The science of healthy ageing
Does Your Metabolism Really Slow Down With Age?
Energy use, body composition and metabolic health are not the same thing.
The short version
Daily energy use does not simply fall with every adult birthday. A large study found a broadly stable adult pattern from about 20 to 60 after accounting for body composition, followed by a later-life decline. [2]
That is one part of the story. What your body contains and how it responds to insulin are different questions.
The explanation feels familiar. Your clothes fit differently. Keeping the same routine no longer seems to give you the same result. Somewhere along the way, you begin to suspect that your metabolism has slowed down.
It is an understandable conclusion. It is also a much bigger claim than it first appears. Are we talking about how much energy your body uses, the balance of muscle and fat it contains, or how it responds to a meal? Those questions often get bundled together, even though they describe different things.
The science offers a more useful picture than a metabolism that simply gets slower with every birthday. There are changes worth understanding, but “faster” is not always the right goal.
01 · Understanding metabolism
Your body is working.
Even when you are still.
Metabolism is the collection of processes that keep a body working: converting and using energy, sustaining organs and supporting movement. “How many calories do I burn?” captures part of that activity, not the whole meaning of the word. [8]
Exercise is the most visible way we use energy. You can feel your breathing change on a hill and your legs working on a flight of stairs. Less obvious is the energy needed when you are sitting quietly: maintaining body temperature, circulating blood and keeping tissues functioning.
Stillness is not inactivity inside the body
The absence of visible movement can be misleading. Breathing and circulation continue while you sit quietly, and the brain and nervous system remain active. Resting expenditure describes the energy needed under resting conditions. It is a baseline measurement of ongoing work, not a body switched off. [1] [8]
The work is not all visible.
Keep functioning
Resting needs, including breathing and circulation.
Process food
Digesting, processing and storing what we consume.
Move through life
Exercise and the movement around it.
Categories, not equal shares or an estimate of your calorie needs. [1]
Food processing is work, too
Eating supplies energy, but handling a meal also requires it. Digestion, processing nutrients and storing consumed food contribute to daily expenditure. This is why food processing belongs in the accounting even on a day without a workout. [1]
Movement extends beyond a session
Exercise is only one part of physical activity. Ordinary tasks also involve movement. A training diary can accurately record a walk while saying very little about the rest of the day. [1]
Imagine two days with the same evening walk. One includes errands on foot and an afternoon of gardening; the other is spent mostly at a desk. Describing both as “a day when I exercised” leaves out much of the activity around that session. It is an example of why the label we give a routine can be less informative than its details.
The distinction becomes useful when we compare measurements. Select a question below to see what its answer can, and cannot, tell us.
02 · Understanding metabolism
The middle-age slowdown
is not the whole story.
In 2021, an international research team analysed daily energy expenditure in 6,421 people across 29 countries. The data covered almost the entire lifespan, from infancy to the nineties. After adjusting for body composition, average daily expenditure was broadly stable between approximately 20 and 60. Later in life, it declined by about 0.7% per year. 2
What changes when researchers account for body composition?
A larger body and a smaller body are not the same starting point for an energy comparison. Nor are bodies with different amounts of fat and fat-free tissue. Comparing only daily totals mixes those differences into the answer.
Adjustment asks how expenditure varies with age after accounting for those tissues. The adult plateau therefore does not mean body composition stopped changing. It describes the age pattern in a comparison that takes composition into account. [2]
Measure the daily total
Energy used during everyday life.
Account for body composition
Compare age patterns while considering fat and fat-free mass.
A long plateau.
A later decline.
Broadly stable from about 20 to 60; declining in later life.
Stable does not mean unchanged
An adjusted average and an individual’s experience answer different questions. A changed routine or body composition can matter even when the population’s adjusted age pattern is broadly level. That is a reason to look more closely at a change, rather than dismiss it or attribute everything to age.
Later life is not a birthday cliff
The later decline remained after adjustment, but that does not identify one cause or set a deadline for an individual. [2]
The finding is not a personal instruction to eat less. Health, activity and nutritional needs belong in decisions about food, rather than a population line on a chart.
How can researchers measure a normal week?
These were measurements of people going about daily life, using a technique called doubly labelled water. Participants consume water containing stable isotopes of hydrogen and oxygen. Tracking how these leave the body allows researchers to estimate carbon dioxide production and, from that, energy expenditure. The method does not require someone to spend their normal week inside a laboratory. 3
This method estimates expenditure across daily life. It does not reveal which individual activity used each calorie, or explain every reason two people differ. That distinction keeps the measurement and its interpretation separate.
03 · Understanding metabolism
A stable weight can hide
a changing body.
A bathroom scale answers a narrow question: how much does the whole body weigh? It does not separate the tissues contributing to that number. A familiar weight can therefore feel like reassurance without answering the question you really wanted to ask.
The total and the tissues tell different stories
The Study of Women’s Health Across the Nation, or SWAN, illustrates the difference. Researchers used repeated body-composition scans in 1,246 women, tracking changes around their final menstrual period. During the menopause transition, fat gain accelerated and lean mass began to decline. Lean mass here is not a direct measurement of muscle alone. Yet the rate of overall weight gain did not accelerate in the same way. 4
One total weight
A scale adds everything together.
Fat tissue
One component of body composition.
Lean tissue
Not a direct measure of muscle alone.
Bone mineral
Another component identified in a DXA assessment.
What repeated scans add
A single weigh-in gives one total at one moment. Repeated scans let researchers examine how the tissue measurements change over time and relate that pattern to the menopause transition. The benefit is a more specific question, rather than another way of ranking bodies.
