Middle-aged adults who maintain a healthy lifestyle can add up to 10 extra disease-free years to their lives, the results of a new study indicate.

People are, on average, living longer worldwide. However, many are doing so with disabilities or chronic diseases, such as heart disease, cancer and diabetes.

While certain lifestyle factors, such as smoking and body weight, are known to affect life expectancy and quality of life, few studies have investigated how a combination of lifestyle factors may impact life expectancy that is free from disease.

US researchers decided to investigate this further. They looked at over 111,000 people who were all free from heart disease, cancer and diabetes at the start of the study. All were monitored for over 20 years.

The researchers focused on five low risk lifestyle factors - never smoking, a healthy weight, at least 30 minutes of physical activity per day, moderate alcohol intake and a good quality diet.

Using these factors, the participants were given a healthy lifestyle score of between zero and five - the higher the score, the healthier the lifestyle.

The study found that at the age of 50, women who adopted no low risk lifestyle factors had a life expectancy free of heart disease, cancer and diabetes, of 24 years. However, those who maintained four or five low risk lifestyle factors could expect to live for another 34 years free of disease - that is an extra 10 disease-free years.

Among men aged 50, life expectancy free of these chronic diseases was also 24 years if no low risk lifestyle factors were adopted. However, this increased by seven years if four or five low risk factors were adopted.

The results stood even when potentially influencing factors, such as family medical history and ethnicity, were taken into account.

Overall, men who smoked heavily, i.e. 15 or more cigarettes per day, and obese men and women had the lowest proportion of disease-free life expectancy at the age of 50.

While this was an observational study, so cannot determine cause, the researchers emphasised the large number of people who took part over an extended period.

The team from Harvard University concluded that "adherence to a healthy lifestyle at mid-life is associated with a longer life expectancy free of major chronic diseases".

"Public policies for improving food and the physical environment conducive to adopting a healthy diet and lifestyle, as well as relevant policies and regulations, for example, smoking bans in public places or trans fat restrictions, are critical to improving life expectancy, especially life expectancy free of major chronic diseases," the team said.

Details of these findings are published in the British Medical Journal.