Researchers assessed data from the 1970 British Cohort Study, an on-going study of almost 9,000 people born between 5 and 11 April in 1970 whose health and behaviour have so far been followed up at the ages of 5, 10, 16 and 26 years.

They found that the fatter the adult, the greater the likelihood of asthma.

This effect was particularly pronounced in women. Obese women were almost twice as likely to report asthma as those of normal weight.

Asthma is on the increase

The researchers, writing in the medical journal Thorax, suggest that the link between fatness and asthma might be explained by a sedentary lifestyle, itself a risk factor for asthma.

Obesity also reduces the extent to which airway muscle is stretched, which can lead to narrowing of the airway and difficulty breathing.

The female sex hormone oestrogen may also be partly responsible for causing asthma in obese women, who often have higher levels of the hormone.

Diet blamed

Asthma expert Professor Anthony Seaton, of Aberdeen University, said: "It is just conceivable that being fat gives you asthma, but the most likely explanation is that bad diet is responsible both for obesity and asthma."

Professor Seaton, who is carrying out research into the link between diet and asthma, said there was evidence that the condition was linked to a lack of fresh fruit and vegetables and an excess of fat in the diet.

He said it was also possible that the switch from animal to vegetable fats in the diet could be partially responsible for the increase in asthma cases in the richer nations.