If you are above 50 years old, male, and love watching television, chances are, you have a higher proportion of body fat than men who clock less time in front of the screen.

That couch potato lifestyle can also set you on the road to coronary heart disease, a new study has found.

But these health risks are not proven in women, based on the study of about 400 people by the National University of Singapore (NUS).

The participants were Chinese, aged 50 and up, and had never had heart failure or a heart attack.

Their daily TV habit ranged from 45 minutes to more than four hours.

The difference in body fat levels between women who chalked up more television time and women who were lighter TV viewers was too weak to be conclusive.

Similarly, the link between TV viewing time and subclinical atherosclerosis - an indicator of coronary heart disease - was not found to be significant in women.

Both links were statistically significant in men. The findings were published in the July edition of online journal, PLoS One.

"We are more certain about the men, but this does not mean that women can watch an indefinite amount of television," said Associate Professor Tai E Shyong of the Saw Swee Hock School of Public Health, the study's principal investigator.

The findings for women have to be taken with a pinch of salt, he said.

One reason is that the test for signs of atherosclerosis used in the study involves measuring the amount of calcium built up inside the coronary arteries of the heart.

Women, especially those in middle age, tend to have low levels of calcium in blood vessels around the heart. However, the mineral may be deposited in other blood vessels.

Thus, the scans, which included only the heart, may have missed calcium deposits in other blood vessels, said Prof Tai.

What is clear, however, is that watching television is uniquely associated with heart health risks.

And it is not because sitting is bad in itself.

Other sedentary activities, such as playing computer games or having a desk-bound job, also involve sitting for long periods, but they do not seem to affect health the same way watching television does.

"It is a peculiar thing. Our data did not suggest that it was about the sitting down," said Prof Tai.

The study went on to show that light to moderate physical activity did not help to negate the detrimental effects that watching TV had on cardiovascular health.

Physical activity includes going from one place to another, such as by walking or cycling; household tasks that require exertion; type of occupation; and leisure activities, which include workouts or sports.

Other lifestyle factors, such as smoking, were also accounted for, but did not affect the results.

The answer may therefore lie in what people do while watching TV that sets it apart from other sedentary behaviour, said Prof Tai.

Snacking, for instance, could be a big culprit here.

Commercial breaks could be yet another factor, given that they are unique to TV viewing, though their role is unclear.

All this does not mean that people - especially men - should quit watching television , said Prof Tai.

"If you spend a lot of time watching television, it is important to watch your diet and exercise to avoid obesity. It's about the other things that you do," he said.

While physical activity was not shown to significantly impact cardiovascular health in this study, exercise remains beneficial for other major health conditions.

For instance, vigorous or high- intensity exercise can mitigate other harmful effects of a sedentary lifestyle, such as diabetes risk.

A 2013 study by NUS found that the insulin resistance of light TV viewers who did not exercise was comparable to that of those with a heavier TV habit but who spent an hour or more each week doing vigorous exercise like running. Insulin resistance points to an early stage in the development of diabetes.

Therefore, exercise should thus not be dismissed based on the latest study's results, said Prof Tai.

Besides, participants in the latest study were engaged only in light and moderate exercise. It could be that they needed to do high-intensity exercise to experience a positive impact, he suggested.

Vigorous exercise is generally safe, but those with sedentary lifestyles should start with moderate exercise, said Changi Sports Medicine Centre registrar Ho Boon Hor.

"The person should consistently do moderate-intensity exercises for at least 150 minutes a week, for a few months," he said. "This will help to establish the appropriate form and muscle strength to reduce one's risk of musculoskeletal injury."

People with a family history of heart disease, or have hypertension, diabetes, abnormal cholesterol levels and obesity, as well as smokers, may want to seek clearance from a doctor before doing vigorous exercise, advised Dr Ho.