In this prospective analysis, we have confirmed our previous findings that higher levels of physical activity are strongly and inversely associated with levels of fat mass in 12 year old children.13
An increase of 15 min/day in moderate-vigorous physical activity at age 12 is associated with around 10% lower fat mass in girls and a 12% lower fat mass in boys at age 14. This percentage difference is equivalent to a reduced fat mass of about 1 kg.
Strengths and limitations of study
The strengths of this study are the large sample size, the well characterised cohort, the longitudinal design, the high levels of measurement precision in both the physical activity and body composition measures, and the availability of an extensive array of possible confounding variables from both the children and their parents.
As with all observational studies, there are some potential sources of error. Residual confounding and cohort attrition can lead to bias. Though the observed associations could be due to confounding, we think this is unlikely as physical activity in this cohort was only weakly (and negatively) associated with higher social position, and the associations were largely unaltered by adjustment for various confounding factors, including social position. Cohort attrition leads to reduced power, which is not a particular problem in a study of this size. However, more importantly, missing data can lead to bias if the association between physical activity and obesity is different in the children who did not take part. While we cannot exclude bias due to missing data, the fact that the associations were not altered by adjustment for factors associated with missing data—such as social position—provides some reassurance.
The study also has only two time points, so even in this longitudinal design, inferring direction of causality must remain tentative.
Finally, the accelerometer cannot be worn for swimming and does not capture cycling activities adequately. However, we have previously shown that when analyses were repeated in children who did not report any swimming in the week of measurement and in children who did not report cycling in the week of measurement, associations were unchanged.13
Comparison with other studies
To our knowledge, only three prospective studies have examined the association between an accelerometer measure of physical activity and obesity in children.29 30 31
These studies were relatively small and measured adiposity by skinfold thicknesses or bioimpedence, which are less accurate measures than the DEXA used in our study. The first study measured physical activity in 103 US children aged 3-5 years for 3-5 days with a Caltrac accelerometer.29
This accelerometer gives only a measure of total activity and cannot measure different intensities of activity. Measurements were taken annually for eight years. Activity levels during the period of measurement were prospectively associated with subsequent sum of skinfold thicknesses. The second study measured 454 American Indian children (average age 7.5 years) using a Tritrac R3D tri-axial accelerometer on one day and followed them up three years later.30
There was a prospective association between total activity and obesity in children who were normal weight at baseline but not in those who were overweight at baseline. The third study measured the extent to which physical activity (measured by Actigraph accelerometer) at the government-recommended intensity (three times resting metabolic rate) is associated with changes in body mass index, fatness (measured by skinfold thickness), and metabolic health in 212 prepubertal children.31
The time spent in activities of sufficient intensity at age 5 was not associated with changes in body mass or fatness in either sex during ages 5-8 years. Some associations were seen with metabolic parameters.
These studies of younger children were smaller than ours. Hence, they had limited ability to compare sexes or dimensions of activity (total activity and moderate-vigorous physical activity). Our study therefore adds to these, as we have precise measures of both physical activity and body composition and greatly increased power to conduct analyses by sex, account for a greater range of confounders, and compare different dimensions of physical activity.
Implications of results
Overweight and obesity are normally defined in terms of body mass index. Although DEXA measurement of fat mass is a much more accurate estimate of adiposity, there are currently no health related criterion levels of fat mass for either adults or children. It is therefore difficult to assess fully the clinical implications of the fat mass levels and changes over time seen in this study. The essential point, however, is that we have observed strong prospective associations between physical activity level and subsequent fat mass over a two year period during early adolescence.
In broad terms, we observed a difference in fat mass of about 10% between active and inactive children. At this early stage of the lifespan, this difference is unlikely to be trivial, given that overweight and obesity track over time and levels of adiposity increase with age. It should also be noted that 12 year old children in this analysis who meet current health related recommendations of 60 minutes of moderate-vigorous physical activity a day would be expected to have around 4.3 kg less fat mass at age 14 than children who do no moderate-vigorous physical activity.
