Surprising Research Findings on Big Breakfasts, Hunger, and Weight Loss

Front-loading calories early in the day reduces hunger but does not affect weight loss.

When dieting, there is the old saying that one should “eat breakfast like a king, lunch like a prince and dinner like a pauper”. It is based on the belief that consuming most of your daily calories in the morning optimizes weight loss by burning calories more efficiently and quickly. However, according to a new study published September 9 in the journal Cellular metabolismHowever, the way a person’s body metabolizes calories is not affected by whether they eat their largest meal earlier or later 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 encourage easier real-world weight loss.

“There are a lot of myths around when to eat and how it might influence body weight or health,” says lead 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 largely driven by the field of circadian rhythm. But we in the field of nutrition have wondered how this could be possible. Where would the energy go? We decided to take a closer look at how the time of day interacts with metabolism.”

For this study, the researchers recruited healthy subjects who were overweight or obese to monitor their diets and measure their metabolism over a period of time. There were 16 men and 14 women who completed the study. Each participant was randomly assigned to eat a morning-loaded or evening-loaded diet for four weeks. The diets were isocaloric (equal calories), with a balance of 30% protein, 35% carbohydrate, and 35% fat. Each participant then switched to the opposite diet for four weeks, after a week-long washout period in between in which calories were evened out throughout the day. With this method, each participant acted as their own study control.

Throughout the study, subjects’ total daily energy expenditures were measured using the doubly labeled water method. This is an isotope-based technique that analyzes the difference between the turnover rates of hydrogen and oxygen in body water as a function of carbon dioxide production. 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 morning- and evening-loaded diets. The subjects lost an average of just over 3 kg (about 7 pounds) during each of the four-week periods.

Secondary endpoints were subjective appetite control, glycemic control, and body composition. “Participants reported that their appetites were better controlled on days they ate a larger breakfast and that they felt satiated throughout the rest of the day,” says Johnstone. “This could be quite useful in the real world environment, compared to the research environment we were working in.”

A limitation of the research is that it was carried out under free-living conditions rather than in the laboratory. Also, certain metabolic measurements were only available 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 on this type of diet to consume their calories.

In the future, the group plans to expand their research on how time of day affects metabolism by conducting studies similar to the one described here in shift-working subjects. Due to the disruption of their circadian rhythms, it is possible that these individuals may have different metabolic responses. “One thing that’s important to keep in mind is that when it comes to timing and diet, there may not be one diet that fits everyone,” Johnstone concludes. “Solving this will be the future of diet studies, but it’s very hard to measure.”

Reference: “The 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 A.N. Filipe, Graham W. Horgan, Klaas R. Westerterp, Jonathan D. Johnston, and Alexandra M. Johnstone, September 9, 2022, Cellular metabolism.
DOI: 10.1016/j.cmet.2022.08.001

This study was funded by the Medical Research Council and the Scottish Government’s Rural and Environmental Sciences and Analytical Services Division.