A New Study Says *This* Might Be The Best Time To Exercise For Weight Loss

runner at sunrise
This Timing May Be Key To Exercise For Weight LossCourtesy / Christine Giordano - Getty Images

Are you someone who gets up early for a sweat sesh or are you more of a nighttime gym person?

Now, a new study suggests that the specific time of day you exercise could affect your ability to lose weight.

A September 2023 study by the Obesity Society found that "moderate to vigorous" workout between the hours of 7 a.m. to 9 a.m. appears to create the "most favorable" relationship between exercise and weight loss. Translation: There *could* be a correlation between earlier workouts and greater weight loss.

But the study also has a lot of caveats, and the findings were inconclusive, say experts.

Curious to know what the study could mean for your workout sched? Here's what the study found, how to interpret the findings, and what experts have to say about it.

Meet The Experts: Dr. Holly Lofton, MD, director of the medical weight management program at New York University Langone Health, Dr. Jeff Toll, MD, internal medicine physician based in Los Angeles.

Who participated in the study?

The study included 5,285 people who participated in the National Health and Nutrition Examination Survey between 2003 and 2006. This survey seeks to understand the physical conditions of children and adults in the United States, and includes both interviews and physical exams, per the U.S. Department of Health and Human Services.

The male and female participants were at least 20 years old, and no pregnant people took part in the study.

What was the study testing?

Researchers wanted to see if exercising at a particular time of day enhanced weight loss. So, they grouped participants by the time of day they typically exercised—morning, afternoon, and night—then cross-analyzed the groups to see if their habits had any correlation with their body mass index or waist circumference.

It's worth noting that body mass index is no longer considered the best way to understand if a person is overweight, and weight circumference provides a clearer measurement.

What did the study find?

Researchers analyzed three years of data, and found that the morning workout group had a mean body mass index (BMI) of 25.9, which was lower than the two other groups. (FYI: A BMI between 25 and 30 is considered "overweight," according to the Center for Disease Control and Prevention.) The mean BMIs of the groups who worked out midday and in the evening were 27.6 and 27.2, respectively.

The waist circumference of the group that worked out in the mornings was also the smallest, at 91.2 centimeters, compared to 95.8 centimeters for the midday group and 95 centimeters for the nighttime group.

Overall, the study found a "strong linear association between moderate-to-vigorous physical activity and obesity" in the morning group and "a weaker curvilinear association" in the other two groups. This suggests that the early morning—specifically between 7 a.m. and 9 a.m.—could be the best time to exercise for weight loss, given the study's results.

However, in speaking with NBC, the study's lead researcher, Tongyu Ma, said that these results could be "due to a number of factors beyond exercise timing."

Ma, who is a research assistant professor at The Hong Kong Polytechnic University, added that it was his "cautious suggestion" from the study "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."

What does this mean for my exercise routine?

Before you change up your routine, experts (and the study itself) say that more research needs to be done.

While the researchers added control variables—such as sex, sedentary behavior and age—into their analysis, the results were still inconclusive, because longer studies and randomized clinical trials need to be completed before confirming the weight loss benefits of morning workouts.

"There needs to be a control group before you can [officially] recommend morning exercise morning exercise versus midday or evening," says Dr. Holly Lofton, a director of the medical weight management program at New York University Langone Health. "Because it's an observational study, it's inconclusive whether morning activity is more effective than evening in reducing obesity."

Experts say you shouldn't immediately switch up your routine based on this study, but some do say morning exercise is better because of the other health benefits it can provide.

For starters, you typically sleep better after working out in the morning versus at night, says Dr. Jeff Toll, an internal medicine physician based in Los Angeles. You'll also burn even more fat throughout the day if you workout before eating in the morning.

"Humans are probably meant to be active in the morning for evolutionary reasons, so it typically makes the most sense to burn calories and workout in the morning," he says. "When people do that and haven't eaten anything yet, creates something called a ketosis where we're burning fat rather than burning sugar."

Plus, when you start your day with movement, "it lowers stress and anxiety, which often leads to better food choices," explains Dr. Toll.

What should I focus on if I'm exercising to lose weight?

Both Dr. Lofton and Dr. Toll emphasize the importance of individualizing your routine, as there's no one-size-fits-all formula for exercising to lose weight. Instead, it's most important to find a routine that works for you.

"You should do at least 150 minutes of exercise a week to get you started [on a weight loss journey]," says Dr. Lofton. In the study, not all participants met that movement requirement, which probably skewed results, she explains.

And if you are trying to lose weight through exercise, you should make sure it's a "moderate to vigorous" workout.

Dr. Lofton explains that you should be able to "talk, but not sing"during your workout and also recommends bumping your fitness routine up to 240 minutes per week (or roughly 35 minutes per day) once you start losing weight, to maintain your rhythm.

"In the end weight loss is really about discipline and consistency," says Dr. Toll. "Whatever gets someone to that—whether it's in the morning or afternoon is going to be most effective [for them.]"

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