Changes in body mass index and smoking habits have a different impact on hemoglobin concentration in men and women: a longitudinal follow-up of the Tromsø Study, 1994-2002.

Background: Body mass index (BMI) and smoking have been positively associated with hemoglobin concentration, and both are risk factors for cardiovascular disease.

Objective: The aim of this study was to assess whether there were sex differences in how changes in BMI and smoking habits influenced hemoglobin concentration.

Methods: In 1994-95 and 2001-02, a longitudinal, population-based study was conducted in the municipality of Tromsø, in northern Norway. Inhabitants aged > or =25 years were invited to participate. Participants replied to a questionnaire regarding health, physical activity, coffee and alcohol consumption, and smoking habits. Blood samples were drawn to analyze hemoglobin concentration. All analyses were performed separately for each sex. Differences between 1994-95 and 2001-02 were examined with t or chi(2) (McNemar) tests for paired data. Cross-sectional comparisons were made using 2-sample t tests. Different models of univariate and multiple linear regression analyses were used to investigate the impact of the various variables on hemoglobin change.

Results: Data from a total of 2105 men and 2945 women were examined. At baseline, mean age was 58.9 years for men (range, 25-78 years) and 57.8 years for women (range, 25-82 years); mean BMI was 26.1 kg/m(2) for men and 25.8 kg/m(2) for women. In men, hemoglobin decreased with age, on average from 147.5 to 145.1 g/L. In women, hemoglobin decreased from 135.6 to 134.7 g/L, but increased with increasing age up to 54 years, and thereafter decreased gradually. Mean BMI increased 0.8 kg/m(2) in men and 1.2 kg/m(2) in women. In total, 394 of 2057 men (19%) and 499 of 2889 women (17%) stopped smoking or smoked fewer cigarettes per day. In a univariate regression model, an increase of 1 kg/m(2) in BMI was associated with an increase in hemoglobin of 1.1 g/L (95% CI, 0.84 to 1.27) in men and 0.4 g/L (95% CI, 0.30 to 0.56) in women. In another univariate model, smoking cessation was associated with a decrease in hemoglobin of 1.9 g/L (95% CI, -3.32 to -0.56) in men and 1.7 g/L (95% CI, -2.93 to -0.56) in women. In men who smoked less and had a BMI increase of >2.5 kg/m(2), hemoglobin decreased 0.3 g/L. In contrast, hemoglobin decreased 3.4 g/L in men who smoked less and lost weight (P for trend, < 0.001 by changing BMI). Women who smoked less had a decrease in hemoglobin independent of BMI changes.

Conclusions: The positive association between an increase in BMI and hemoglobin was stronger in men than in women. The effect of smoking reduction on hemoglobin was attenuated with increasing BMI in men, but not in women.