With a busy schedule, children to feed, and a job that requires constant sitting behind a desk, growing older can make it difficult to keep a lean body. Even though your body naturally goes through changes in middle age, there are ways you can fight the onset of weight gain when you hit your 40s.

As you get older, your body transforms brown fat into white fat more frequently. Infants are packed with brown fat, which is considered the “good fat,” to keep them warm; this almost always disappears by adulthood. Brown fat is used to generate heat, and white fat stores energy – but too much of white fat can lead to obesity.

Accumulating white fat around the abdomen in particular can lead to a higher risk of heart disease and can narrow and harden arteries, whereas buildup of fat around the hips doesn’t have the same effect. This extra abdominal fat can also cause diabetes and other metabolic diseases, and is more likely to occur with the onset of middle age, which brings hormonal changes and a drop of metabolism with it.

Dr. Francis Collins, director of the National Institutes of Health (NIH), wrote on the NIH Director’s Blog that brown fat doesn’t necessarily completely disappear in adulthood; some reserves in our shoulders remain. People who harbor more brown fat than white are more likely to be leaner, and researchers hope to find a way to increase brown fat in the body.

“It would be potentially therapeutic if we could transform some of our white fat into brown,” Collins wrote. “Determining which genes control the development of white and brown fat may be the first step toward developing game changing treatments for diabetes and obesity.”

Muscle loss, lower metabolism, a decrease in brown fat, and simultaneous increase in white fat all play a part in causing waist lines to grow as we age. Become less active and add stress-eating to the pot, and it becomes much easier to gain weight compared to the youthful, beer-guzzling, pizza-eating 21-year-old you once were.

“According to Dr. [Pamela] Peeke [author of Fight Fat Over 40]… once we turn 40, our metabolic rate — in other words our ability to burn calories — drops,” Lucy Cavendish wrote in the Daily Mail. “We lose muscle tone. We get stressed and eat more calories than we need, when what we should be doing is exercising more and controlling portion sizes.”

Cavendish’s realization that she simply couldn’t eat the way she did in her 20s and 30s was what prompted her to make a change in her lifestyle and steer away from foods rich in fat, and also stay away from the stress-eating time period between 3 p.m. and 12 a.m., when energy levels drop and women in particular tend to overeat unthinkingly.

But is it ever possible to remain the same weight you once were in your 20s? For Michael Symonds, professor of developmental physiology at the University of Nottingham, exercise is key.

"I've got two allotments, six children and I bike 20 miles a day," he told the BBC. "I'm the same weight as I was when I was in my 20s.”

Symonds also mentioned the importance of sleep patterns, growing your own vegetables, finding a hobby that requires physical activity, and even reducing work stress as ways to keep the waistline slimmer for those approaching middle age.