Body Mass Index, the Most Widely Used But Also Widely Criticized Index: Would a Criterion Standard Measure of Total Body Fat Be a Better Predictor of Cardiovascular Disease Mortality?

Abstract

Objectives: To examine whether an accurate measure (using a criterion standard method) of total body fat would be a better predictor of cardiovascular disease (CVD) mortality than body mass index (BMI).

Participants and methods: A total of 60,335 participants were examined between January 1, 1979, and December 31, 2003, and then followed-up for a mean follow-up period of 15.2 years. Body mass index was estimated using standard procedures. Body composition indices (ie, body fat percentage [BF%], fat mass index [FMI], fat-free mass [FFM], and FFM index [FFMI]) were derived from either skinfold thicknesses or hydrostatic weighing. For exact comparisons, the indices studied were categorized identically using sex-specific percentiles.

Results: Compared with a medium BMI, a very high BMI was associated with a hazard ratio (HR) of 2.7 (95% CI, 2.1-3.3) for CVD mortality, which was a stronger association than for BF% or FMI (ie, HR, 1.6; 95% CI, 1.3-1.9 and HR, 2.2; 95% CI, 1.8-2.7, respectively). Compared with a medium FFMI, a very high FFMI was associated with an HR of 2.2 (95% CI, 1.7-2.7) for CVD mortality, with these estimates being markedly smaller for FFM (ie, HR, 1.2; 95% CI, 0.9-1.6). When the analyses were restricted only to the sample assessed with hydrostatic weighing (N=29,959, 51.7%), the results were similar, with even slightly larger differences in favor of BMI (ie, HR, 3.0; 95% CI, 2.2-4.0) compared with BF% and FMI (ie, HR, 1.5; 95% CI, 1.2-1.9 and HR, 2.1; 95% CI, 1.6-2.7, respectively). We estimated Harrell's c-index as an indicator of discriminating/predictive ability of these models and observed that the c-index for models including BMI was significantly higher than that for models including BF% or FMI (P<.005 for all).

Conclusion: The simple and inexpensive measure of BMI can be as clinically important as, or even more than, total adiposity measures assessed using accurate, complex, and expensive methods. Physiological explanations for these findings are discussed.

Figure 1. Hazard ratios for mortality due to cardiovascular disease (CVD) according to body mass index (BMI), percent body fat (BF%) and fat mass index (FMI) groups in the whole study sample (N=57,910) and in the sub-sample with hydrostatic weighing assessment (N=29,959)

Body weight/body fat groups were estimated based on sex-specific centiles: Very low if < percentile 5^th, Low if percentile 5^th-15^th, Middle if percentile 15^th-85^th, High if percentile 85^th-95^th and Very high if above percentile 95^th. All the models were adjusted for age, sex, examination year, smoking, alcohol consumption, inactivity and parental history of CVD.

Figure 2. Hazard ratios for mortality due to cardiovascular disease (CVD), according to fat-free mass (FFM) and to fat-free mass index (FFMI) groups in the whole study sample (N=57,910) and in the sub-sample with hydrostatic weighing assessment (N=29,959)

FFM and FFMI groups were estimated based on sex-specific centiles: Very low if < percentile 5^th, Low if percentile 5^th-15^th, Middle if percentile 15^th-85^th, High if percentile 85^th-95^th and Very high if above percentile 95^th. All the models were adjusted for age, sex, examination year, smoking, alcohol consumption, inactivity and parental history of CVD.

Figure 3. Graphical illustration of the main findings of the present study and plausible physiological interpretation

BMI indicates body mass index; BF%, percent body fat; FMI, fat mass index; FFM, fat-free mass; FFMI, fat-free mass index; CVD, cardiovascular disease; LV, left ventricular; RV, right ventricular. Part of the physiological interpretation shown is adapted with permission from Lavie et al. . More detailed information about obesity and its relationship with pathophysiology and hemodynamics of CVD is provided elsewhere ^,,.