ROCHESTER, MN — A larger waist, irrespective of body-mass index (BMI), is associated with an increased risk of mortality, according to a new pooled analysis of more than 650 000 men and women from 11 cohort studies[1]. Researchers suggest that waist circumference should be measured in combination with BMI, even for those within the normal BMI range, to help stratify patients who might be at risk for obesity-related mortality.

"Our results strongly suggest that BMI and waist circumference jointly serve as important predictors of mortality in the general population, so clinically it may not be useful to try to select one measure over the other," write Dr James Cerhan (Mayo Clinic, Rochester, MN) and colleagues March 13, 2014 in the Mayo Clinic Proceedings.

Obesity is frequently measured using BMI, and obese individuals have higher all-cause mortality than individuals with normal BMI, which is defined as 18.5 to 24.9 kg/m2. However, the relationship between BMI and mortality is not linear—studies have documented lower BMI being associated with higher mortality—and is confounded by tobacco use, preexisting illness, weight loss, or short follow-up, explain the researchers. Given these and other limitations, the group sought to assess the independent effect of waist circumference on mortality across the entire BMI range.

In an analysis adjusted for multiple demographic and lifestyle characteristics, physical-activity levels, and BMI, each 5-cm increment in waist circumference was associated with a 7% higher risk of all-cause mortality in men and a 9% higher risk in women. The elevated risk with higher waist circumferences was evident for both men and women across the entire range of BMI levels. The association with waist circumference was stronger for respiratory and cardiovascular mortality than for deaths from cancer.

Adjusting the analysis for BMI strengthened the association between waist size and mortality, which investigators suspect might be because this decreased "confounding by preexisting diseases, pathologic conditions, or general frailty, all of which are associated with low lean body mass."

Clinically speaking, the results suggest "that current recommendations regarding waist circumference, which are predicated on using a single sex-specific cut point and evaluation only in the BMI range of 25 to 34.9 kg/m2, should be broadened as part of risk assessment for premature mortality," conclude Cerhan and colleagues.