Ultrasound bone measurements in pre-adolescent girls: interaction between ethnicity and lifestyle factors.

AIM: We obtained calcaneal ultrasound measurements in 198 girls between 7.5 and 11.7 y of age, representing ethnic groups (black [n = 80], white [n = 41], mixed ancestral origin [n = 77]) in South Africa.
METHODS: Anthropometry was assessed. Demographics, physical activity, habitual dietary calcium intake and pubertal development were quantified by questionnaires. Broadband ultrasound attenuation (BUA) and speed of sound (SOS) of the left calcaneus were measured. Girls in Tanner breast stage 5 and/or those menstruating were excluded from analysis.
RESULTS: Black girls were lighter than white girls (31.4 +/- 7.8 vs 34.8 +/- 7.5 kg; p < 0.05), and shorter than girls of mixed ancestral origin (1.29 +/- 0.08 vs 1.34 +/- 0.07 m; p < 0.001) and white girls (1.35 +/- 0.07 m; p < 0.001), after adjusting for age. Reported calcium intake scores were higher in black than white girls (21.6 +/- 11.1 vs 16.1 +/- 8.4; p < 0.01). Total peak bone strain score (TPBSS) was higher in white compared to black girls (6.8 +/- 4.8 vs 5.0 +/- 4.7; p < 0.05), while walking energy expenditure (MET h/wk) was higher in black girls compared to the other groups (p < 0.001). BUA and SOS were higher in the black girls (59.6 +/- 13.7 dB/MHz; 1575.1 +/- 22.6 m/s; p < 0.001) and girls of mixed ancestral origin (59.0 +/- 12.5 dB/MHz; 1567.8 +/- 26.1 m/s; p < 0.01) than in the white girls (50.4 +/- 8.7 dB/MHz; 1552.1 +/- 19.5 m/s). Co-varying for age and weight did not affect these results. Walking energy expenditure (r = 0.20) and calcium score (r = 0.17) were correlated (p < 0.05) with SOS for the whole group.
CONCLUSION: Ultrasound parameters were lower in the white compared to the black girls, who consumed more calcium on average, but who were lighter, shorter and performed less impact activity. This suggests that interactions between ethnicity and lifestyle factors determine bone quality in premenarcheal girls.