The influence of cigarette smoking on the association between body weight and mortality. The Framingham Heart Study revisited.

PURPOSE

To calculate for two measures of obesity, the Metropolitan Relative Weight (MRW) and body mass index (BMI), the value at which minimum mortality occurs. This was done to retest the hypothesis, in the Framingham Heart Study data, that the association between obesity and mortality can be obscured by an interaction between the measure of obesity and smoking. In the original analysis of the Framingham data it was suggested that there was a U- or J-shaped relationship between MRW and death in smokers but a linear relationship in nonsmokers. The design and setting were those of the NHLBI Framingham Heart Study.

METHODS

The 5209 members of the Framingham Heart Study underwent a baseline examination in 1948-1952 (Exam 1) and they were reexamined at approximately two-year intervals over a 30-year period. The study included both men (n = 2336) and women (n = 2873) in the age range of 28 to 62 years. After excluding persons with missing baseline data, the analytic sample size was 5163. Additional analyses were conducted by deleting persons with cardiovascular disease (CVD) at baseline (n = 135), the sample used by the original paper by Garrison and colleagues, and persons who died within the first four years of follow-up (n = 62). The main outcome measures consisted of thirty-year survival through Exam 16, approximately in 1980, as influenced by MRW or BMI, age, and smoking status at baseline (Exam 1).

RESULTS

We were able to show that the sample sizes of male nonsmokers were too small to test the hypothesis within age groups < 40 and 40-49 years. In men ages 50-62 there was a significant age-adjusted quadratic relationship between BMI or MRW, and risk of death. The estimated BMI at the minimum risk of death for smokers (24.5) and nonsmokers (23.8) were not statistically different. Identical results were found for MRW (minimum: smokers = 112.5, nonsmokers = 111.4). In men and women ages 28-62 there appeared to be a u- or j-shaped relationship between the 30-year crude mortality rate and MRW. After excluding persons with missing data, CVD at baseline, and persons who died within the first four years of follow-up, the age adjusted estimated BMI value at the minimum risk of death was nearly identical for men and women and for smokers and nonsmokers (Men: smokers = 22.8, nonsmokers = 22.8; Women: smokers = 22.9, nonsmokers = 23.3). Additionally, the estimates of the minimum were always below the mean. Identical results were found without deleting persons with CVD at baseline and deaths in the first four years of follow-up. Identical results were found for MRW.

CONCLUSIONS

Reanalysis of the Framingham Heart Study data does not support the hypothesis that there is an interaction between smoking and measures of obesity. Moreover, the estimated BMI or MRW at the minimum risk of death was similar for men and women smokers and nonsmokers alike even after deleting prevalent cases of CVD and deaths within the first four years of follow-up.