So a hearty breakfast alone is far from being a weight-loss booster. The results do, however, provide further evidence that when it comes to metabolism, it’s not just what and how much we eat that matters—but possibly also when. And they demonstrate once again just how many factors come into play when it comes to weight loss.