Association of body mass index with all-cause and cause-specific mortality: findings from a prospective cohort study in Mumbai (Bombay), India

Background The association between extremes of relative body weight and mortality has not been well characterized in developing countries. A prospective cohort study was conducted in Mumbai (India) to study the association of relative body weight, as estimated by body mass index [BMI = weight (kg)/height(2) (m(2))], with mortality. Methods Using the voters list as the selection frame, 148 173 men and women aged >= 35 years were recruited during 1991-97 in Mumbai city and were followed-up during 1997-2003. Results During 774 129 person-years of follow-up, 13 261 deaths were observed. After adjusting for the potential confounders, increased mortality was observed in all under-weight categories [relative risk (RR) = 1.94 for BMI < 16.0 kg/m(2), 1.38 for BMI 16.0 to 17.0 and 1.24 for BMI 17.0 to 18.5 among women; the corresponding values for men were 2.24, 1.45 and 1.27, respectively] when compared with the rate in the normal weight category (BMI 18.5 to <25.0). Extremely thin (BMI <16.0 kg/m(2)) cohort members were at highest risk for death due to tuberculosis (TB) (RR = 7.20 and 14.94 in women and men, respectively), cancer (RR = 1.87 and 2.44, respectively) and respiratory diseases (RR = 3.46 and 4.35, respectively). Subjects with above normal BMI had lower mortality risk than those with normal BMI values. Over-weight (BMI 25.0 to <30.0) women (RR = 0.89) and men (RR = 0.87) were at a lower risk; however, obese (BMI >= 30.0) men < 60 years of age had an increased mortality risk (RR = 1.22). Conclusion In Mumbai, under-weight was associated with an increased risk of pre-mature death. Despite the growing concerns regarding the gradual transition toward increasing rates of obesity, under-nutrition remains a major health problem in India. This study suggests the need for public health policies focusing on reducing under-nutrition.