A new study explains why we tend to gain weight as we age, even if our caloric intake and exercise regimens remain unchanged.

Somewhere in that box of photographs squirreled away in our basement is a snapshot of me lounging on the grass in front of the duplex where My Lovely Wife and I resided prior to our nuptials. I’m wearing a pair of cut-off jeans and a baggy T-shirt, and I desperately need a haircut. MLW has described this as my “skinny-hippie” phase, a period in my mid-20s when I weighed in at about 125 pounds no matter how much brown rice and veggies or beer I consumed.

I’ve packed on about 30 pounds in the 40-odd years since then and, like a lot of my geezer peers, lament the fact that it seems to be a natural consequence of aging. There’s plenty of research suggesting that lagging metabolism and surging sarcopenia (muscles melting into fat) can conspire to raise those numbers on the scale as the years accumulate. But a Karolinska Institute study released last week offers a more nuanced perspective on aging and weight gain, pointing to the body’s ability to efficiently process lipid molecules in fat cells.

“The results indicate for the first time that processes in our fat tissue regulate changes in body weight during aging in a way that is independent of other factors,” study coauthor Peter Arner, PhD, said in a statement. “This could open up new ways to treat obesity.”

Arner and his team examined the fat cells in 54 men and women over a period of 13 years and found that lipids (AKA triglycerides) lingered in those cells longer as the participants aged. As a result, those who didn’t cut calories or ramp up their workouts gained, on average, 20 percent more weight.

That might explain why you can maintain a stable caloric intake and a regular exercise regimen into and beyond middle age and still find yourself weighing more than you did in your 20s or 30s.

Exercise — especially strength training — can mitigate the effects of your lounging lipids while also fending off sarcopenia. I was carrying a bit over 160 pounds when I returned to the gym a dozen years ago and have been holding pretty steady around 155 ever since. My “skinny-hippie” days are well behind me, but I can’t say I miss them. I may be a lot heavier than I was in my youth, but at least I can afford a decent haircut.