Don't worry, be happy -- you may just live longer.

A large existing body of research shows that negative emotions -- including depression, stress and anxiety -- can have a detrimental effect on our physical well-being.

Andrew Steptoe, the director of the Institute of Epidemiology at University College London, decided to examine the flip side of the coin: Can a healthy, happy, energized mental state have a positive physical effect? "We were interested in seeing whether positive well-being might have a protective effect on age-related changes," Steptoe says.

To find out, Steptoe and his co-authors analyzed data on 3,199 people, 60 and older, and recorded their attitudes about how much they enjoyed life. Respondents were split into three groups: about 21 percent were categorized as having a positive mental outlook, 56 percent a medium level and 23 percent a low level.

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The study, published in the Canadian Medical Association Journal, tracked respondents over an eight-year period. Inevitably, as participants aged, difficulty performing day-to-day tasks -- which included getting dressed, climbing stairs, and cutting food -- increased as mobility became more limited.

At the end of the study, about 4 percent of those with the most positive outlooks developed two or more new functional impairments, compared with 17 percent of those who enjoyed life the least.

The authors tried, as much as possible, to control for variables such as baseline health, age, wealth, education and other similar categories. "Obviously, there's a correlation between people who report lower-life enjoyment and those who are chronically ill or have mobility issues," Steptoe says. "We tried to measure all the other factors that could impact the results, and then adjust the data accordingly."

While initial differences in baseline health and mobility significantly influenced the results, "even when we controlled for these variables, a strong association between a positive mental outlook and physical health remained," Stephoe says.

After the data was adjusted, people who responded with low-life satisfaction levels were still 83 percent more likely to develop two or more day-to-day age related problems than those who reported high-life satisfaction.

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While Steptoe admits that the results should be read with a grain of salt -- "this is a correlational study, which means we can't definitively say that enjoyment of life is causing these changes" -- he does believe that, at the very least, they illuminate the close working relationship between our body and our mind.

"Our emotional state is not only important in itself, but may have an effect on our ability to function later in life," he says.

The way we approach to senior health, he explains, is often strictly reactionary: a problem arises, so we take grandma or grandpa to the doctor. But this may be the wrong strategy.

"We should be focusing on simulating higher levels of enjoyment and activity in seniors because it could be very physically beneficial in the long run," he says. "We all know 90-year-olds who are very spry and we also know people in their 70s who have major health problems. What this study suggests is that this difference is not solely a function of their physical health, but of their mental state as well."

Positivity and enjoyment of life, it seems, may indeed slow the ageing process.

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