Two new studies suggest it might be easier to make a significant difference to people's risk of death than we think. Researchers in Taiwan found that just 15 minutes exercise a day appears to be enough to lengthen lifespan, even for people with cardiovascular disease, while researchers in Australia found that long hours spent watching TV can shorten lifespan.

Both studies are published online this week, the first in The Lancet, the second in the British Journal of Sports Medicine.

For the The Lancet study, Chi-Pang Wen of the Institute of Population Science at Taiwan's National Health Research Institutes, and colleagues, write that the benefits of physical activity, including links with longer lifespan, are well known, but we know little about the effects of exercising less than the widely recommended 150 minutes a week.

To look into this further, they assessed the benefits of various amounts of exercise in over 400,000 people (approximately half men and half women) living in Taiwan who underwent standard medical screening between 1996 and 2008.

As part of the screening the participants had filled in questionnaires about exercise, so that from their responses, the researchers were able to categorize them according to five levels of weekly exercise volume: inactive, low, medium, high, and very high.

Then over an average follow up of 8 years, they noted any deaths among the cohort and calculated the risk of death of each of the four active groups compared to the inactive group, and the life expectancy of each group.

They found that compared with the inactive group, the low volume exercise group had a 14% reduced risk of death from all causes (Hazard Ratio HR = 0·86, 95% Confidence Interval CI =0·81 to 0·91) and their life expectancy was 3 years longer. This group exercised on average 92 minutes a week (95% CI from 71 to 112 minutes a week) or 15 minutes a day. (SD 1.8).

Bringing in the other active groups, they found that every extra 15 minutes of exercise per day above the 15 minutes per day of the low volume group, reduced risk of death from all causes by another 4% (95% CI from 2.5 to 7.0), death from all cancers by 1% (0.3-4.5).

These results applied to all age groups, both men and women, and to participants with risks for cardivascular disease.

Wen and colleagues also found that participants in the inactive group had a 17% higher risk of death compared with participants in the low-volume exercise group (HR 1.17, 95% CI 1.10 - 1.24).

In the British Journal of Sports Medicine paper, researchers in Australia found that watching TV or videos for an average of six hours a day could shorten a person's lifespan by nearly five years.

Lennert Veerman of the School of Population Health at the University of Queensland, and colleagues, used data from the Australian Diabetes, Obesity and Lifestyle Study, a national population-based observational survey that started in 1999-2000. This was a cross-sectional survey of Australian adults aged 25 and over that included information about TV viewing habits.

They then cross-referenced the data against the Australian Bureau of Statistics national population and mortality figures for 2008.

Using a "life table model" approach, they modelled the effect of changes in population average TV viewing time on life expectancy at birth: effectively estimating how long a person can expect to live depending on how many hours they spend watching the TV every day.

Their results showed that compared to people who don't watch TV at all, those who spend an average of 6 hours a day watching TV can expect their lives to be 4.8 years shorter.

They also found that every hour of TV viewing after the age of 25, took 22 minutes off a person's life expectancy.

The researchers said these findings suggest the effect of TV viewing time on loss of life is comparable to that of other major chronic disease risk factors such as obesity and lack of exercise.

Written by Catharine Paddock PhD