Background Low heart rate variability (HRV) is associated with increased cardiovascular disease risk. South Asians are at increased risk of cardiovascular disease.

Methods This cross-sectional, observational study compared long-term HRV between South Asian and White European (EU) children. 155 children (92 White EU, 63 South Asian) aged 8±1 years wore a combined heart rate and activity monitor for 7 days. From 24-h beat-to-beat heart rate recordings, long-term HRV was calculated as the SD of the average RR intervals in each 5 min period (SDANN). Physical activity, daily energy expenditure, percentage body fat and maximal oxygen consumption (VO_2max) were calculated using published or commercial algorithms.

Results SDANN was significantly lower during the 24-h period in South Asian compared with White EU children (113±27 vs 130±28 ms, p<0.01, mean±SD) and during sleep (44±12 vs 54±16 ms, p<0.01, mean±SD). Ethnic differences in HRV were associated with lower levels of physical activity, daily energy expenditure, VO_2max, and higher percentage body fat and resting heart rate, in South Asian children. Ethnic differences in HRV persisted despite statistical adjustment for these factors. SDANN was significantly lower in girls, but sex differences were no longer apparent when VO_2max, physical activity and energy expenditure were statistically controlled for.

Conclusions Our data confirm that South Asian children have lower HRV and reveal the novel information that the reduction is partially independent of physical activity and aerobic fitness.