“My metabolism just isn’t what it used to be.”
We hear this almost every week.
Often it’s in the subtle jokes members, usually female, say to each other in class or out of frustration with a kind of resigned shrug. Like it’s settled.
Like the body made a decision without consulting her and now she just has to live with it.
It’s one of the most widely believed things in health, and it turns out it’s mostly wrong.
Clearly this is a big claim so let’s look at what the research actually shows, because the real story is a lot more hopeful than the one you’ve been told.
What the Biggest Study on This Found
In 2021, a team led by Dr. Herman Pontzer at Duke published what is still the most comprehensive look at human energy expenditure ever assembled. It ran in the journal Science.
The scale is worth appreciating. They analyzed 6,421 people across 29 countries, ages 8 days to 95 years, using doubly labeled water, which is the gold standard for measuring how much energy a body actually burns in a day. Not an estimate from a calculator. Actual measured expenditure.
They found four distinct phases of human metabolism:
Birth to about age 1. Babies are metabolic furnaces. By age one, their size-adjusted energy expenditure is running about 50% above adult levels. Something remarkable is happening in those cells.
Age 1 to about 20. A slow, steady decline down toward adult levels. And here’s a surprise the researchers themselves flagged: puberty didn’t cause a spike. There was no detectable jump in energy expenditure during the teen years, which contradicts what most of us assumed.
Age 20 to 60. That’s forty years of adulthood, and once you account for body size and composition, energy expenditure just holds steady. This includes pregnancy, where calorie needs were no higher than you’d predict from the added body mass.
After 60. A gradual decline begins, roughly 0.7% per year but that’s it.
Read that third phase again. Your twenties, thirties, forties, and fifties were the most stable decades of your entire life.
Specifically Women
This part matters, because our members are largely women, and the “my metabolism slowed down at menopause” story is everywhere.
Sixty-four percent of the people in that study were women. And when researchers adjusted for body size and composition, sex had no detectable effect on the pattern. The plateau from 20 to 60 held for women just as it did for men. The decline didn’t start early for women. There was no menopause cliff in the data.
We want to be really careful here, because this is where the conversation usually goes sideways in one of two directions.
We are not saying menopause and perimenopause aren’t real, or that they don’t change things.
They 100% play a big role in the game.
Sleep gets disrupted. Hot flashes and night sweats wear you down. Mood shifts, and so does motivation. Cravings change. Recovery slows. Body fat tends to redistribute toward the midsection, which changes how you look even at the same weight. Joint aches show up. For a lot of women, it becomes genuinely harder to eat well, sleep enough, and stay consistent with training.
All of that is real, and if you’re living it, you don’t need us to tell you.
What the evidence says is that the cells themselves aren’t burning less. What changes is everything around them, and everything that makes healthy habits harder to hold onto. That’s a meaningful distinction, and it’s not a small one.
Because if your metabolism breaks, there’s nothing to do.
However, if your circumstances get harder, those are things we can actually work on.
The Honest Nuance Some Forget
Here’s the part that usually gets left out of the headlines, and we want you to have it so you’re not confused when your own experience seems to disagree.
The stability in that study is in size-adjusted expenditure. Meaning per pound of lean tissue, your metabolism holds steady.
But if you’ve lost muscle over the last twenty years, and most adults do without deliberate strength training, then your total daily burn genuinely has gone down. Not because your metabolism slowed, but because you’re carrying less metabolic machinery to do a part of the work.
That distinction changes everything. A slowing metabolism is something happening to you. Losing muscle is something you can do something about.
Muscle is metabolically active tissue. By protecting it (and building it!), you defend your daily energy needs.
This is the entire reason we build our programs around strength training rather than just cardio. It’s not about lifting heavier for the sake of it. It’s about not quietly giving away the tissue that keeps your engine running.
And that’s why we say move up in weight 😉
So What Actually Changed?
If it isn’t your metabolism, what happens between 25 and 50?
Life happened.
You move less. Not “you stopped exercising” less, likely a lot less. At 25 you had a job that kept you on your feet, or you walked places, or you played something on weekends. Now there’s a desk, a commute, a car, and a couch that’s earned its keep. The workout is one hour. The other twenty-three moved a lot less than they used to.
