Diet and obstructive lung diseases.
The results presented in this review suggest that the impact of nutrition on obstructive lung disease is most evident for antioxidant vitamins, particularly vitamin C and, to a lesser extent, vitamin E. By decreasing oxidant insults to the lung, antioxidants could modulate the development of chronic lung diseases and lung function decrement. Antioxidant vitamins could also play an important role in gene-environment interactions in complex lung diseases such as childhood asthma. Data also suggest that omega-3 fatty acids may have a potentially protective effect against airway hyperreactivity and lung function decrements; however, relevant data are still sparse. Although epidemiologic data suggest that consumption of fresh fruit may reduce risk of noncarcinogenic airway limitation, there are no clear data on which nutrients might be most relevant. While some studies evaluate daily intake of vitamin C, other studies use fruit consumption as a surrogate for antioxidant intake. Given the dietary intercorrelations among antioxidant vitamins, particularly vitamin C, beta-carotene, and flavonoids, as well as other micronutrients, it may be difficult to isolate a specific effect. Some population subgroups with higher levels of oxidative stress, such as cigarette smokers, may be more likely to benefit from dietary supplementation, since some studies have suggested that antioxidant intake may have a greater impact in this group. Studies of lung function decrement and COPD in adults suggest that daily intake of vitamin C at levels slightly exceeding the current Recommended Dietary Allowance (60 mg/day among nonsmokers and 100 mg/day among smokers) may have a protective effect (20). In the Schwartz and Weiss (85) and Britton et al. (87) studies, an increase of 40 mg/day in vitamin C intake led to an approximate 20-ml increase in FEV1. Daily mean vitamin C intakes in these studies were 66 mg and 99.2 mg, respectively, and the highest intake level (178 mg/day) was approximately three times the Recommended Dietary Allowance. Although the amplitude of the effect was modest, if these effects accumulate over 20-30 years, they could have a meaningful impact on the rate at which pulmonary function declines, particularly in symptomatic subjects (85). Longitudinal data support the hypothesis that fresh fruit consumption has a beneficial impact on the lung (95). Among children, consumption of fresh fruit, particularly fruit high in vitamin C, has been related to a lower prevalence of asthma symptoms and higher lung function (64). This effect was observed event at low levels of fruit consumption (one or two servings per week vs. less than one serving per week), which suggests that a small increase in dietary intake could have a beneficial effect. Consumption of fish has also been related to lower airway hyperreactivity among children (75) and higher lung function in adults (100); however, longitudinal data do not provide evidence that increased omega-3 fatty acid intake protects against lung disease (101). Experimental studies of persons with asthma suggest that magnesium infusion may have a place in the acute treatment of asthma, but it does not seem to have long-term benefits. The studies of sodium, selenium, and fish oils do not show convincing evidence of clinical benefits. Studies of vitamin C supplementation suggest a short-term protective effect on airway responsiveness and pulmonary function. It remains to be proven whether consistent use of vitamin C would have a protective effect on the evolution of chronic asthma. Results from supplementation studies conducted among subjects exposed to high levels of oxidants (57-60) suggest that daily intake of antioxidant vitamins exceeding the Recommended Dietary Allowance may have a beneficial effect on lung airways and that intake higher than the Recommended Dietary Allowance should be recommended for populations chronically exposed to photooxidant air pollutants (such as ozone), cigarette smoking, or vigorous exercise. It is difficult to determine the amounts of antioxidant vitamins that people should consume. In particular, although vitamin C was shown to have maximum bioavailability when given in a single dose of 200 mg (102), experiments on which this finding was based were conducted under normal conditions. Guidelines from the US National Cancer Institute (103) recommend consumption of five servings of fruit and vegetables daily, corresponding to a vitamin C intake exceeding 200 mg. Dietary surveys carried out in the US population indicate that less than 12 percent of US children and adults meet this recommended level of intake (104). Diet appears to be an important cofactor in the development of obstructive lung disease, although data are still sparse. There is a need for further research in experimental and epidemiologic settings to better understand the physiologic effects of antioxidant vitamins, omega-3 fatty acids, and other nutrients on lung tissues. The impact of diet on the incidence and evolution of asthma and COPD should be investigated using a cohort design that accounts for known risk factors. This will allow researchers to evaluate the exposure-disease relation over an adequate time frame and obtain insight into the causality of the relation. Some of these studies should enroll infants and young children to determine the impact of early diet on respiratory health. Research should also focus on the equally challenging policy issues--namely, finding effective methods of convincing people to increase their daily consumption of fresh fruits and vegetables, to stop smoking cigarettes, and to minimize their environmental and occupational exposure to pollutants and other agents that cause respiratory disease.