I hear it all the time. People claim that when they started getting a little older their metabolism stopped working. The issue with that common thought is that it didn't actually slow down the moment you hit 30.

The data doesn't back that up… not even a little.

In 2021 researchers ran the biggest study on human energy expenditure ever done, published in the journal, Science. They tracked daily calorie burn in more than 6,400 people across 29 countries using doubly labeled water (this is the gold standard for measuring real-world energy use; see image below). Adjusted for lean body mass, total expenditure holds flat from age 20 to 60…
It doesn't start dropping until after 60, and even then it's slow, being around 0.7 to 1 percent a year.

So if your metabolism isn’t actually dropping due to age, why does it feel like it is?
Well, a few things are probably going on here. And it’s safe to say, none at which care how old you are. Let’s get into it.

You're losing muscle.
Muscle is considered a high-demanding metabolic tissue. It burns more energy (calories) at rest than fat. Less of it means a lower baseline burn, metabolism aside. Adults lose 3 to 8 percent of their muscle mass per decade starting around 30, faster after 60. However, if you routinely resistance train and exercise muscle atrophy is significantly reduced as you age. Meaning that sitting at a desk all day, not picking up a barbell, not walking your inclines, and avoiding progressive overload will in fact cause atrophy of muscle to happen much quicker in life.

You quit moving.
Let’s talk NEAT (non-exercise activity thermogenesis), which covers everything that isn't sleep, food, or a workout. This encompasses walking to your car, standing/pacing on a call, and even fidgeting at your desk. Mayo Clinic research found sedentary obese adults on average sit about 2.5 hours more a day than sedentary lean adults, of roughly a 350 calorie difference. This may not seem like a crazy amount... but over time it adds up. Going off of the extra 2.5 hours daily, that’s 912.5 hours a year. Then if you go off the average calories, it comes out to an extra 127,750 calories a year.

Your cells become less efficient.
Smaller factor than people assume, but real all the same. Mitochondria take on more oxidative damage with age and squeeze out less usable energy per unit of fuel. ATP output drops roughly 8 percent per decade, and older adults run about 1.5 times lower oxidative capacity than younger ones. Then you take in lifestyle factors the average person partakes in nowadays as in bad sleep, high stress inflammation, and processed food all speed the damage up. Although, training in any sort of exercise has a high potential to slow it down.

Your hormones shift the math.
Insulin sensitivity drifts down with age. Falling testosterone or estrogen pushes the body toward storing fat around the organs instead of holding onto muscle. Stress and high-glycemic eating make both worse. Real, but not the whole story, and not a life sentence.

"My metabolism crashed because I’m getting older" is bullshit. Resting rate per pound of lean mass holds flat through midlife. And nothing naturally begins to significantly drop until after 60, unless you aren’t taking care of yourself.
"Age makes weight loss impossible" is also wrong, most of this fat increase comes from less movement, less protein, and calorie consumption skyrocketing. Not to mention, if you have been overweight for years and have been dealing with health issues, don’t expect things to change in weeks.

Lift heavy, about two or three times a week, and focus on progressive overload.
Do a high-intensity aerobic/anaerobic movement at minimum once every other week.

Eat enough protein. If you're regularly training, I would aim for roughly .8 to 1 gram of protein per pound of body weight. Split this across multiple meals (I take whatever my goal is, divide it by how many meals I’ll have, then shoot for having that amount or protein in each meal). This combined with exercise will help you hold onto your muscle, giving you a fighting chance against any significant decline in muscle mass.

You need to move more during the day. A 10-minute walk after meals does some pretty sweet work on blood sugar and digestion. Plus, walking does wonders for any fitness level and I personally feel it doesn’t get the credit it deserves.

So don’t panic because you’re getting a little older.
It's lifting something heavy, eating enough protein, and not sitting on your ass all day.

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