1. ¹Health Analysis Division, Statistics Canada, Ottawa, Ontario, Canada
2. ²Canadian Institute for Health Information, Ottawa, Ontario, Canada
3. ³Department of Geography, McGill University, Montréal, Québec, Canada
4. ⁴School of Community Health, Portland State University, Portland, Oregon, USA
5. ⁵Institute of Health Economics and Department of Economics, University of Alberta, Edmonton, Alberta, Canada
6. ⁶Kaiser Permanente Northwest Center for Health Research, Portland, Oregon, USA
7. ⁷Department of Psychiatry, Oregon Health and Science University, Portland, Oregon, USA

Correspondence: Heather Orpana (horpana@uottawa.ca)

Received 17 December 2008; Accepted 11 May 2009; Published online 18 June 2009.

Abstract

Although a clear risk of mortality is associated with obesity, the risk of mortality associated with overweight is equivocal. The objective of this study is to estimate the relationship between BMI and all-cause mortality in a nationally representative sample of Canadian adults. A sample of 11,326 respondents aged 25 in the 1994/1995 National Population Health Survey (Canada) was studied using Cox proportional hazards models. A significant increased risk of mortality over the 12 years of follow-up was observed for underweight (BMI <18.5; relative risk (RR) = 1.73, P < 0.001) and obesity class II+ (BMI >35; RR = 1.36, P <0.05). Overweight (BMI 25 to <30) was associated with a significantly decreased risk of death (RR = 0.83, P < 0.05). The RR was close to one for obesity class I (BMI 30–35; RR = 0.95, P >0.05). Our results are similar to those from other recent studies, confirming that underweight and obesity class II+ are clear risk factors for mortality, and showing that when compared to the acceptable BMI category, overweight appears to be protective against mortality. Obesity class I was not associated with an increased risk of mortality.