“My cautious suggestion from this study is that if we choose to exercise in the early morning, before we eat, we can potentially lose more weight compared to exercise at other times of the day,” said lead researcher Tongyu Ma, a research assistant professor at The Hong Kong Polytechnic University.
Ma and his team used two years of data from the Centers for Disease Control and Prevention‘s national health and nutrition examination survey, which included the exercise, eating and lifestyle habits of more than 5,200 adults who were at least 20 years old. The people in the study wore accelerators to capture their activity levels.
The data showed that people who did moderate to vigorous exercise in the morning had lower body mass indexes than people who exercised at midday or in the evening, though Ma said this could be due to a number of factors beyond exercise timing.
On average, people who performed moderate to vigorous exercise in the morning were more than a decade older than those who exercised later in the day.
Although some younger adults were morning exercisers, the older age might explain the findings, Ma said. The observed benefits “may not be due to activity patterns, it might be due to their age and different response to exercise.”
Other factors aside from exercise timing may also play a role: Morning exercisers were predominantly white and female, had a college degree and had never consumed tobacco or alcohol. They were also more likely to work out at the same time every day. Compared to those who exercised later, people who did so in the morning consumed fewer calories and, interestingly, spent more time sedentary than people in the other groups.
Ma cautioned that only a clinical trial, which the new study was not, can truly determine if morning exercise is best. In a clinical trial, researchers could assign people of the same age, and who had the same lifestyle and eating habits, a time of day to exercise and then directly compare the results.
“The results are interesting for science, but aren’t secure enough to be used to recommend morning exercise for optimal health,” said Cameron Mitchell, an assistant professor of kinesiology at The University of British Columbia in Vancouver, Canada, who was not involved with the new research.
Ross Andersen, a professor of exercise physiology, medicine and nutrition at McGill University in Montreal, who was also not involved in the study, said it was difficult to pin the observed benefits on exercise alone. The people who exercised in the morning likely also practiced a number of other healthy behaviors that contributed to their lower weight and better health, he said.
Why morning may be best
In the past decade or so, a slew of studies have explored what time of day people should aim to work out to reap the biggest benefit — and the latest study probably won’t have the final say.
But there are several reasons in favor of exercising in the mornings.
Andersen said he always encourages his patients to get their workout in early. While there are some physiological reasons scientists believe this could be beneficial, there’s also a logical one.
“It’s easy to put off self-care and to say, ‘I’ll just let my exercise go today because I’m too busy and something has to go,’” he said. But if exercise is the first thing a person does in the morning, they may be more likely to do it every day, he added.
The body’s molecular clock may also be a reason morning exercise could help with weight loss. In every cell in the body, particularly the muscles, there is a molecular clock that prompts certain bodily functions, including those that have to do with how fat is metabolized in the body, at different times of day, Mitchell said.
Some early evidence has found that exercise can reset this clock, bringing it back into its normal rhythms, which could benefit metabolic health. But it’s not yet understood whether exercising at certain times of day would have a bigger impact.
Another possible explanation is that exercising in the morning, specifically before eating, may also help the body burn fat.
Since people are fasting when they are asleep, exercising before eating breakfast can help the body burn fat instead of carbohydrates, Ma said. The benefits of exercise also last for hours after the workout is done.
“Our metabolism doesn’t just return to baseline after a tough workout, it remains a little higher. If people are walking around most of the day with a higher metabolic rate, it’s a good thing,” Anderson said.
For those who do get their workout in early, experts said, that’s not the only change they need to make if weight loss is the goal. A healthy diet is essential.
“It’s a whole lifestyle approach,” Mitchell said, adding that exercise is a key part of the approach, especially since it helps people maintain muscle mass as they lose fat.
If morning exercise doesn’t fit into a person’s schedule, Andersen said not to fret.
“The best time of day for people who are exercising is the time of the day they like to do it,” he said. “I wouldn’t dictate that people exercise in the morning if they really don’t like exercising in the morning, but if they are indifferent, I would recommend the morning so they get it in.”Follow NBC HEALTH on Twitter & Facebook.