MILLIONS of people who try to keep fit by jogging, cycling or going to the gym could be wasting their time, a study revealed today.

The international research, led by the University of London, found that aerobic exercise does not benefit everyone in equal measures, and its usefulness is determined by a person's genes.

According to the results, published in the Journal of Applied Physiology today, 20 per cent of people do not receive any health benefits from aerobic exercise.

The study, which stretched from London to Ontario, saw an international team of researchers from 14 institutions examine the human genome to find a way of predicting who would benefit the most from exercise.

The work built on the belief among researchers that one of the best predictors of health was a body’s ability to take in and use oxygen during maximum exercise.

In theory the more blood a heart can pump, and the more oxygen muscles use, the less risk there would be of early disease and death.

James Timmons of the Royal Veterinary College at the University of London, who headed the study, said aerobic exercise would not help certain people ward off heart disease, diabetes and other potential ailments.

Mr Timmons argued this new research could help advance and improve healthcare.

“If a patient is not likely to benefit much from aerobic exercise, the physician could turn to other types of exercise or alternative therapies. This would be one of the first examples of personalised, genomic-based medicine,” he said.

Alternative types of exercise include anaerobic pursuits such as weightlifting, push-ups and pull-ups.

Participants in the study were asked to undergo rigorous aerobic training, while researchers took muscle tissue samples before and after. Using new procedures the team then identified a set of about 30 genes that predicted the increase of oxygen their body consumed.

By the end of the study 20 per cent saw their maximum oxygen increase by less than five per cent.

About 30 per cent showed no increase in insulin sensitivity, meaning that the exercise did not reduce their risk of diabetes.

“We know that low maximal oxygen consumption is a strong risk factor for premature illness and death, so the tendency is for physicians and public health experts to automatically prescribe aerobic exercise to increase oxygen capacity," Mr Timmons said.

"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,” he added.