Question

Share an example of how the body’s measurement may vary by ethnicity.

Answer 1

Bioelectrical Impedance Analysis (BIA) has the potential to be used widely as a method of assessing body fatness and composition, both in clinical and community settings. BIA provides bioelectrical properties, such as whole-body impedance which ideally needs to be calibrated against a gold-standard method in order to provide accurate estimates of fat-free mass. UK studies in older children and adolescents have shown that, when used in multi-ethnic populations, calibration equations need to include ethnic-specific terms, but whether this holds true for younger children remains to be elucidated. The aims of this study were to examine ethnic differences in body size, proportions and composition in children aged 5 to 11 years, and to establish the extent to which such differences could influence BIA calibration.

Methods

In a multi-ethnic population of 2171 London primary school-children (47% boys; 34% White, 29% Black African/Caribbean, 25% South Asian, 12% Other) detailed anthropometric measurements were performed and ethnic differences in body size and proportion were assessed. Ethnic differences in fat-free mass, derived by deuterium dilution, were further evaluated in a subsample of the population (n = 698). Multiple linear regression models were used to calibrate BIA against deuterium dilution.

Results

In children <11 years of age, Black African/Caribbean children were significantly taller, heavier and had larger body size than children of other ethnicities. They also had larger waist and limb girths and relatively longer legs. Despite these differences, ethnic-specific terms did not contribute significantly to the BIA calibration equation (Fat-free mass = 1.12+0.71*(height2/impedance)+0.18*weight).

Conclusion

Although clear ethnic differences in body size, proportions and composition were evident in this population of young children aged 5 to 11 years, an ethnic-specific BIA calibration equation was not required.

Comparison of body size and proportions between ethnic groups

There were marked ethnic differences in body size, these differences being similar in both populations (Table 1). After adjusting for age and sex, Black African/Caribbean children were significantly heavier, taller and with greater BMI than those from other ethnic groups. On average, Black African/Caribbean children had the largest body girths (MUAC, calf and waist) across all age groups, but the lowest sitting/standing height ratio when compared to other ethnicities, indicating relatively longer legs . With the exception of height and sitting/standing height ratio, ethnic differences in body size increased after 9 years of age, particularly between Black African/Caribbean and South Asian children.

Comparison of body composition between ethnic groups

FMI and FFMI for each child in the BIA calibration population were plotted on Hattori charts (Figure 3). For a particular BMI value, there was substantial individual variability in the proportions of FFMI and FMI (Figure S4). The standard deviation of FFMI for all children was 1.66 kg/m2 and of FMI was 2.62 kg/m2. This indicates that after adjusting for height, overall between-subject variability in FM was over 1.6 times greater than that in FFM. In contrast, after adjusting for height, ethnic differences in FFM were more evident than ethnic differences in FM . There were also clear ethnic differences in the distribution of FM and FFM, i.e. after adjusting for height, as FFM increased, the FM in South Asian and Black African/Caribbean children increased more than in White children, as demonstrated by steeper regression lines. FMI was generally higher in South Asian children than children of other ethnicities for both boys and girls. The ethnic-specific relationship between FMI and FFMI also differed in boys and girls, with a much steeper relationship in girls, indicating that after adjusting for height, girls had greater FM per unit of FFM than boys.