Survival of the fittest: Researchers say our DNA could also have an affect on our ability to burn off fat through exercise Spent hours sweating it out in the gym but don't feel any fitter? Blame your parents.
One in five of us has inherited 'unfitness genes' that mean no matter how often we pound the treadmill, we'll still be out of puff.
The researchers focused on aerobic fitness but believe our DNA could also affect our ability to burn off fat through exercise.
The finding paves they way for a simple blood test that will tell us whether we should be getting on our bikes to get fit - or taking a less strenuous route to good health.
Researchers from 14 institutions around the world asked more than 600 people to take up cycling and tracked subsequent improvements in their aerobic fitness.
This involved measuring the amount of oxygen they took in, an indicator of the heart's ability to pump blood and one of the best predictors of a long and healthy old age.
Running, cycling and brisk walking all increase the body's ability to take in and use oxygen, boosting resistance to disease.
By peering into the volunteers' DNA they found 30 genes that affect oxygen uptake and showed that differences in 11 of these genes closely correlated with changes in aerobic fitness after cycling.
Around one in five of those tested had a combination of genes that meant their oxygen uptake hardly changed, despite weeks of fast pedalling.
'The change in aerobic fitness was so small you could miss it,' said researcher James Timmons.'People will say that we only looked at one measure of fitness and there are other benefits of aerobic exercise.
'That is absolutely true but this is the hard and fast one that really predicts death.'
Another, more fortunate, 10 per cent saw a massive improvement in aerobic fitness, according to a report of the research in the Journal-of Applied Physiology.
The researchers have applied for the patent for a blood test that would pinpoint those carrying the ' unfitness genes'.
They believe it could be on the market within a year.
They envisage GPs or gym instructors using the test results to tailor a person's exercise programme.
Those found to carry the ' unfitness genes' would be advised to concentrate on changing their diet or taking cholesterol-busting statins to improve their heart health.
They could also benefit from different types of exercise, such as weight training.
source: dailymail