You sit more. Meetings, screens, driving kids places, sitting in the car, evenings in front of the TV that you genuinely need after that kind of day.
You sleep less, and worse. Kids, aging parents, stress, perimenopause, a phone next to the bed. Poor sleep drives hunger hormones in the wrong direction, tanks next-day activity, and makes every good intention harder to follow through on.
You carry more stress. Career pressure, money, teenagers, caring for parents. Chronic stress affects appetite, sleep, recovery, and the mental capacity you have left for planning meals and getting to the gym.
And your priorities shifted, which is not a character flaw. Most of our members spent two decades taking care of everyone else. That’s not laziness. That’s a full life. But it does mean the things that kept you lean at 25 quietly disappeared from your calendar one at a time, and nobody announced it.
None of those are metabolic problems. Every single one of them is a lifestyle problem, which is to say: a solvable one.
But Why is This is Good News
We say this to members all the time and we mean it.
If your metabolism were broken, you’d be stuck. There’s no fixing a broken furnace with better habits.
But that’s not what’s happening. What’s happening is that the inputs changed. And inputs are the one thing you actually have leverage over.
You are far more in control than you’ve been led to believe. The narrative that your body betrayed you sometime around 40 isn’t just discouraging, it’s inaccurate. And believing it costs people years of effort they could have spent on things that work.
What Can You Do About It
Protect your muscle. Strength training two to three times a week (plenty!) is the single highest-return thing you can do. It protects the lean tissue that determines your daily energy needs, and the benefit compounds every decade you keep it up.
Move more outside the gym. This is the biggest lever most people ignore. Walk after dinner. Take calls standing. Park farther out. Get up every hour. The workout matters, but daily movement adds up to far more calories over a week than your training sessions do.
Protect your sleep like it’s part of the program, because it is. Poor sleep undermines appetite regulation, recovery, and next-day activity all at once. If one thing on this list is quietly wrecking your results, this is usually it.
Address stress honestly. Not as a wellness buzzword. As a real variable that affects your eating, your sleep, and whether you show up. Walks, downtime, saying no to things, and actual support all count.
Get your protein. It supports the muscle you’re trying to keep, and it keeps you fuller. This gets more important with age, not less.
Stop waiting to feel motivated by a fixed metabolism. Your metabolism is fine. Go build the habits.
Key Points
Human energy expenditure is remarkably stable from age 20 to 60, then declines gradually at about 0.7% per year.
There was no metabolic cliff at menopause in the data, and 64% of the study participants were women.
Menopause absolutely affects sleep, mood, cravings, recovery, and body composition. It just doesn’t slow your cells down.
If your total daily burn has dropped, it’s usually because you’re carrying less muscle, not because your metabolism failed.
The real barriers are moving less, sitting more, sleeping less, and carrying more stress.
All of those are things you can change. That’s the good news.
Where We Come In
If you’ve been operating under the assumption that your body stopped cooperating, we’d love to show you otherwise.
Building strength, moving more, and putting some structure around nutrition works at every age we serve, and we’ve watched it work for people in their fifties, sixties, seventies, and eighties. If you’re not sure where to start, reach out or talk to one of our trainers. We’ll help you figure out which of our programs makes sense for where you’re actually at.
You don’t need a faster metabolism. You need a plan you’ll stick with.
Reminder
This article is educational and isn’t medical advice. Menopause, thyroid conditions, medications, and other health issues can genuinely affect weight, energy, and body composition. If something feels off, talk with your physician, OB-GYN, women’s health nurse practitioner, or a registered dietitian. We’re always glad to work alongside what your provider recommends.
References
Pontzer H, Yamada Y, Sagayama H, et al. Daily energy expenditure through the human life course. Science. 2021;373(6556):808-812.
Rhoads TW, Anderson RM. Taking the long view on metabolism. Science. 2021. (Perspective accompanying Pontzer et al.)
Harvard Health Publishing. Surprising findings about metabolism and age. 2021.
Lauersen JB, Andersen TE, Andersen LB. Strength training as superior, dose-dependent and safe prevention of acute and overuse sports injuries. British Journal of Sports Medicine. 2018.
Pontzer H. Burn: New Research Blows the Lid Off How We Really Burn Calories, Lose Weight, and Stay Healthy. Avery. 2021.
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