Front-loading calories early in the day reduces hunger but does not affect weight loss.
In dieting, there’s the old saying that one should “breakfast like a king, lunch like a prince, and dine like a pauper.” It is based on the belief that consuming the bulk of daily calories in the morning optimizes weight loss by burning calories more efficiently and quickly. However, according to a new study that was published on September 9 in the journal Cell Metabolism, the way a person’s body metabolizes calories is not affected by whether they eat their largest meal early or late in the day. On the other hand, the study did find that people who ate their largest meal in the morning reported feeling less hungry later in the day, which could foster easier weight loss in the real world.
“There are a lot of myths surrounding the timing of eating and how it might influence either body weight or health,” says senior author Professor Alexandra Johnstone. She is a researcher in the field of appetite control at the Rowett Institute at the University of Aberdeen in Scotland. “This has been driven largely by the circadian rhythm field. But we in the nutrition field have wondered how this could be possible. Where would the energy go? We decided to take a closer look at how time of day interacts with metabolism.”
For this study, the researchers recruited healthy subjects who were overweight or obese to have their diets controlled and their metabolisms measured over a period of time. There were 16 men and 14 women who completed the study. Each participant was randomly assigned to eat either a morning-loaded or an evening-loaded diet for four weeks. The diets were isocaloric (having the same number of calories), with a balance of 30% protein, 35% carbohydrate, and 35% fat. Then each participant crossed over to the opposite diet for four weeks, after an intermediate washout period of one week in which calories were balanced throughout the day. With this method, each participant acted as their own study control.
Throughout the study, the subjects’ total daily energy expenditures were measured using the doubly labeled water method. This is 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. The primary endpoint of the study was energy balance measured by body weight. Overall, the investigators 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.
The secondary end points were subjective appetite control, glycemic 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 satisfied throughout the rest of the day,” Johnstone says. “This could be quite useful in the real-world environment, versus in the research setting that we were working in.”
One limitation of the research 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.
Johnstone notes that this type of experiment could be applied to the study of intermittent fasting (also called time-restricted eating), to help determine the best time of day for people following this type of diet to consume their calories.
In the future, the group plans to expand its research into how the time of day affects metabolism by conducting studies similar to the one described here on subjects who do shift work. Due to the disruption of their circadian rhythms, it’s possible that these individuals could have different metabolic responses. “One thing that’s important to note is that when it comes to timing and dieting, there is not likely going to be one diet that fits all,” Johnstone concludes. “Figuring this out is going to be the future of diet studies, but it’s something that’s very difficult to measure.”
Reference: “Timing of daily calorie loading affects appetite and hunger responses without changes in energy metabolism in healthy subjects with obesity” by Leonie C Ruddick-Collins, Peter J Morgan, Claire L Fyfe, Joao AN Filipe, Graham W Horgan , Klaas R. Westerterp, Jonathan D. Johnston and Alexandra M. Johnstone, 9 September 2022, Cell Metabolism.
This study was funded by the Medical Research Council and the Scottish Government, Rural and Environment Science and Analytical Services Division.