The researchers, from the University of Medicine Bichat, Paris, also report that heavy smokers who had high intakes of beta-carotene and vitamin E reduced their loss of lung function.
"These results strongly suggest that beta-carotene protects lung function in the general population and that beta-carotene and also vitamin E have a protective effect in heavy smokers," wrote lead author Armelle Guénégou in the journal Thorax (Vol. 61, pp. 320-326).
As we age, our lung function declines. This decline is even more pronounced in smokers where tobacco smoke increases the oxidative stress on the lung tissue. By increasing the intake of antioxidants it has been suggested that this decline could be slowed.
The study followed-up 535 participants from the European Community Respiratory Health Survey (ECRHS) started in 1992. Fifty per cent of the volunteers were men, 20 per cent were 'moderate' smokers, and 10 per cent were heavy smokers.
Blood samples were taken in order to determine the level of antioxidants, and lung function was measured by the volume of air that could be forcibly blown out in one second, the so-called forced expiratory volume (FEV1).
Subjects with the highest blood levels of beta-carotene (between 0.437 and 3.298 micromoles per litre) reduced their decline in FEV1 by eight millilitres per year, compared to those with the lowest beta-carotene blood concentrations.
For heavy smokers, defined as people who smoked more than 20 cigarettes per day, low blood levels of both beta-carotene and vitamin E increased the loss of lung function by a whopping 50 ml per year.
No association between blood levels of other antioxidants like alpha-carotene and vitamin A was observed.
The researchers offer two explanations for the protective effects of beta-carotene with or without vitamin E, based on either the storage of the antioxidant in the lungs or the ability of these specific antioxidants to scavenge damaging superoxide anions.
Previous research has suggested that beta-carotene and, to a lesser extent, vitamin E are found in relatively high concentrations in lung tissue and come into contact with harmful oxidizing species. Alpha-carotene and vitamin A, argue the researchers, are found only in low concentrations in the lung tissue and are thus less effective at reducing oxidative stress.
The superoxide mechanism is proposed due to the ability of both beta-carotene and vitamin E to scavenge the reactive species, and protect against damage to the lining of the lungs.
"Subjects who sustain heavy smoking and continue to have low antioxidants levels are probably at very high risk of developing chronic obstructive pulmonary disease (COPD)," said Guénégou.
The advice from the scientists for smokers to increase their beta-carotene and vitamin E intake to protect against lung aging is in stark contrast to recommendations from the European School of Oncology, who based recommendations on evidence from other epidemiological studies..
"Beta-carotene supplementation may increase lung cancer mortality in smokers," says the ESO on its website.