BMI, Lung Cancer, Inversely Associated in Current and Former Smokers

Body mass index (BMI) appears to be inversely associated with the risk for lung cancer, even after smoking status is carefully taken into account. A new prospective study found this association was restricted to current and former smokers and was actually stronger after adjustments for smoking were made.

Cigarette smoking's association with lower BMI has always been a significant confounding factor

Many earlier studies have found that increasing BMI is associated with decreasing risk of lung cancer, but cigarette smoking’s association with lower BMI has always been a significant confounding factor. In the new study, led by senior author Gretchen L. Gierach, PhD, MPH, researchers examined this association in 448,732 men and women between the ages of 50 and 71 who were recruited during 1995-1996 for the National Institutes of Health-AARP Diet and Health Study. The results were published in the Journal of the National Cancer Institute.

The cohort consisted of 271,238 men and 177,494 women; the median BMI of men was 26.6 kg/m2, and 25.8 for women. Less than 30% of men had a BMI that is considered normal weight, while 43.1% of women were of normal weight; 70% of men and 56% of women were either current or former smokers-former smokers had the highest BMI among both men and women, while current smokers were the leanest group.

Over a mean follow-up period of 9.7 years, 6,093 men and 3,344 women were diagnosed with lung carcinomas. Of those, there were 166 men and 249 women diagnosed who had never smoked. The overall incidence rate of lung cancer was 233 per 100,000 person-years among men, and 192 per 100,000 person-years among women.

In a multivariate model that adjusted for smoking status, BMI was significantly and inversely associated with the risk of lung cancer in both men and women. For men, the hazard ratio for men with a BMI of 35 or higher vs. men with a BMI between 22.5 and 24.99 was 0.81 (95% CI, 0.70-0.94). For women, the decreased risk was even more pronounced, with a HR of 0.73 (95% CI, 0.61-0.87.)

When stratified by smoking status, it became clear that the inverse association between BMI and lung cancer risk was only present in current and former smokers. “Among never smokers, we observed no association between BMI and the risk of lung cancer in either age-adjusted or multivariable-adjusted models,” the researchers wrote. The inverse risk was slightly stronger among current smokers than among former smokers.

The investigators also wanted to examine any potential residual effects of smoking on lung cancer risk. They tested the BMI association in models stratified by amount of time since quitting smoking and the number of cigarettes smoked per day; the inverse association remained intact except in men who quit smoking 20 cigarettes or fewer at least 10 years ago, and women who quit the same number of cigarettes between 5 and 9 years ago.

“This finding suggests the need to consider alternative explanations for the inverse association between BMI and risk of lung cancer,” the researchers wrote.

One of these possible explanations involves increased levels of estrogen among obesity. Studies have shown decreased risk of lung cancer with the use of menopausal hormones, so it may be possible that the increased hormones in obese individuals directly lead to the reduction in cancer risk. The authors noted that the stronger inverse association in this study among women would support such a hypothesis.