The old saying goes "breakfast like a king, lunch like a prince, and dine like a pauper". It’s based on the notion that eating the bulk of daily calories in the morning optimizes weight loss by burning calories more efficiently and quickly.
According to a new study, though, whether a person eats their largest meal early or late in the day doesn’t affect the way their body metabolizes calories. On the plus side –people who ate their largest meal in the morning did report feeling less hungry later in the day, which could facilitate easier weight loss in the real world.
There have been lots of recommendations about timing of meals for losing weight and overall health. These hve been driven largely by the field of chronobiology, or circadian rhythms. Circadian rhythms are physical, mental and behavioral changes that follow a 24-hour cycle. These natural processes respond primarily to light and dark and affect most living things.
About the study
For the new study, researchers from the University of Aberdeen, Scotland decided to take a closer look at how time of day interacts with metabolism.
The investigators recruited healthy people who were overweight or obese to have their diets controlled and their metabolisms measured over a period of time. Sixteen men and 14 women completed the study.
Each participant was randomly assigned to eat either a morning-loaded or an evening-loaded diet for four weeks. The diets contained the same number of calories and provided 30% protein, 35% carbohydrate and 35% fat.
After a washout period of one week in which calories were balanced throughout the day, each participant crossed over to the opposite diet for four weeks. In that way, each participant acted as their own study control.
Throughout the study, the subjects' total daily energy expenditures were measured using the doubly labelled water method, an isotope-based technique that looks at the difference between the turnover rates of the hydrogen and oxygen of body water as a function of carbon dioxide production.
No difference in weight loss
The primary endpoint of the study was energy balance measured by body weight.
Overall, the researchers found that energy expenditures and total weight loss were the same for the morning-loaded and evening-loaded diets. The subjects lost an average of just over 3 kg (about 7 pounds) during each of the four-week periods.
Big breakfast eaters report less hunger
The researchers also looked at appetite control, blood sugar control and body composition.
“The participants reported that their appetites were better controlled on the days they ate a bigger breakfast and that they felt satiated throughout the rest of the day”, the researchers said. "This could be quite useful in the real-world environment, versus in the research setting that we were working in."
One limitation of the study is that it was conducted under free-living conditions rather than in the lab. Additionally, certain metabolic measurements were available only after breakfast and not after dinner.
The researchers note that this type of experiment could be applied to the study of intermittent fasting, specifically time-restricted eating, to help determine the best time of day for people following this type of diet to consume their calories.
The research group plans to expand its research into how the time of day affects metabolism by conducting studies similar to the one described here in people who do shift work. It's possible these individuals could have different metabolic responses due to the disruption of their circadian rhythms.
Source: Cell Metabolism, September 9, 2022.
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