Background

The definition of obesity is over-accumulated body fat. However, the present standards of overweight and obesity were based on BMI cutoff points. This study is proposed to assess the diagnostic performance of BMI in Chinese adults' adiposity using the World Health Organization BF % reference standard for obesity as gold standard, and to explore the appropriate BMI cut-off for Chinese adults health intervention.

Methods

689 adults aged from 18 to 79-year-old consented to participate study. Their height and weight were measured, BMI was calculated according to the formula BMI = weight(kg)/height(m)2 . Dual Energy X-ray Absorptiometry (DEXA) was used to estimate participants' body fat percentage (BF%). We assessed the diagnostic performance of BMI standard for Chinese adults’ obesity (³28) using the World Health Organization reference standard for obesity of BF % > 25% in men and > 35% in women. Receiver Operating Characteristic analysis was applied in cut-off point estimate with the lowest value for the formula: (1-sensitivity)2+(1-specificity)2.

Results

(1) The standard of BMI ≥ 28 had a higher specificity (95%) , but a poor sensitivity (37% for men and 18 % for women respectively) and higher false negative rate (especially for women: 82%) to detect BF %-defined obesity. (2) The reliability evaluation of BMI showed that crude consistency rate with “gold standard” was higher in men (0.67) and lower in women (0.58); and the consistency in men is good (Kappa = 0.32), in women is poor (Kappa = 0.13). (3) Youden index of BMI standard is lower, especially for women (0.13), the positive predictive value and positive likelihood are higher, and the negative predictive value is lower, negative likelihood is relatively high. (4) Using the percentage of body fat determining obesity as gold standard, the diagnostic performance of BMI is medium (AUC=0.822, P=0.000), and gets better when analysis were conducted by men and women separately (AUC is 0.844 for men and 0.868, for women, P=0.000); BMI cut-off were 25.24 and 23.98 for men and women respectively.

Conclusions

Under the background of high prevalence of physical inactivity, the existing Chinese adults’ BMI obesity threshold might be too high, especially for women. In smaller sample studies, it is not appropriate using BMI as obesity classify standard; To establish health indicators-referenced, body fat composition-considered and sex and age specified obesity criterion is necessary; BMI screening oriented by health interventions should take improving sensitivity, and reducing specificity appropriately as principle.