When it comes to staying trim, timing may be everything.

That's according to new research that found adults who routinely engaged in moderate-to-vigorous exercise early in the morning were less likely to be overweight or obese than those who worked out later in the day.

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“For individuals who exercise regularly, their body mass index [BMI] is 2 units lower and waist circumference is 1.5 inches shorter if they exercise in the early morning than in other times of day,” said study author Tongyu Ma, an assistant professor of exercise physiology at Franklin Pierce University in Rindge, N.H.

BMI and waist circumference are considered two key measures of obesity risk.

The upshot, said Ma, is that a “morning workout is a promising tool for weight management.”

To explore the question, investigators tracked obesity status among nearly 5,300 adult men and women. All were enrolled in the ongoing U.S. National Health and Nutrition Examination Survey, either between 2003 to 2004 or between 2005 to 2006.

Each of the participants wore a hip accelerometer whenever they were awake for somewhere between four to seven consecutive days, including at least one day over the weekend.

Based on activity routines, they were then categorized into one of three exercise groups: morning (642 participants), midday (2,456) or evening (2,187).

In turn, waist circumferences were measured and BMI scores were calculated. BMI is a measurement based on height and weight.

The result: Exercising between 7 a.m. and 9 a.m. turned out to be associated with a lower risk for being overweight or obese compared with exercising later in the day.

BMI scores revealed that all of the participants in the study were overweight, meaning that their BMIs registered somewhere between 25 and 29.9.

But while those in the morning group had an average BMI of 25.9, those in the midday and evening exercise groups had BMI averages of 27.6 and 27.2, respectively.

Similarly, while the average waist circumference seen among morning exercisers was pegged at about 36 inches, that number landed between 37 and 38 inches among the midday and evening groups.

Ma stressed that for now the research can only point to an association between morning workouts and weight, rather than a direct cause and effect.

And in an unexpected twist, the team also observed that those in the morning exercise group actually spent the most time being sedentary.

But Ma pointed out that while the morning group might spend more time on the couch they also probably engaged in a more “solid morning workout session” compared with the type of activity expended by midday and evening exercisers.

“Think about an office worker who works out every morning versus a janitor who is busy all day. The former is probably at lower risk of obesity,” Ma explained.

Another factor: The morning group also tended to eat healthier overall, and ultimately took in fewer calories than their midday and evening group peers.

So what is it about a morning workout that might offer a leg up on staying in shape?

Perhaps because, “in the early morning, our body is in a low emerge state after the overnight fast,” suggested Ma. "We don’t have a lot of carbs to burn, therefore our body will rely more on lipid [fat] to produce the energy needed for exercise. That’s probably why morning exercise may be better for weight management.”

The findings were published recently in the journal Obesity.

Connie Diekman is a food and nutrition consultant and former president of the Academy of Nutrition and Dietetics.

She agreed that more research is needed to pinpoint what’s afoot. But at the same time, she expressed little surprise with the overall findings.

“We have long recommended that exercise, first thing in the morning, provides more benefit,” said Diekman. “The evidence to this point focused on the benefit of exercise in boosting metabolism, which remains elevated once the activity is completed, so calorie burn stays higher during the day. However, the strength of the evidence was never strong enough to say ‘You must work out in the morning.'”

Her advice: Work out “when it works for your schedule.”

“The most important recommendation from my viewpoint as a registered dietician is that in order to support a healthy body, physical activity needs to be a part of our routines,” Diekman said. “Therefore, each week try to get at least 150 minutes of physical activity, in a manner that best fits your daily/weekly routine.”