Scientists have discovered that obesity greatly accelerates ageing of the liver, which could explain why obese people have a greater risk of developing many age-related diseases, such as liver cancer.

According to Prof Steve Horvath of UCLA in the US, while it has long been suspected that obesity causes a person to age faster, until now it has not been possible to prove this.

The US scientists, working with a team from University Hospital Dresden in Germany, used a recently developed biomarker of ageing known as an epigenetic clock. This uses a previously unknown time-keeping mechanism in the body to gauge the age of human organs, cells and tissues.

The scientists used this clock to measure the biological age of a number of tissues. They found that while the biological and chronological age of the tissues matched in people of a normal weight, the biological age of liver tissues tended to be ‘older' in obese people.

In other words, the liver appeared to be biologically older than it should have been.

"This is the first study that evaluated the effect of body weight on the biological ages of a variety of human tissues. Given the obesity epidemic in the Western world, the results of this study are highly relevant for public health," Prof Horvath commented.

The research involved almost 1,200 human tissue samples, including 140 samples from the liver. It found that obesity did not appear to affect the epigenetic age of fat, muscle or blood tissue. However, the epigenetic age of the liver increased by over three years for every 10 BMI (body mass index) units.

BMI uses a person's weight and height to assess whether their weight may be posing a risk to their health. A BMI of 18-24.9 is considered normal, 25-29.9 is overweight and 30 or over is considered obese.

As an example, the scientists pointed out that if a woman was 5 feet, five inches tall and weighed 140lbs, she would have a BMI of 23.3. A woman of the same height who weighed 200lbs would have a BMI of 33.3. This study found that the liver of the heavier woman would be three years ‘older' than the slimmer woman.

"This does not sound like a lot, but it is actually a very strong effect. For some people, the age acceleration due to obesity will be much more severe, even up to 10 years older," Prof Horvath insisted.

The scientists want to carry out more research into the exact mechanisms behind this ageing process, which are so far unknown.

"The increased epigenetic age of liver tissue in obese individuals should provide insights into common liver-related comorbidities of obesity, such as liver cancer. These findings support the hypothesis that obesity is associated with accelerated ageing effects and stresses once more the importance of maintaining a healthy weight," they added.

Details of these findings are published in the journal, Proceedings of the National Academy of Sciences.