Physical activity in the morning more effective in preventing obesity than at night

Moderate-to-vigorous physical activity (MVPA) in the morning appears to be more protective against obesity, particularly for individuals who are sedentary for most of the day, a study has found.

“[T]he diurnal pattern of MVPA influences the association between MVPA level and obesity,” the investigators said. “Early morning (07:00‒09:00) appears to be the most favourable time of day to enhance the association between MVPA and obesity.”

This cross-sectional study was designed to examine the impact of the diurnal pattern of MVPA on the relationship between MVPA and obesity among 5,285 participants in the 2003‒2006 National Health and Nutrition Examination Survey (NHANES).

The investigators performed a K-means clustering analysis to classify the diurnal pattern of the objectively measured MVPA into the following groups: morning (n=642), midday (n=2,456), and evening (n=2,187). They then tested the associations of MVPA levels and the diurnal pattern with obesity.

The morning cluster showed a robust linear association between MVPA and obesity, while the midday and evening clusters had a weaker curvilinear association between MVPA and obesity.

Among participants who met the physical activity guidelines, the adjusted means for body mass index (BMI) were 25.9 kg/m² (95 percent confidence interval [CI], 25.2‒26.6) in the morning group, 27.6 kg/m² (96 percent CI, 27.1‒28.1) in the midday group, and 27.2 kg/m² (95 percent CI, 26.8‒27.7) in the evening group. [Obesity 2023;31:2638-2647]

Additionally, the corresponding adjusted means for waist circumference were 91.5 cm (95 percent CI, 89.4‒93.6), 95.8 cm (95 percent CI, 94.7‒96.9), and 95.0 cm (95 percent CI, 93.9‒96.1).

“Our findings substantiated the role of morning MVPA in weight management,” the investigators said. “However, owing to the observational nature of our data, it remains inconclusive whether morning MVPA is more effective than evening MVPA in reducing obesity.”

The current findings supported those of the Midwest Exercise Trial 2, which found a greater decrease in body mass and fat mass among individuals who attended >50 percent of their training sessions in the morning than those who attended >50 percent of their training sessions between 15:00 and 19:00. [Int J Obes (Lond) 2020;44:114-124]

In the study by Creasy and colleagues, results showed that a high amount of MVPA in the morning was a crucial factor among those who successfully maintained their weight loss. The finding further supported the positive effect of morning MVPA on weight management. [Int J Obes (Lond) 2021;45:2074-2082]

Mechanism

“The mechanism by which the diurnal pattern of MVPA influences the association between MVPA and obesity is not completely understood,” the investigators said.

Two experiments performed by Iwayama and colleagues using indirect calorimeter-equipped metabolic chambers demonstrated that morning exercise in a fasting state increased fat oxidation over 24-hours following exercise. On the other hand, postlunch and postdinner exercise did not significantly change 24-hour fat oxidation. [PLoS One 2017;12:e0180472; EBioMedicine 2015;2:2003-2009]

Moreover, other studies also observed a greater fat oxidation during fasted-state than fed-state morning exercise, indicating a significant role of the fasted-state exercise in promoting fat oxidation. [J Nutr Metab 2016;2016:1984198; Br J Nutr 2013;110:721-732]

“The reduced glycogen stores after an overnight fast may cause a shift of substrate use toward fatty substrates,” the investigators said. “It appears that the favourable obesity-protective outcomes of MVPA in the morning cluster could be driven by fasted-state exercise.”

To confirm the benefits of morning MVPA on obesity, prospective studies and randomized clinical trials must be conducted, according to the investigators.