Vitamin A is found in beta carotene, a substance found in green vegetables and carrots.

Some forms of the vitamin are thought to protect against cancer because they contain chemicals called antioxidants.

But a large-scale trial has shown that beta carotene increases the risk of lung cancer in heavy smokers and asbestos workers.

Explaining the paradox

Dr Marvin Legator led a team of researchers at the University of Texas in Galveston.

Publishing their findings in the journal Nature, they offered a possible explanation as to why the paradox exists.

They said it could be down to an enzyme found in beta carotene that activates particles found called polycyclic aromatic hydrocarbons, or PAHs.

PAHs are found in tobacco smoke and are known to cause cancer.

Rats fed high doses of beta carotene were found to have increased levels of the enzyme - called CYP - in their lungs.

The researchers said: "In humans, correspondingly high levels of CYPs would predispose an individual to cancer risk from the widely bioactived tobacco-smoke procarcinogens."

Large-scale studies

A study involving nearly 30,000 participants found that there were 18% more lung cancers and 8% more deaths in smokers taking beta carotene.

Another, involving more than 18,000 people, found 28% more lung cancers and 17% more deaths in smokers and asbestos workers who took beta carotene and vitamin A supplements.