Vigorous exercise can be good for your health, but you shouldn't overdo it, according to US researchers.

They say this type of exercise should be limited to a maximum amount of between 30 and 50 minutes per day.

The idea that a high level of vigorous and intense exercise, such as regularly running marathons, can only do you good, is a myth - and a potentially dangerous one, say the US cardiologists.

Looking at the accumulated research evidence, the doctors from Kansas and Louisiana concluded that large amounts of high intensity exercise over prolonged periods are more likely to do harm than good to your heart.

"If you really want to do a marathon or full distance triathlon, etc, it may be best to do just one or a few and then proceed to safer and healthier exercise patterns," they write in the journal Heart.

"A routine of moderate physical activity will add life to your years as well as years to your life. In contrast, running too far, too fast, and for too many years may speed one’s progress to towards the finishing line of life."