The team from the University of Helsinki said that both vitamin E and beta-carotene affect various measures of immune function but only few controlled trials have tested whether they influence the predisposition of humans to infections.
They found that neither affected the incidence of pneumonia in men who smoke but vitamin E may benefit men who start smoking at a relatively late age.
Research has found that people who smoke more than 20 cigarettes per day are almost three times more likely to acquire pneumonia than non-smokers.
The Finnish researchers used data from the Alpha-Tocopherol Beta-Carotene Cancer Prevention study, which examined the effects of daily vitamin E (50mg) and beta-carotene (20mg) on the rate of lung cancer in 29,133 men who smoked at least five cigarettes per day at the start of the study. The subjects were randomly assigned to receive the supplements or placebo for about six years.
The new analysis of the data, looking at cases of hospital-treated pneumonia, found that neither vitamin E nor beta-carotene supplementation had any overall effect on the incidence of pneumonia.
"Only the age of smoking initiation was a significant modifying factor for both vitamin E and beta-carotene supplementation," write the authors in the February issue of Chest (125:557-565).
Vitamin E supplementation decreased the risk of pneumonia by 35 per cent among subjects who had started smoking at the age of 21 or older. In contrast, beta-carotene supplementation increased the risk by 58 per cent in this population.
The finding 'warrants further investigation', said the researchers.