Growing older and being overweight are not necessarily associated with a decrease in mental well-being, according to a cross-cultural study looking at quality of life and health status in the US and the UK.

The study, led by Warwick Medical School at the University of Warwick, analysed lifestyle and health patterns in more than 10,000 people in both countries and their links to participants' mental and physical quality of life and health status.

Quality of life was evaluated using a measure which takes in eight different factors including perception of general health, pain, social functioning and mental health.

The researchers found that people reported better mental quality of life as they age, despite a decrease in physical quality of life.

This is in line with previous research, for example by Professor Andrew Oswald, also at the University of Warwick, which suggests that happiness levels follow a U-shape curve with their lowest point in the mid-40s after which they rise as people move into older age.

Supportive results were found in this cross-cultural comparison study in the UK and US -- two countries which have different welfare and health-care systems, factors which could impact on people's quality of life.

The researchers also found that being overweight or obese did not have a significant impact on mental well-being levels, with people with a BMI of more than 30 showing similar mental quality of life levels to those considered to be a healthy weight.

For women in the US, low levels of physical exercise did not appear to impact on their mental well-being. This was not the case for men, where limited physical exercise had a significant adverse impact on their mental quality of life.

Dr Saverio Stranges, who led the study at Warwick Medical School at the University of Warwick, along with Dr Kandala Ngianga-Bakwin, said: "It's obvious that people's physical quality of life deteriorates as they age, but what is interesting is that their mental well-being doesn't also deteriorate -- in fact it increases."

"We suggest that this could be due to better coping abilities, an interpretation supported by previous research showing older people tend to have internal mechanisms to deal better with hardship or negative circumstances than those who are younger.

"It could also be due to a lowering of expectations from life, with older people less likely to put pressure on themselves in the personal and professional spheres.

"With regard to our findings on excess body weight and its lack of significant impact on mental-wellbeing -- this has been reported in previous research, i.e. the so-called "jolly fat" hypothesis, although not consistently."

The study also looked at the effect of sleep on quality of life, and found there was an optimum window of sleep duration.

Those who sleep between six and eight hours per day tended to have both better physical and mental health scores than those who slept on average less than six hours or more than eight hours.

Owing to its cross-cultural nature, the study was also able to identify differences between the quality of life of US and UK respondents.

In the US, respondents' social background was more likely to affect their quality of life, with those in higher socio-economic groups reporting better mental and physical quality of life.

The researchers suggested this could be due to the presence of universal healthcare in the UK, which has a levelling effect on well-being.

Also, levels of reported physical quality of life tended to be higher in the UK population while mental quality of life was higher in the US group -- but researchers suggested this could be due to the slightly younger average age of the UK group and other intrinsic differences in the groups surveyed..