Overweight: The Body Mass Index Category With an Identity Crisis

Decades of epidemiologic research have shown that body mass index (BMI), the most commonly used anthropometric index of adiposity, is associated with risk for death (1). In addition, BMI has been a simple and useful metric to document the growth in prevalence of overweight and obesity worldwide (2). Both ends of the BMI spectrum (underweight [BMI <18.5 kg/m2] and obesity [BMI 30 kg/m2]) are clearly associated with increased risk for death, resulting in the often-illustrated J-shaped curve between BMI and mortality. However, uncertainty remains about the optimal BMI associated with the lowest risk for death. This debate was further fueled by a meta-analysis conducted by Flegal and colleagues (3), which concluded that overweight persons had a lower risk for death than normal-weight personsa finding that received extensive media attention and even led the lay press to suggest that the risks of overweight and obesity had been exaggerated. Yu and associates (4) explore this question further using 3 large and well-established prospective cohorts: the NHS (Nurses' Health Study) I and II and the HPFS (Health Professionals Follow-Up Study). In each of these cohorts, BMIs were reported by participants on biennial questionnaires, providing a unique opportunity to examine the importance of weight history rather than merely a single measurement. For each participant, the investigators identified the highest BMI over an initial follow-up of 16 years (maximum BMI), as well as a single baseline BMI recorded after this initial 16-year weight history period. They then compared the associations of the baseline and maximum BMIs with mortality during the subsequent follow-up (20 years in the NHS, 8 years in the NHS II, and 10 years in the HPFS). At the baseline measurement, participants in the HPFS were, on average, older (68 years) than those in the NHS (58 years) and the NHS II (50 years). The prevalence of overweight and obesity was lower when baseline BMIs were used than when the maximum BMIs during the previous 16 years were used. This difference was particularly marked in the older HPFS cohort (61% for baseline BMI vs. 74% for maximum BMI), suggesting that some elderly participants in the HPFS may have started to unintentionally lose weight at the time of their baseline measurement. Similar to prior findings (3), Yu and associates found that the overweight category as defined by a single baseline BMI was not associated with increased risk for death in the 3 cohorts. In contrast, when maximum BMI was used, overweight was clearly associated with a significant, albeit modest, increase in risk. These results show the limitations of using a single BMI measurement, particularly among older persons. In addition, they address the potential for reverse causation as the likely explanation for the misclassification of risk for death in the overweight BMI category (that is, an illness might cause weight loss and thus a lower BMI as well as increased mortality). As the authors note, because of the high prevalence of overweight (about 25% to 33%, depending on the region of the globe considered), there is a clear need to better define the health and mortality risk associated with this BMI category. Although this elegant analysis addresses the issue of misclassification associated with a single BMI measurement, it was unable to provide an inexpensive clinical alternative to refine the assessment of risk for death associated with the overweight BMI category. However, opportunities lie ahead. For example, although both the normal and obese BMI categories generally track well with a low versus a high body fat content, respectively, the overweight category has the largest individual variation in body composition and body fat distribution. Sahakyan and colleagues (5) have shown that 37% of overweight men in the NHANES III (Third National Health and Nutrition Examination Survey) cohort were characterized by excess central adiposity (as assessed by an elevated waist-to-hip ratio) and that these overweight and centrally obese men were at even greater risk for death than obese men without an elevated waist-to-hip ratio. Another pooled analysis similarly revealed that a simple anthropometric measurementwaist circumferencecould be associated with dramatic modulation in risk for death within every BMI category considered (6). Two other behavioral risk factors also modulate morbidity and mortality associated with a given BMI: level of physical activity or cardiorespiratory fitness (which can have a profound effect on body composition and can reduce visceral adiposity, ectopic fat, and related health risk) and overall nutritional quality (which can substantially modulate health risk at a given BMI) (7, 8). From a practical standpoint, how could clinical practice be improved for patients in this overweight BMI category that still has an identity crisis? These recent findings support our previous proposal that adding waist circumference to BMI may be the simplest way in clinical practice to refine evaluation of the health risk of overweight when no precise record of weight history is available. Simple and standardized questions, such as the EPIC-Norfolk Physical Activity Questionnaire (9), have also been shown to further refine evaluation of health risk associated with given BMI and waist circumference categories. Assessing global nutritional quality in clinical practice has been a daunting objective, but promising food-based tools have been reported (10). We envision that in the near future, clinicians will be able to use overall nutritional quality scores to better discriminate morbidity and mortality risk. The development and implementation of such tools in clinical practice should be a high-priority objective for clinical nutrition experts. In summary, the article by Yu and associates (4) sheds light on what seems to be a reverse causality problem. This lesson suggests that we need to go beyond a single BMI measurement to refine our risk assessment, particularly within this prevalent subgroup of overweight patients who may sometimes read in the lay press that their body weight is OK. Adding waist circumference to the BMI may represent a simple but key step in the introduction of a new and relevant vital sign in clinical practice.