Welcome to your forties: where the warranty on your body quietly expires and things start making weird noises. It isn’t the end of the world. But it sure feels like the end of eating junk food at midnight without consequences. I’m a 44-year-old guy who used to laugh at “dad bod” memes, until my own body became the joke.

The Betrayal Begins

You hit 40 and suddenly, your metabolism, that once-trusty furnace of calorie-burning might, decides to it’s time to take things easy. Permanently. In my twenties, I was a human garbage disposal. I could eat a whole pack of breadsticks dipped in condensed milk, washed down with coke at midnight, and wake up still just bones and muscles (never managed to gain weight, no matter what I ate). Now? I just look at a croissant and gain a kilogram. I swear my metabolism sees food and just forwards it directly to my love handles without even trying to process it.

Why does metabolism slow down? Science says muscle mass naturally declines and hormones shift. I say it’s because the universe hates me. And it’s not just my imagination, unfortunately. Research suggests that your metabolic rate begins to slow as early as your late 20s or early 30s, with a more noticeable change typically happening by the age of 40.

Muscle Mass

One of the key reasons metabolism slows with age is the loss of muscle mass. Muscle burns more calories than fat, even at rest, so a decrease in muscle mass means your body uses fewer calories.
The body’s resting energy expenditure decreases, meaning you burn fewer calories just existing. Translation? Your body becomes spectacularly efficient at turning any food into belly fat.

Hormones

After 40, levels of key hormones like testosterone, DHEA and growth hormone start to drop, which can lead to more body fat, less muscle and a slower metabolism overall. Insulin resistance creeps in, making carbs hit like a sugar-coated bombs. This hormonal shift doesn’t just make it harder to keep weight off or stay strong; it also increases the risk for chronic issues like diabetes, heart disease and even cognitive decline.

Metabolic Cliff

Recent research shows our bodies don’t just gradually slow down — they hit a dramatic shift in the mid-40s, with major changes in how we process fats, alcohol and even caffeine. Scientists found that around age 44, there’s a drop in thousands of molecules and microbes that affect heart health, immune function and muscle maintenance. This isn’t just about burning fewer calories; your entire metabolic machinery starts working differently, which can show up as sudden changes in weight, energy or how you react to foods and drinks you once tolerated easily.

The Takeaway (Besides Regret)

Yes, 40 is when your body swaps “effortless cool” for “effortful everything.” The good news? Regular exercise and a healthy, calorie-conscious diet are proven ways to fight back against these age-related changes, helping to maintain muscle, keep metabolism humming and support overall health as the years add up.

I’ve stepped in to a gym fist time in my life a year ago and started lifting weights. My muscles? Still invisible. My ego? Bruised. My ability to laugh at myself? Thankfully intact.

Being 40 isn’t the end, but it is definitely the start of needing to take action.
Stay tuned for Part 2, where we discuss the mental side of aging!