BMI and Mortality: Analysis of 1.46 Million White Adults

Introduction to BMI as a Health Indicator

Body Mass Index (BMI) is widely regarded as an indicator of overall health. Health researchers frequently include it as a possible predictor of specific outcomes of interest, such as death or the incidence of a particular disease. Studies establishing an association between BMI and the incidence of cardiovascular disease, for example, are quite common in the current literature; however, such studies do not always account for other possible contributors to cardiovascular disease, such as smoking or other latent conditions.

In the study Body-Mass Index and Mortality among 1.46 Million White Adults, researchers were interested in determining whether an optimal BMI level exists. The investigators pooled 19 prospective trials — originally designed to address cancer-related inquiries — in order to arrive at a better understanding of the association between all-cause mortality and BMI. Specifically, the researchers' primary interest was "to assess the optimal BMI range and to provide stable estimates of the risks associated with being overweight, obese, and morbidly obese (BMI ≥40.0), with minimal confounding due to smoking or prevalent disease" (de Gonzalez, A.B., Hartge, P., Cerhan, J.R., et al., 2010, p. 2212).

Study Design and Statistical Methods

Prospective studies are those designed to observe a predefined population over time. In a prospective analysis, a particular outcome of interest — such as death or the development of disease — is documented, and exposure or non-exposure to certain risk factors is ascertained as events occur during the course of the study (Gordis, L., 2004, p. 152).

Because the study design was "time-to-event" in nature and the variable "time" was itself a variable of interest, the authors applied a statistical model from the survival analysis branch of statistics: the proportional hazards model. Cox regression was used to estimate hazard ratios, which, in plain terms, can be understood as a ratio of the probability of the event occurring in the exposed group versus a non-exposed group.

Conclusion and Limitations

The age at which a subject's BMI was measured contributed greatly to the varied risk of mortality across each BMI stratum. This was true even for subjects who fell within the "optimal" category of 20.0 to 24.9. The researchers found that "for a BMI of 22.5 to 24.9, the hazard ratios were higher for participants whose height and weight were ascertained at 20 to 49 years of age than for those whose height and weight were ascertained after the age of 70 years (P = 0.005 for trend across categories of age)" (de Gonzalez, A.B., Hartge, P., Cerhan, J.R., et al., 2010, p. 2214). An optimal BMI was more indicative of a lifetime of good fitness when the subject's BMI was measured and found to fall within the optimal range at an advanced age.

The researchers concluded that their results supported previous studies conducted to establish an optimal BMI. The study found that both overweight and obesity (and possibly underweight) categories were associated with increased all-cause mortality when the subject group was limited to those who had never smoked and had not been diagnosed with cancer or heart disease.

It should also be noted that this study included only non-Hispanic white subjects drawn from the pooled trials, because, as referenced by the authors, "the relationship between BMI and mortality may differ across racial and ethnic groups" (de Gonzalez, A.B., Hartge, P., Cerhan, J.R., et al., 2010, p. 2212). The investigators acknowledged that the results of the study are most applicable to white people living in affluent countries.

de Gonzalez, A.B., Hartge, P., Cerhan, J.R., et al. (2010). Body-mass index and mortality among 1.46 million white adults. N Engl J Med, 363, 2211–2219.

Gordis, L. (2004). Epidemiology (3rd ed.). Elsevier Inc.