Gaining weight as we age has become completely normalised in modern society. Men and women in their late 30s and beyond are almost expected to gain weight, even though we've never understood exactly why. While various genetic and lifestyle factors influence weight gain, there are age-related considerations that can't be accounted for by eating or exercise habits alone. As it turns out, how we store and process fat changes substantially as we get older.

According to new research from Karolinska Institutet in Sweden, lipid turnover in human fat tissue decreases during ageing, which makes it easier to gain weight. The study, published in the journal Nature Medicine, sheds some much-needed light into the middle age spread. The scientists studied the fat cells in 54 men and women over an average period of 13 years. The researchers also examined lipid turnover in 41 women who underwent bariatric surgery to see how the lipid turnover rate affected their ability to keep weight off. 

During the study period, all subjects showed decreases in lipid turnover within the fat tissue. Whether we exercise or not, the rate at which lipid is removed and stored seems to slow as the years progress. Subjects who didn't make compensations by eating less or exercising more gained weight by an average of 20 percent. When you combine this factor with things like menopause, sedentary lifestyles, and decreased muscle mass, it's easy to see why so many people experience middle-age weight gain and increased belly fat.

According to Peter Arner, professor at the Department of Medicine in Huddinge at Karolinska Institutet, "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... This could open up new ways to treat obesity." Obesity is at record levels in the West, with a massive 67% of Australians either overweight or obese, and 66% of Kiwis in the same situation. In order to reduce these rates, we need to change our eating patterns and increase our exercise routines. The study supported previous research that the best way to speed up lipid turnover in fat tissue was to exercise more.

According to Kirsty Spalding, senior researcher at the Department of Cell and Molecular Biology at Karolinska Institute and another of the study's main authors, "Obesity and obesity-related diseases have become a global problem... Understanding lipid dynamics and what regulates the size of the fat mass in humans has never been more relevant." While this understanding alone is not enough to cause a global shift, it presents a clear cut answer to the question of why we gain weight as we get older. Rather than making excuses or throwing our hands up in the air, we all need to do that little bit extra if we want to maintain a youthful body shape for as long as possible.