Surprising research findings on big breakfasts, hunger and weight loss

New research finds that people who eat their biggest meal in the morning don’t metabolize their food more efficiently. However, they are less hungry later in the day, which could help in weight loss efforts.

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

In diet, there is the old saying that you should “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 that was published on September 9 in the journal Cellular 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 found that people who ate their biggest meal in the morning reported feeling less hungry later in the day, which could make real-world weight loss easier.

“There are many myths surrounding the timing of eating 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 circadian rhythm field. But we in the nutrition field have wondered how this could be possible. Where would the energy go? We decided to look more closely at how the time of day interacts with metabolism”.

For this study, researchers recruited healthy overweight or obese subjects to monitor their diet 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 either a morning or evening 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. Each participant then switched to the opposite diet for four weeks, followed by a one-week washout period in which calories were balanced throughout the day. With this method, each participant acted as their own study control.

Throughout the study, subjects’ total daily energy expenditure was 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 main objective of the study was energy balance as measured by body weight. Overall, the researchers found that energy expenditure and total weight loss were the same for the morning and evening loaded diets. 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 the days they ate a larger breakfast and that they felt fuller for the rest of the day,” says Johnstone. “This could be very useful in the real-world environment, compared to the research environment we were working in.”

A limitation of the research is that it was conducted under free-living conditions rather than in the laboratory. In addition, certain metabolic measures 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 on 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’s not likely to be a one-size-fits-all diet,” concludes Johnstone. “Achieving this will be the future of diet studies, but it’s something that’s very difficult to measure.”

Reference: “Timing of Daily Calorie Load Affects Hunger and Hunger Responses Without Changes in Energy Metabolism in Healthy Obese Subjects” 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 Sep 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 Science and Analytical Services Division.

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