The Relationship Between Time of Day of Physical Activity and Obesity in Older Women

Background: Physical activity is important for maintaining healthy weight. The time of day when exercise is performed a highly discretionary aspect of behavior may impact weight control, but evidence is limited. Thus, we examined the association between the timing of physical activity and obesity risk in women. Methods: A cross-sectional analysis was conducted among 7157 Women's Health Study participants who participated in an ancillary study begun in 2011 that is measuring physical activity using accelerometers. The exposure was percentage of total accelerometer counts accumulated before 12:00 noon and the outcome was obesity. Results: Mean (+/- SD) BMI among participants was 26.1 (+/- 4.9) kg/m(2) and 1322 women were obese. The mean activity counts per day was 203,870 (95,811) of which a mean 47.1% (+/- 11.5%) were recorded in the morning. In multivariable-adjusted models, women who recorded < 39% (lowest quartile) of accelerometer counts before 12:00 noon had a 26% higher odds of being obese, compared with those recording >= 54% (highest quartile) of counts before noon (P-trend = 0.02). Conclusions: These study findings-that women who are less active during morning hours may be at higher risk of obesity-if confirmed can provide a novel strategy to help combat the important health problem of obesity.