Background Links between physical activity and body mass index (BMI) are well-recognised. However, the evidence used to quantify this relationship and inform physical activity guidelines is largely based on self-reported activity data. This usually captures specific types of moderate to vigorous activity, less commonly undertaken at older ages, and is subject to reporting bias. In addition, the changing energy requirements of activities with changes in both age and BMI are often overlooked. The possibility that BMI may influence ability to remain active as well as the reverse, especially in later life, is also not always considered. Improving our understanding of the relationship between BMI and physical activity across the full range of activity intensity at older ages is thus important. We aimed to examine the associations of contemporaneous BMI and duration of exposure to obesity across adulthood with daily patterns of monitored physical activity in a nationally representative cohort at age 60–64 years.

Methods Up to 1634 men and women from the MRC National Survey of Health and Development, a British cohort followed-up prospectively since birth in 1946, who had worn a physical activity monitor (Actiheart) for up to five days and had a valid measure of BMI at age 60–64 years were included in analyses. Linear regression analyses were used to estimate the associations of contemporaneous BMI and obesity history (categorised as: not obese in adulthood; obese by age: 36; 43; 53; 60–64) with cumulative daily log counts of activity. Adjustments were made for gender, socioeconomic position, long-term limiting illness and anxiety and depression.

Results Higher BMI at age 60–64 years was associated with lower cumulative daily activity counts; in fully-adjusted analyses a 1 kg/m² increase in BMI was associated with a mean difference in cumulative daily log activity counts of -31.10 (95% CI:  –40.23, –21.98). Greater duration of exposure to obesity was also associated with lower cumulative daily activity counts with those participants who were obese by age 36 recording the lowest number of daily activity counts; in fully-adjusted analyses the mean difference in cumulative daily log activity counts was  –423.11 (95% CI: –641.05, –205.18) when comparing those participants who were obese by age 36 with those who were never obese.

Conclusion These findings highlight the strength of associations of BMI and obesity history with physical activity in early old  age. They have important implications for two major public health challenges currently faced; population ageing and obesity.