BMI misclassification, leptin, C-reactive protein, and interleukin-6 in young women with differing levels of lean and fat mass.

OBJECTIVE

To describe the misclassification of obesity based on BMI criteria, in women with similar fat mass but differing lean mass levels, and to describe how the misclassification relates to circulating serum leptin, high sensitive C-reactive protein (hsCRP) and interleukin-6 (IL-6).

DESIGN

Cross-sectional study.

SUBJECTS

Healthy women aged 18-35 (n = 255).

MEASUREMENTS

Body fat was measured by dual X-ray absorptiometry (DXA). Obesity was defined using WHO cut-off points for BMI and body fat % (BF%) ≥35. Participants were grouped as: (1) high fat mass/high lean mass, (2) high fat mass/low lean mass, (3) low fat mass/high lean mass, and (4) low fat mass/low lean mass. Serum leptin, hsCRP and IL-6 were assayed using commercial kits.

RESULTS

According to BMI, 18.8.5% of the subjects (n = 48) were overweight and 13.7 (n = 35) were obese; in contrast, according to BF%, 48.6% (n = 124) were obese. While, the BF% of the high-fat groups was virtually identical (at 42.7% [sd = 7.0] and 41.8% [sd = 4.2], respectively), the BMI of the high fat/high lean group was significantly greater than that of the high fat low/low lean group (30.0 ± 5.2 and 24.9 ± 1.9, respectively). Leptin, hsCRP and IL-6 concentrations in the high-fat groups were not significantly different from each other, but were significantly greater than in the low-fat groups.

CONCLUSIONS

BMI underestimates obesity in young women; misclassifying women with high fat mass and low lean mass as 'normal' when BF%, leptin, hsCRP and IL-6 suggest they are obese.