It’s no secret that as we age, it becomes harder to keep the weight off. But why does this happen?

New research from Sweden suggests that “lipid turnover” – the rate at which lipid (or fatty acids) in the fat cells is removed and stored – decreases during ageing. This makes it easier to gain weight, even if the amount we eat and exercise stays the same.

For the study, published in the journal Nature Medicine, scientists analysed the fat cells in 54 men and women over an average period of 13 years.In that time, all subjects – regardless of whether they gained or lost weight – showed decreases in lipid turnover in their fat tissue.

Those who didn’t compensate for that by eating less calories gained weight by an average of 20%, according to the study.

Researchers also examined lipid turnover in 41 women who underwent bariatric (weight loss) surgery to see how the rate affected their ability to keep the weight off after surgery. Only those who had a low lipid turnover rate before surgery managed to increase their lipid turnover and maintain their weight loss.

“The results indicate for the first time that processes in our fat tissue regulate changes in body weight during ageing in a way that is independent of other factors,” said Peter Arner, professor at Karolinska Institutet, Sweden, and one of the study’s main authors.

“This could open up new ways to treat obesity.”

Previous studies have shown that one way to speed up the lipid turnover in the fat tissue – and thus slow down weight gain – is to exercise more. This new research supports that notion, said the authors.

“Obesity and obesity-related diseases have become a global problem,” added Kirsty Spalding, senior researcher at Karolinska Institutet and study co-author. “Understanding lipid dynamics and what regulates the size of the fat mass in humans has never been more relevant.”