Big American Breakfast Food

Surprising research findings on big breakfasts, hunger and weight loss

New research reveals 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 them in their weight loss efforts.

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

When it comes to diet, there’s the old saying, “Breakfast like a king, lunch like a prince, and dinner like a pauper.” It is based on the belief that consuming the majority of daily calories in the morning optimizes weight loss by burning calories more efficiently and faster. However, according to a new study published 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 earlier or later 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 may support easier weight loss in the real world.

“There are many myths surrounding when you 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. “It was largely driven by the circadian rhythm field. But we in nutrition wondered how that 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 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 loaded diet in the morning or a loaded diet in the evening for four weeks. The diets were isocaloric (having the same number of calories), with a balance of 30% protein, 35% carbohydrates and 35% fat. Then each participant switched to the opposite diet for four weeks, after 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, the subjects’ total daily energy expenditure was measured using the doubly labeled water method. It is an isotope-based technique that examines the difference between the hydrogen and oxygen turnover rates 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 researchers found that energy expenditure and total weight loss were the same for morning and evening 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 appetite was better controlled on days when they ate a larger breakfast and that they felt full throughout the day,” says Johnstone. “It could be very useful in the real-world environment, relative to the research context we were working in.”

One of the limitations of the research is that it was conducted in free-living conditions rather than in the laboratory. Also, some 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 into how time of day affects metabolism by conducting studies similar to the one described here on subjects who work shifts. Due to the disruption of their circadian rhythms, these people may have different metabolic responses. “One thing that’s important to note is that when it comes to timing and diet, there probably won’t be one diet that’s right for everyone,” Johnstone concludes. “Understanding this is going to be the future of diet studies, but it’s something very difficult to measure.”

Reference: “Time of daily caloric load affects appetite and hunger responses without altering energy metabolism in obese healthy 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, September 9, 2022, Cell metabolism.
DOI: 10.1016/j.cmet.2022.08.001

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


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