, published in Cell Metabolism, from the University of Aberdeen. The idea that ‘front-loading’ calories early in the day might help dieting attempts was based on the belief that consuming the bulk of daily calories in the morning optimizes weight loss by burning calories more efficiently and quickly.
“There are a lot of myths surrounding the timing of eating and how it might influence either body weight or health,” said senior author Alexandra Johnstone, PhD, a researcher at the Rowett Institute, University of Aberdeen, who specializes in appetite control. “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.”
Her team undertook a randomized crossover trial of 30 overweight and obese subjects recruited via social media ads. Participants – 16 men and 14 women – had a mean age of 51 years, and body mass index of 27-42 kg/ m2 but were otherwise healthy. The researchers compared two calorie-restricted but isoenergetic weight loss diets: morning-loaded calories with 45% of intake at breakfast, 35% at lunch, and 20% at dinner, and evening-loaded calories with the inverse proportions of 20%, 35%, and 45% at breakfast, lunch, and dinner, respectively.
Each diet was followed for 4 weeks, with a controlled baseline diet in which calories were balanced throughout the day provided for 1 week at the outset and during a 1-week washout period between the two intervention diets. Each person’s calorie intake was fixed, referenced to their individual measured resting metabolic rate, to assess the effect on weight loss and energy expenditure of meal timing under isoenergetic intake. Both diets were designed to provide the same nutrient composition of 30% protein, 35% carbohydrate, and 35% fat.
All food and beverages were provided, “making this the most rigorously controlled study to assess timing of eating in humans to date,” the team said, “with the aim of accounting for all aspects of energy balance.”
No optimum time to eat for weight loss
Results showed that both diets resulted in significant weight reduction at the end of each dietary intervention period, with subjects losing an average of just over 3 kg during each of the 4-week periods. However, there was no difference in weight loss between the morning-loaded and evening-loaded diets.
The relative size of breakfast and dinner – whether a person eats the largest meal early or late in the day – does not have an impact on metabolism, the team said. This challenges previous studies that have suggested that “evening eaters” – now a majority of the U.K. population – have a greater likelihood of gaining weight and greater difficulty in losing it.
“Participants were provided with all their meals for 8 weeks and their energy expenditure and body composition monitored for changes, using gold standard techniques at the Rowett Institute,” Dr. Johnstone said. “The same number of calories was consumed by volunteers at different times of the day, with energy expenditure measures using analysis of urine.
“This study is important because it challenges the previously held belief that eating at different times of the day leads to differential energy expenditure. The research shows that under weight loss conditions there is no optimum time to eat in order to manage weight, and that change in body weight is determined by energy balance.”