Exercise Timing May Dictate Obesity, Type 2 Diabetes Risk

Liam Davenport

September 21, 2023

Exercising in the morning may have the biggest impact on the likelihood of having obesity, whereas morning and afternoon exercise appear to reduce the risk of developing type 2 diabetes, suggest two studies.

Tongyu Ma, PhD, research assistant professor with the Health Sciences Department, Franklin Pierce University, Rindge, New Hampshire, and colleagues studied data on almost 5300 individuals, finding a strong association between moderate to vigorous physical activity (MVPA) and obesity.

The research, published in Obesity, showed that people who exercised in the morning had a lower body mass index than those who exercised at other times, even though they were more sedentary.

For the second study, Chirag J. Patel, PhD, associate professor of biomedical informatics at Harvard Medical School, Boston, Massachusetts, and colleagues examined more than 93,000 individuals and found that morning and afternoon, but not evening, exercise reduced the risk for type 2 diabetes.

However, the results, published in Diabetologia, also indicated that people who undertook at least MVPA were protected against developing type 2 diabetes no matter what time of day they exercised.

Along with considering the timing of exercise, the authors suggest that it is "helpful to include some higher intensity activity to help reduce the risk of developing diabetes and other cardiovascular disease."

Morning Exercisers Perform Less Physical Activity

Ma and colleagues highlight that "although a beneficial association among the levels of physical activity with obesity has been frequently reported, the optimal timing of physical activity for decreasing obesity remains controversial."

The researchers analyzed data from the National Health and Nutrition Examination Survey for the 2003-2004 and 2005-2006 cycles, because accelerometry was implemented in those periods.

They included 5285 individuals aged ≥ 20 years who had physical activity measured via an accelerometer worn on the right hip during waking hours for 7 consecutive days.

The diurnal pattern of MVPA was classified into three clusters by the established technique of K-means clustering analysis: morning (n = 642), midday (n = 2456), and evening (n = 2187).

The association between MVPA, diurnal pattern, and obesity was then assessed in linear regression models taking into account a range of potential confounding factors.

Overall, participants in the morning cluster were older and more likely to be female than those in the other clusters (P < .001 for both). They were also more likely to be nonsmokers (P = .007) and to have less than high school education (P = .0041).

Morning cluster individuals performed less physical activity and were more sedentary than those in the midday and evening groups (P < .001 for both), although they were more likely to be healthy eaters (P = .004), with a lower calorie intake (P < .001).

Individuals in the morning cluster had, on average, a lower body mass index than those in other clusters, at 27.4 vs 28.4 in the midday cluster and 28.2 in the evening cluster (P for interaction = .02).

Morning cluster participants also had a lower waist circumference than participants in the midday or evening cluster: 95.9 cm, 97.9 cm, and 97.3 cm, respectively (P for interaction = .06).

The team reports that there was a strong linear association between MVPA and obesity in the morning cluster, whereas there was a weaker curvilinear association in the midday and evening clusters.

"This is exciting new research that is consistent with a common tip for meeting exercise goals — that is, schedule exercise in the morning before emails, phone calls or meetings that might distract you," commented Rebecca Krukowski, PhD, professor, Public Health Sciences, University of Virginia School of Medicine, Charlottesville, Virginia, in a release.

However, she noted that the cross-sectional nature of the study means that it is "not known whether people who exercise consistently in the morning may be systematically different from those who exercise at other times, in ways that were not measured in this study.

"For example, people who exercise regularly in the morning could have more predictable schedules, such as being less likely to be shift workers or less likely to have caregiving responsibilities that impede morning exercise," said Krukowski, who was not involved in the study.

No Association Between Evening Activity and Type 2 Diabetes Risk

In the second study, the team studied 93,095 persons in the UK Biobank, with a mean age of 62 years and no history of type 2 diabetes, who wore a wrist accelerometer for 1 week.

The movement data were used to estimate the metabolic equivalent of task, which was then summed into the total physical activity completed in the morning, afternoon, and evening and linked to the development of incident type 2 diabetes.

After adjustment for potential confounding factors, both morning and afternoon physical activity were associated with a reduced risk of developing type 2 diabetes, at hazard ratios of 0.90 (P = 7 × 10-8) and 0.91 (P = 1 × 10-5), respectively.

However, there was no association between evening activity and the risk for type 2 diabetes, at a hazard ratio of 0.95 (P = .07).

The team found, however, that MVPA and vigorous physical activity were associated with a reduced risk for type 2 diabetes at all times of day.

Patel's study was supported in part by National Institutes of Health grants. No other funding was declared. No relevant financial relationships were declared.

Ma T, et al. Obesity. Published online September 4, 2023. Full text

Tian C, et al. Diabetologia. Published online September 20, 2023. Full text

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