This was an observational study, and patterns were not identical across all groups. Its lesson is not that menopause explains every change in a woman’s body. It is that changes in tissue composition and changes on a scale are not interchangeable. 4
Why muscle deserves its own question
“What do I weigh?” is not the same question as “What can I comfortably do?” Clothing fit and a scale reading can be useful observations, but neither directly measures strength or function. Those deserve attention in their own right, rather than being treated only as clues about calorie use.
Muscle is part of this story, but it deserves more than a role as a calorie-burning tool. Our guide to why you lose muscle as you get older explores the distinct questions of muscle size, strength and function.
04 · Understanding metabolism
Metabolic health is not
a calorie-burning contest.
The body can use energy and respond to nutrients in different ways. Insulin is a hormone involved in regulating blood glucose. When the body does not respond to it appropriately, that is insulin resistance. It is a question about the response to a signal, not simply the total number of calories used in a day. 5
A response to a signal is not a calorie total
Imagine two reports: one describes energy used across a day, and the other describes how a body responds to insulin. Both concern metabolism, but neither can stand in for the other. That is why “better metabolic health” needs an outcome attached to it.
Human evidence · Randomised trial
One exercise trial.
Two different kinds of progress.
136 previously sedentary older adults with abdominal obesity
6 months of aerobic, resistance, combined exercise or non-exercise control
The study assessed insulin resistance and functional limitation separately. Its results show why one broad promise to “boost metabolism” can obscure the outcomes that matter. [6]
| Exercise group | Insulin resistance | Functional limitation |
|---|---|---|
| Aerobic | Improved | Improved |
| Resistance | No significant improvement | Improved |
| Combined | Improved | Improved |
Direction of reported outcomes, not effect sizes or a ranking of every exercise programme. [6]
Why the outcomes belong side by side
Resistance training’s functional result should not disappear because its insulin-resistance result was different. Likewise, a metabolic measurement does not tell the full story of capability. Looking at both outcomes makes the evidence more useful than reducing exercise to calorie burning.
What this does not prescribe
These were specific programmes in a defined population, followed for six months. The findings inform the question; they do not select an exercise prescription for every reader or show that a “faster metabolism” is the mechanism.
05 · Understanding metabolism
Where NMN research fits.
NMN is involved in the body’s production of NAD+, a molecule used in cellular metabolism. That biological connection is why researchers investigate it. It does not make every claim containing the word “metabolism” equivalent. 7
From a biological role to a human result
NMN is a precursor used to make NAD+. The question for a human trial is not simply whether that pathway exists, but whether supplementation changes a measurable outcome in the people receiving it. In the study below, the relevant outcome was a response to insulin, not a measure of daily calorie expenditure. [7]
Yoshino et al. · 2021
A focused test of insulin sensitivity.
25 postmenopausal women with overweight or obesity and prediabetes completed a ten-week double-blind, placebo-controlled trial. The NMN group received 250 mg daily. [7]
Improved
Muscle insulin sensitivity.
No improvement reported in these outcomes
Body composition, liver insulin sensitivity or adipose-tissue insulin sensitivity.
The distinction between tissues matters: a result in muscle is not automatically a result throughout the body. Nor does an insulin-sensitivity finding establish faster calorie burning, weight loss or longer life. This was a small study in a particular group, not a trial of Eternum Labs’ finished product. [7]
For a closer look at the underlying biology, read Inside NMN: How It Supports NAD+ and Cellular Energy.
06 · Understanding metabolism
Start with the change.
Then ask the question.
“My metabolism has slowed down” can feel like an explanation, but it often arrives before we have identified what changed. Start with the observation. A change in weight, a different fit of clothing, a harder staircase and a blood-test result are not interchangeable pieces of information.
Keeping them separate does not make the experience less real. It makes the next question more useful. You do not need to solve the whole subject of metabolism before describing what is happening in your own life.
A different weight or fit
Ask what changed, over what period, and alongside which changes in routine. A scale gives a total; clothing fit gives a different observation. Neither establishes the tissue changes or their cause.
Less comfortable movement
Describe the task: stairs, carrying bags or a familiar walk. That is more informative than only saying “less fit”, and keeps strength and function in the conversation.
A health-check result
Ask which measurement changed and what it means in your health context. A glucose-related result is not a calorie-burn measurement, and should not be interpreted as one.
Bring the change, not a self-diagnosis
For a persistent or unexplained change in weight or energy, a qualified health professional can help put the timing, symptoms, medicines and broader health picture together. A brief description of what has changed is a more useful starting point than deciding in advance that metabolism is broken.
Keep the goal bigger than calorie burn
Healthy ageing is not a contest to use the most energy. The clearer aim is to understand what is changing, preserve the ability to do what matters, and interpret research without asking one measurement to explain everything.
Sources
- Roberts SB, Das SK. Energy requirements of men and women. 2004. Scientific book chapter; author abstract hosted by USDA Agricultural Research Service, not a single clinical trial.
- Pontzer H, et al. Daily energy expenditure through the human life course. Science. 2021;373:808–812. Human observational analysis.
- Global Doubly Labeled Water Database. Method and database overview. Research database resource.
- Greendale GA, et al. Changes in body composition and weight during the menopause transition. JCI Insight. 2019;4:e124865. Longitudinal human cohort.
- National Institute of Diabetes and Digestive and Kidney Diseases. Insulin Resistance and Prediabetes. Health information.
- Davidson LE, et al. Effects of exercise modality on insulin resistance and functional limitation in older adults. Archives of Internal Medicine. 2009;169:122–131. Randomised controlled trial.
- Yoshino M, et al. Nicotinamide mononucleotide increases muscle insulin sensitivity in prediabetic women. Science. 2021;372:1224–1229. Randomised, double-blind, placebo-controlled trial.
- MedlinePlus Medical Encyclopedia. Metabolism. Reviewed 2024. General physiology overview.
General educational information, not personalised medical advice.
