Epidemiological studies on the relation between diet and COPD

Chronic Obstructive Pulmonary Disease (COPD) is an important cause of morbidity and mortality around the world. In the early 1990's several dietary factors were suggested to protect against COPD, based on proposed biological mechanisms and a small number of epidemiological studies. Antioxidants (e.g. vitamin C and E,β-carotene, flavonoids) and foods rich in antioxidants (e.g. fruits, vegetables, whole grains) may protect the airways against oxidant-mediated damage. Alcohol and n-3 fatty acids (mainly present in fish) are thought to have anti-inflammatory effects.

To gain more insight into the relation between diet and COPD, we analysed data from two large-scale population-based epidemiological studies. Data from the Seven Countries Study (1960-1990) allowed us to study the relation longitudinally in an international setting. However, less sophisticated methods than available today were used to examine mainly ever-smoking middle-aged and older men. In the cross-sectional MORGEN-study (1993-1997), Dutch men and women (20-59 yr.) with a large variation in smoking habits were examined using modern, high-quality methods. All associations were adjusted for age, height (for pulmonary function only), gender, smoking, body mass index and total energy intake.

In the 16 cohorts of the Seven Countries Study, we observed an inverse ecological association of 25-yr COPD mortality with baseline fruit and fish consumption. At the individual level, not energy-adjusted baseline fruit and vitamin E intake were inversely associated with 20-yr COPD mortality in men from three European countries. Alcohol consumption showed a U-shaped curve with 20-yr COPD mortality, with the lowest rate in light drinkers (>1.4,≤30 g/day). This U-shaped curve was supported by cross-sectional data on alcohol and pulmonary function. In all three countries, men with intake of both fruits and vegetables above the median had a higher pulmonary function (FEV ₁ or FEV _0.75 ) than those with a low intake of both foods. Finally, bread intake was positively associated with pulmonary function in the three countries.

In participants of the MORGEN-study, intake of catechins, flavonols and flavones was positively associated with the FEV ₁ and inversely associated with the prevalence of chronic cough and breathlessness. Catechin intake, not derived from tea, was independently associated with both the FEV ₁ and all studied COPD symptoms. Flavonol and flavone intake, however, was independently associated with chronic cough only. At the food group level, solid fruit (=apples, pears), but not tea, intake was beneficially associated with COPD. Furthermore, we observed independent beneficial associations of a favourable intake of fruits and whole grains (above the median) and alcohol (1-30 g/day) with COPD. The 2578 subjects with a favourable intake of all three foods had a 139 ml higher FEV ₁ and a lower prevalence of COPD symptoms (OR = 0.44) compared to those with unfavourable intakes of the three foods. This was also observed in never smokers.

The studies described in this thesis mainly suggest a protective effect of a high intake of fruits and whole grains and of light alcohol consumption (up to 3 drinks/day) against COPD. Causality of the observed relations is supported by an apparently temporal relation and by plausible biological mechanisms. Smoking did not seem to explain our findings. Confounding by other health related lifestyle factors can, however, not be excluded. The observed effect of dietary factors was estimated to reduce COPD with 10 to 30% at the population level and is, if causal, certainly of public health relevance.