Toronto, Feb 25 (IANS): Just as most therapies have a dose-response relationship whereby benefits diminish at high doses and the risk of adverse events increases, high level of intense exercise may also be bad for the heart, suggests a new study.

The researchers reviewed studies that looked into the relationship between exercise and heart problems and found that there is growing evidence that high levels of intense exercise may be cardiotoxic and promote permanent structural changes in the heart.

There is already fairly compelling evidence supporting the association between long-term sports practice and increased prevalence of atrial fibrillation -- abnormal heart rhythm characterised by rapid and irregular beating.

"Much of the discussion regarding the relative risks and benefits of long-term endurance sports training is hijacked by definitive media-grabbing statements, which has fuelled an environment in which one may be criticized for even questioning the benefits of exercise," explained study author Andre La Gerche from the Baker IDI Heart and Diabetes Institute, Melbourne, Australia. 

"This paper discusses the often questionable, incomplete, and controversial science behind the emerging concern that high levels of intense exercise may be associated with some adverse health effects," La Gerche noted.

The study was published in the Canadian Journal of Cardiology.

"The answers regarding the healthfulness of 'extreme' exercise are not complete and there are valid questions being raised," La Gerche said. 

"Given that this is a concern that affects such a large proportion of society, it is something that deserves investment. The lack of large prospective studies of persons engaged in high-volume and high-intensity exercise represents the biggest deficiency in the literature to date, and, although such work presents a logistical and financial challenge, many questions will remain controversies until such data emege," La Gerche observed.