This analysis is novel in that, as far as the author is aware, this is the first longitudinal analysis of weight-change in association with an objective measure of physical activity, from a rural South African setting.
The main finding of this analysis was that there was no consistent, significant pattern of high sedentary and physical inactivity prevalence in those who remained overweight-obese or moved from underweight-normal weight to obese, and high physical activity levels in those who remained normal weight or lost weight. In fact, irrespective of the weight change status, ambulatory physical activity was high. However, there was a tendency for the weight loss group (OW/OB→UW/NW) and the UW/NW weight-stable group to accrue higher average daily ambulation within the 420 min.wk–1 and ≥10 000 steps.day–1 physical activity guidelines, with a low proportion in the 150 min.wk–1 guideline. The low sample size might have obscured more definitive and significant patterns.
Interestingly, very similar proportions of those not meeting any energy expenditure-based physical activity guidelines were seen between quite disparate groups (OW/OB→UW/NW and remained OW/OB: ≈18%; UW/NW→OW/OB and remained UW/NW: ≈5%). Moreover, more than 80% of any weight-change group adhered to a physical activity guideline, whether energy expenditure-based or step-based. Previous cross-sectional analyses of the 2005–7 survey data, have shown that irrespective of increased BMI levels, the ambulation levels and the prevalence of meeting physical activity guidelines are high [15,19,21].
Meeting physical activity guidelines, especially 150 min.wk–1 (5 days.wk–1, moderate-to-vigorous intensity), was not associated with weight loss or being weight-stable. Nearly 90% of those who gained weight met the 150 min.wk–1 guideline. These findings are in agreement with Dickie et al. who found that in a group of 57 urban African women, body mass increased over a period of 5.5 years, whether classified as physically active (150 min.wk–1) or physically inactive using a self-report measure . The overall body mass increase was +7.3 kg , which is 6-fold higher than the 10 year body mass change in the current rural African sample. However, those meeting physical activity guidelines were metabolically healthier than those classified as physically inactive .
Similarly, in a prospective cohort study (mean follow-up 13.1 years), Lee et al. showed that weight gain was the same in those meeting or not meeting physical activity guidelines (150 min.wk–1) . The overall mean weight gain was 2.6 kg, which is more than two-fold compared to the current sample. Weight stability was evident only in women attaining 420 min.wk–1 of moderate-to-vigorous physical activity . In the current analysis, only the weight-stable UW/NW group showed a significantly greater prevalence of meeting 420 min.wk–1 physical activity guidelines (p<0.05). The physical activity guideline of 420 min.wk–1  addresses issues around weight loss and prevention of weight gain after weight loss , unlike the physical activity guideline of 150 min.wk–1  which addresses risk reduction for mortality and morbidity, and metabolic health [4,5,23].
In contrast, an increase in BMI over a 10 year period in 430 urban African women, was significantly, inversely (p = 0.02) related to vigorous physical activity (assessed using a self-report measure). The overall increase in body mass was 5.17 kg . In a more recent analysis, this group has shown the relationship between moderate-to-vigorous physical activity (150 min.wk–1) and changes in BMI to be part of a complex interaction, with significant direct and indirect effects via socio-economic status. Change in moderate-to-vigorous physical activity was directly and inversely related to socio-economic status .
The generally high levels of physical activity coupled with high levels of obesity highlighted in the current analysis, are in agreement with the assertion that higher levels of physical activity do not necessarily attenuate weight gain . In a two-year prospective cohort (1 943 adults of African origin), which included 8-day accelerometry, neither meeting physical activity guidelines (150 min.wk–1) nor sedentary time were associated with weight gain, suggesting the likelihood that nutritional factors might be of greater importance [9,10].
In conclusion, this report presents longitudinal weight-change data, incorporating an objective measure of physical activity, from a rural African setting, which suggests that meeting public health physical activity guidelines is not tightly associated with weight change or stability.