It's never too late for couch potatoes to start exercising and cut their risk of heart disease, according to research on Tuesday.

Neither does it have to be strenuous activity - even just walking can make a difference.

"You don't have to go to the gym. Just get off the couch," said Dr Dietrich Rothenbacher of the University of Heidelberg in Germany. "It is never too late to start exercising," he told Reuters.

The researchers studied the impact of physical activity on patients with coronary heart disease (CHD) and a group of healthy volunteers of the same age and sex.

They found that people who exercised throughout their lives had the lowest risk of the illness, which is one of the biggest killers in industrialised countries.

"But we also found that people who changed their physical activity patterns in late adult life also reduced their risk for coronary heart disease," added Rothenbacher, an epidemiologist at the university.

The scientists re-evaluated data they had previously collected on patients and volunteers ranging in age from 40 to 68 who had been questioned about their habits and exercise patterns.

Smoking, diabetes and high blood pressure, which are risk factors for heart disease, were more common in the patients with the illness than in the healthy volunteers.

People who said they had been active throughout their lives had about a 60 percent lower risk of being diagnosed with coronary heart disease.

Couch potatoes who changed their ways and began exercising after the age of 40 were about 55 percent less likely to be diagnosed with the illness than people who had always been inactive.

"Our results suggest that a more active physical activity pattern is clearly associated with a reduced risk of coronary heart disease, and that changing from a sedentary to a more physically active lifestyle even in later adulthood may strongly decrease CHD risk," Rothenbacher said in the study published in the journal Heart.