Body composition unaltered for African women classified as 'normal but vulnerable' by body mass index and mid-upper-arm-circumference criteria

Objective: To test the hypothesis that 'normal but vulnerable' adults, as d efined by body mass index (BMI) in combination with mid-upper-arm-circumfer ence (MUAC), are closer to normal than to malnourished ones. For that purpo se body composition measurements were compared between normal and low BMI c ategories and according to MUAC value in an African context and for differe nt age groups. Design: Reanalysis of data from a previous cross-sectional cluster sample n utrition survey. Setting: A rural area of the Republic of Congo, Central Africa. Subjects;: A representative sample (n = 544) of non-pregnant women. Main outcome measures: Arm muscle area was calculated from measurements of triceps skinfold thickness and MUAC. Peripheral body fat was assessed by th e sum of four skinfold thicknesses. The ratio of resistance at high and low frequencies was derived from whole body measurement of multifrequency bioe lectrical impedance analysis and used as the extracellular to total body wa ter ratio index. Results: The prevalence of thinness decreased from 18.7% as defined by BMI alone to 9.0% as defined by BMI and MUAC. This difference was due to the gr oup of subjects classified as 'normal but vulnerable' (9.7%). prevalence of thinness increased with age When assessed by BMI alone, but no longer when assessed by BMI and MUAC. Comparison with the BMI greater than or equal to 18.5 kg/m(2) category showed that in 'normal but vulnerable' subjects lowe r BMI was accompanied by lower both fat and lean compartments, in absolute values, but the equilibrium of body water compartments was not altered. In BMI < 18.5 women, low MUAC was associated with altered lean tissues, at per ipheral and whole body level, whereas fat tissue did not differ. Conclusions: 'Normal but vulnerable' subjects appeared as 'thin but healthy ' rather than malnourished, at all ages, even though their BMI was lower th an 18.5 kg/m(2). The new classification of thinness based on BMI and MUAC p rovides a more specific index of nutritional status when restricting the th in category to more at-risk subjects.