Beta carotene belongs to a group of antioxidant substances called carotenoids, which give fruits and vegetables their red, yellow, and orange colourings.
Researchers assessed the FEV1 and the blood levels of carotenoids and vitamin E in almost 1200 adults aged between 20 and 44 in 1992.
Eight years later the same measurements was taken in 535 of the original participants, half of whom were men.
FEV1 measures the volume of air forced out of the lungs in one second after taking a deep breath, and is a critical indicator of lung power. It naturally declines with age.
Between 1992 and 2000, the average decline in FEV1 was just under 30 ml a year. But the drop off was significantly slower in those in whom levels of beta carotene were highest to begin with and in those in whom levels rose over the eight years.
Alpha carotene, vitamin A, and vitamin E levels had no impact on the rate of decline of FEV1.
In 1992, men had lower levels of carotenoids in their blood than women, and the decline in FEV1 was significantly steeper in men than in women over the eight years.
But the steepest declines in FEV1 of more than 52 ml a year were seen in those smoking a pack or more of cigarettes a day, and with low levels of vitamin E and beta carotene in their blood.
The authors suggest that beta carotene compensates for some of the damage caused by oxygen free radicals, while both it and vitamin E may help to lessen the damaging effects of heavy smoking on the airways.
And they warn that heavy smokers whose dietary intake of antioxidants is low, "are probably at very high risk of developing chronic obstructive pulmonary disease (COPD)."