Some people are immune to the benefits of exercise, according to a major new study.

Claude Bouchard, of Louisiana State University, has revealed the results of a study of more than 700 people.

"There is astounding variation in the response to exercise. The vast majority will benefit in some way, but there

 will be a minority who will not benefit at all," Bouchard told New Scientist.

The volunteers, who had not exercised regularly for six months, were put through a strict 20-week endurance

 training programme.

By the end of the programme, there were working strenuously on exercise bikes for 50 minutes three times a

week. The team found that training improved maximum oxygen consumption, a measure of a person's ability to

work, by 17% on average.

But some volunteers gained over 40% and some showed no improvement at all. Similar patterns were seen with

cardiac output, blood pressure, heart rate and other markers of fitness.

The impact of training on insulin sensitivity - a marker of risk for diabetes and heart disease - also varied.

It improved in 58% of the volunteers following exercise, but in 42% it showed no improvement or, in a few cases,

 may have got worse.