Smokers with a bad diet are particularly at risk

Dr Nick Wareham, Cambridge University

They have calculated that an increase of 20 �mol/litre in vitamin C concentration in the blood plasma can reduce the risk of airway obstruction by as much as 13%.

The average person has a vitamin C concentration of 50 �mol/litre, but 30% of the population has a concentration of less than 20 �mol/litre.

The finding is based on data from a major long-term study of the impact of diet on cancer that started in 1993 and which involves 8,000 volunteers.

Heavy smokers

Heavy smokers, who have smoked the equivalent of a packet a day for over 20 years, with a vitamin C level in the blood of only 25 �mol/litre are the most at risk of respiratory disease.

They run six times more risk of experiencing airway obstruction than non-smokers.

However, if the concentration of vitamin C is doubled, the risk is reduced by approximately half.

Vitamin C is known to neutralise the damaging effect that electrically-active oxygen particles have on the tissues of the body.

These particles, known as free radicals, are released by smoking.

Dr Wareham told BBC News Online: "Like everybody else, smokers should attempt to increase their intake of fresh fruit and vegetables.

"However, a good diet does not obliterate the damage caused by smoking, it is just that smokers with a bad diet are particularly at risk."

The findings are published in the European Respiratory Journal.