The influence of cigarette smoking on circulating concentrations of antioxidant micronutrients.

Cigarette smoke is a significant source of oxidative stress, one potential mechanism for its untoward health effects. The antioxidant defense system is partly comprised of antioxidant micronutrients, making it important to understand the relationship between cigarette smoking and circulating concentrations of antioxidant micronutrients. A synthesis of the literature shows that compared with nonsmokers, on average, active smokers have greater than 25% lower circulating concentrations of ascorbic acid, alpha-carotene, beta-carotene, and cryptoxanthin. The differences in blood concentrations of these micronutrients in former smokers is intermediate between never and current smokers, but average circulating concentrations of alpha-carotene, beta-carotene, and cryptoxanthin were 16-22% lower in former smokers compared with never smokers. Differences in dietary habits between smokers and nonsmokers could potentially account for these associations. Dietary micronutrient intake is associated with blood micronutrient concentrations. Furthermore, patterns of micronutrient consumption by smoking status mimic the pattern of associations observed for blood concentrations. For example, when pooled across studies intake of vitamin C was 16% lower in current smokers and 2% lower in former smokers than in never smokers; the corresponding figures for beta-carotene were 17 and 4%, respectively. Despite the strong potential for confounding, the differences observed between current smokers and nonsmokers seem to be due to an acute effect of smoking based on results of studies of smoking and antioxidant micronutrient concentrations that have either adjusted for dietary antioxidant micronutrient intake and other potential confounding factors or documented short term changes in circulating antioxidant micronutrient concentrations in smokers before and after smoking cigarettes. The associations observed with active smoking also appear to hold true for passive smoking, implying that even low-dose exposures to tobacco smoke can result in lowered circulating antioxidant micronutrient concentrations. Smoking was more weakly associated with circulating concentrations of vitamin E and the nonprovitamin A carotenoids lutein/zeaxanthin and lycopene. The combined evidence supports the conclusion that cigarette smoking is independently associated with lowered circulating concentrations of ascorbic acid and provitamin A carotenoids. These associations have implications for the design and interpretation of epidemiologic studies of antioxidant micronutrients in relation to health and disease. To the extent that these micronutrients are associated with health and longevity, this evidence documents yet another deleterious consequence of cigarette smoking on human health.