Over the past decades, several reports from long-term population studies have shown that being overweight or obese is associated with an increased risk of premature death. Although the strength of the association between obesity (using body mass index [BMI] defined as the weight in kilograms divided by the square of the height in meters) and mortality risk is very consistent across studies, this association is generally not very strong. Indeed, an important portion of the population with a low BMI has a high mortality risk whereas another portion of the population with a high BMI has a somewhat higher life expectancy. Therefore, the BMI is a good tool to estimate mortality risk of populations but its use for predicting mortality risk in any given individual is questionable. Studies have suggested that this observation could be due to the fact that individuals with similar BMIs come in different shapes and sizes and that the assessment of body composition or body fat distribution may help discriminate between those who will and who will not develop pathologies such as cardiovascular disease (CVD), diabetes or even premature death.
This hypothesis was recently tested by a group of researchers from the Mayo Clinic in the United States. In an article published in the latest issue of Annals of Internal Medicine, the authors used data from 15,184 men and women followed for approximately 14 years who participated in the third National Health and Nutrition Examination Survey (NHANES) to document the respective contributions of both waist and hip circumference measurements (used as a ratio to estimate abdominal obesity) and BMI to total and cardiovascular mortality risk. During the follow-up, 3222 deaths occurred with 1404 of them being attributable to CVD. In participants classified as “normal weight” (BMI below 25 kg/m2), those with abdominal obesity (defined as a waist-to-hip ratio equal or above 0.90 for men and 0.85 for women) were much more likely to die during the follow-up compared to those without abdominal obesity (87% increased risk for men and 48% for women). In fact, normal weight participants with abdominal obesity had the worst long-term survival compared with individuals with any BMI, but without abdominal obesity. Similar relationships were also observed for cardiovascular mortality, in both men and women, even after statistical adjustment for potential confounders.
Although this report was conducted in a U.S. population only, that no imaging of body fat distribution was performed and that other epidemiological studies had documented the health hazards of abdominal obesity in the past, this report is of high relevance as it shows that regardless of body weight, abdominal obesity is associated with higher mortality rates in the general population. It also emphasizes that this relationship is particularly observed in people with a normal body weight, e.g., those people who, according to most guidelines from various health authorities, have an ideal body weight. In other words, people with an ideal body weight may have a shorter lifespan if they have abdominal obesity (a high waistline or a high waist-to-hip ratio). The authors of this report suggested that this observation could be due, at least in part, to the fact that BMI will not capture the “sarcopenic obesity” phenotype (those people who, as they age, tend to decrease their muscle mass in the legs and increase fat storage in the abdomen). Results of this report also suggest that individuals who are classified as overweight or even obese may not be at increased risk of premature death if they do not have abdominal obesity and therefore strongly question the usefulness of the BMI as a health indicator (especially if not combined with an abdominal obesity measurement).
Recent studies clearly identified healthy lifestyle behaviours such as high physical activity levels and a good nutritional quality as strong predictors of CVD in various populations. Yet, although such lifestyle behaviours may influence body composition, their impact on BMI is trivial. Consequently, the pursuit of healthy lifestyle behaviours is what the guidelines and everyone’s goals should be all about no matter if you lose weight or not.