Millions of people who strive to keep fit by jogging, swimming or going to the gym are wasting their time, scientists said.
Researchers have discovered that the health benefits of aerobic exercise are determined by our genes - and can vary substantially between individuals.
Around 20pc of the population do not get any significant aerobic fitness benefit from regular exercise, according to an international study led by scientists at the University of London.
For these people, regular jogging and gym work will do little to ward off conditions like heart disease and diabetes which aerobic exercise is generally thought to resist.
Researchers say they would be better off abandoning their exercise regime and focusing on other ways of staying healthy - such as improving their diet or taking medication.
James Timmons of the Royal Veterinary College at the University of London, who led the study, said that the discovery would pave the way for more personalised treatments, with patients able to take DNA tests to find out the most effective way of keeping their own hearts healthy.
Dr Timmons said the research broke new ground by using the human genome - the genetic map of the body which was decoded by scientists 10 years ago - to suggest improvements to healthcare.
"This would be one of the first examples of personalised, genomic-based medicine," he said.
As part of the research, published in the Journal of Applied Physiology, more than 500 participants in Europe and the US were asked to undergo various aerobic training programmes in line with government advice to do 30 minutes of exercise five times a week.
By the end of the 20, 12 and six week programmes the majority of people had shown a measurable improvement in how much oxygen their body consumes during exercise, a key indicator of aerobic fitness.
But 20pc saw their maximum oxygen increase by less than five per cent - a negligible improvement. Around 30pc showed no increase in insulin sensitivity, meaning that the exercise did not reduce their risk of diabetes.
A pioneering analysis of muscle tissue samples taken from the participants revealed a set of about 30 genes that predicted the increase in oxygen intake. Of these, 11 were shown to have a particular impact on how much a person could benefit from aerobic exercise.
Dr Timmons said: “We know that low maximal oxygen consumption is a strong risk factor for premature illness and death so the tendency is for public health experts to automatically prescribe aerobic exercise to increase oxygen capacity.
"Our hope is that before too long, they will be able to target that prescription just to those who may stand a greater chance of benefiting, and prescribe more effective preventive or therapeutic measures to the others.”
Research published by the British Heart Foundation this week found that one third of adults do their recommended 30 minutes of physical activity a day.
The research was conducted in association with the Human Genomics Laboratory in Louisiana and the Centre for Healthy Ageing at the University of Copenhagen.