The observed inverse association between changes in activity and changes in fat mass through ages 12-14 years suggests that children who are on a rising trajectory of activity during these years are on a declining trajectory of fat mass. This suggests a dynamic relation between physical activity and adiposity, with relatively acute effects of the behaviour on the physiological response.
In the current study, and in keeping with our previous cross sectional analysis,13
differences in moderate-vigorous physical activity were more strongly associated with difference in fat mass than were differences in total activity. Although we selected arbitrary increments of total activity (100 counts/min) and moderate-vigorous physical activity (15 min), the two are broadly equivalent in the amount of activity involved. A difference of 100 counts/min over a 10 hour day (our minimum threshold) represents 60
000 accelerometer counts. A difference of 15 min/day of moderate-vigorous physical activity at the threshold of 3600 counts/min (again our minimum threshold) represents 54
000 additional accelerometer counts. We can therefore be confident that the stronger association observed for moderate-vigorous physical activity is not a function of the moderate-vigorous physical activity variable representing a larger volume of activity than the variable for total activity. This is supported by the fact that, with mutual adjustment of the two activity variables, the effect sizes for total activity disappeared.
It might be expected that total activity measured in counts/min—which captures the majority of movement and hence might be considered to more closely represent total activity energy expenditure—would exhibit the stronger association with fat mass. Moderate-vigorous physical activity represents a relatively small proportion of a child’s daily energy expenditure. For example, at age 12 years, the proportion of physical activity due to moderate-vigorous physical activity was 26.1% (SD 10.9%) for boys and 20.6% (10.3%) for girls. At age 14, the corresponding figures were 29.7% (13.3%) for boys and 24.8% (13.2%) for girls.
One possible explanation for this is that an accelerometer measures only trunk movement and does not measure peripheral movement such as arm exercise or fidgeting. If these activities—known collectively as non-exercise activity thermogenesis32
—contribute substantially to energy expenditure, then the total accelerometer counts may not be that closely related to energy expenditure.
Alternatively, more vigorous activity may stimulate excess post-exercise oxygen consumption—a measurably increased rate of oxygen uptake for a time after strenuous activity. The extra oxygen is used in the homoeostatic processes that restore the body to a resting state including hormonal adjustment, replenishment of fuel stores, and cellular repair. The total energy value of vigorous activity therefore includes both the energy expended during the activity plus additional energy expended after exercise.
At present, however, we are unable to say why more intense exercise is the more important dimension of physical activity with respect to adiposity. Perhaps moderate-vigorous physical activity acts as a better marker of an overall behavioural pattern that is protective against obesity, or perhaps it has a greater effect on appetite regulation. Alternatively, more vigorous activity may lead to the development of more muscle mass and hence more metabolically active tissue. It is also possible that children with higher levels of cardiorespiratory fitness—which has a heritability component—may be more able to perform moderate-vigorous physical activity and hence the association between moderate-vigorous physical activity and adiposity is partially confounded by inherited traits.
The much weaker (but still discernible) associations of physical activity with body mass index indicate that body mass index may have some clinical utility as an indirect proxy for adiposity, but its main attraction may be financial and practical rather than scientific.
We have demonstrated in a large, well characterised group of children that physical activity levels at age 12 are strongly and inversely associated with fat mass two years later. Public health policies to reduce obesity in children should include strategies to promote higher levels of physical activity, particularly activity that is of moderate intensity and above.
What is already known on this topic
- The role of physical activity in the prevention of childhood obesity is not well described, because of a lack of large representative studies using precise measures of both physical activity and fat mass
- The level and type of activity that is most protective against obesity is also unknown
What this study adds
- Higher levels of physical activity, especially activity that is of at least moderate intensity, are strongly associated with lower levels of fat mass two years later
- Physical activity should be a primary target in public health initiatives to prevent obesity