Total amount, not intensity, of exercise key to children's BP control

11 December 2007

MedWire News: Children who keep physically active have lower blood pressure (BP) than their less active peers, report UK researchers who found the total amount of exercise children do is more important than the intensity.

The relationship between physical activity and BP in adults is well established, but findings in children have been inconsistent. This may be because most studies have based physical activity data on self-reported questionnaires, which provide a poor measure in children. By contrast, mechanical techniques such as accelerometers may provide a more accurate measure.

Lead investigator Sam Leary (University of Bristol) said: "Our study provides invaluable data. In addition to its large size, it is one of the few to use accelerometers and the first to our knowledge to compare the volume and intensity of activity with respect to associations with BP in childhood."

The researchers used an accelerometer that detects movement in the vertical plane, recorded as counts, to measure the activity of children aged 11-12 years who were recruited from the Avon Longitudinal Study of Parents and Children.

They looked at two parameters: total physical activity (PA), including all activities at all intensities measured as the average counts per minute (cpm) over the period of valid recording, and moderate to vigorous physical activity (MVPA), which was calculated by the average number of minutes per day above 3600 cpm.

A total of 5505 children had both systolic and diastolic BP measurements plus valid accelerometer measures, defined as at least 10 hours for 3 separate days.

As reported in an advance online publication by the journal Hypertension, the median total PA was 644.7 cpm for boys and 528.6 cpm for girls, while median MVPA measures were 25.4 and 15.7 minutes per day, respectively.

Few of the children had high BP levels. Average systolic BP was 104.8 mmHg among boys and 106.0 mmHg for girls, while the average diastolic BP was 58.3 mmHg and 59.1 mmHg, respectively. Current guidelines of at least 60 minutes MVPA per day were met by just 2.6% of children; mean systolic and diastolic BP levels were 2.4 mmHg and 1.0 mmHg lower in those who did.

Small but significant inverse associations were seen between PA and both BP levels, equivalent to almost 0.5-mmHg lower levels of each for every 100 cpm increment in PA. This association was weakened but not lost by adjustment for the confounding factors social class, maternal health, and body mass index.

Similarly, each 15-minute increase in MVPA was associated with at least 0.5 mmHg lower BP levels. But when PA and MVPA were analyzed in combination, associations between PA and BP levels were weakened only slightly, whereas those between MVPA and BP levels were substantially reduced.

"Any activity other than lying or sitting contributes to the total volume of activity, so if, for example, children chose to walk to and from school instead of riding in a car, their overall level of physical activity would increase," Leary explained. "Moderate to vigorous activity includes anything where you work up a sweat, such as fast walking, running, or jogging."

He added that the findings "suggest that encouraging children to increase their levels of physical activity likely will track into adulthood and be associated with a lower adult BP and thus contribute to a reduction in cardiovascular risk."

Hypertension 2007; Advance online publication