In the study, 12 physically active white men ages 18 to 23 arrived at the researchers' lab at 8 a.m., and were randomly given either oatmeal and orange juice, or no breakfast. Then, at 10 a.m., the men ran on a treadmill for an hour.
A week later, the participants repeated the experiment, but the groups were switched — the men who had eaten breakfast in the previous experiment had no breakfast, and vice versa. The men were given food to take with them to eat for the rest of the day, and were free to eat as much as they wanted. The researchers measured the food that was leftover at the end of the day.
The results showed that the men consumed many more calories overall on the days when they ate breakfast before exercising, according to the findings, published online Sept. 21 in the Journal of Nutrition and Metabolisms. [9 Meal Schedules: When to Eat to Lose Weight]
On average, the participants consumed about 4,500 calories on the days when they ate breakfast before exercising but only 3,600 calories on the days when they exercised before eating, said Jessica Bachman, the lead author of the study and an assistant professor of exercise science at the University of Scranton in Pennsylvania.
The men consumed the majority of those extra calories in the evening, Bachman told Live Science.
"People have been looking for a while at how exercise affects appetite," Bachman said. "What really hasn't been manipulated is whether consuming breakfast affects this at all."
The findings could have implications for people who are trying to lose weight or maintain weight loss, the researchers said. "Energy balance, [which is] the relationship between energy consumed and energy expended, is a simple equation on paper but is a complicated concept in practice that ultimately determines whether an individual’s weight increases, decreases or stays the same," they wrote in their study. [Eating After Workouts: The Science of Timing Meals and Exercise]
In their article, the researchers noted that many observational studies have concluded that people who habitually skip breakfast tend to weigh more than those who don't. However, a review of studies published in 2013 found that the few randomized controlled trials that have looked at the question of whether eating breakfast can help with weight maintenancehave not confirmed the results of those observational studies.
The new results support a growing amount of research that indicates that skipping breakfast may lead people to consume fewer calories throughout the day, the researchers said.
Furthermore, the researchers found that the men burned more fat when they exercised while in a fasted state — in other words, when they went running without consuming breakfast beforehand.
Bachman and her colleagues noted that they tested only a small, homogenous group of people. As such, the results cannot be generalized to other groups. In addition, the researchers noted that it's not clear whether the effects found in the experiment lasted beyond one day.
However, one strength of the study was that the researchers were able to weigh the participants' dietary intake instead of relying on the participants to report what they ate; self-reporting isn't always an accurate and reliable method because people may misremember or misreport information.
Future studies could look at the effect of skipping breakfast before exercising in a wider variety of populations and over a longer period of time. They could also include investigations into the effect of skipping a meal on appetite-regulating hormones such as insulin, leptin and ghrelin.
Ultimately, the research may help to shed light on how exercising in a fasted state could help with weight management, Bachman said.
Originally published on Live Science.