Many people notice that staying lean seems harder after age 40 or 50. They gain weight more easily, lose muscle, or find that eating the way they did when they were younger no longer works like it did before.
The usual explanation is: “My metabolism slowed down.”
But groundbreaking research has revealed that’s not true – at least, metabolism doesn’t slow down with age in the way most people assume.
One of the largest studies ever conducted on human energy expenditure was published in the prestigious journal Science in 2021.
The primary finding was that human metabolism stays remarkably stable from about age 20 to age 60. A measurable, decline based on chronological aging doesn’t typically begin until after 60 – a decade or two later than most people believe.
That doesn’t mean the changes people experience after 40 aren’t real. Your muscle mass can decrease, your body fat can increase, and your daily calorie expenditure can fall. The important question is why.
And that distinction is encouraging, because many of the factors involved are still within your control.
In this article, we’ll take a close look at what the research really says about metabolism as we age, why weight gain after 40 is so often blamed on the wrong culprit, what really does change after 60, and whether metabolism works differently in men and women.
Key Points: Does Metabolism Slow Down With Age?
- Metabolism does not appear to slow down simply because you turn 40 or 50.
- Energy expenditure remains remarkably stable from roughly age 20 to 60.
- Less physical activity and loss of muscle can still reduce how many calories you burn each day.
- After about age 60, an additional age-related decline in energy expenditure starts to appear.
- Maintaining muscle and staying physically active can help minimize the part of the decline you can control.
What Do We Actually Mean By “Metabolism”?
When people talk about having a “fast” or “slow” metabolism, they are usually talking about how much how many calories they burn. Scientists often describe this in terms of energy expenditure. There are two major components:
Resting metabolic rate (RMR) is the amount of energy your body uses simply to keep you alive and functioning at rest.
Total daily energy expenditure (TDEE) includes resting metabolism plus three other sources of calorie expenditure:
- The thermic effect of food,
- Non-exercise activity,
- Deliberate exercise.
Resting metabolism is the largest component, typically accounting for about 60% to 70% of your total daily energy expenditure. But activity can still make an enormous difference in how many calories two people burn in a day.
This distinction becomes important when we talk about aging. If someone burns fewer calories at 50 than they did at 25, we have to ask whether their body actually became metabolically slower because of age – or whether their muscle mass, activity level, body weight or lifestyle changed. Those are not the same thing.
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In Burn the Fat, Feed the Muscle, I explain how metabolism, energy balance, nutrition and training work together, and how to use those principles to lose fat while maintaining or building muscle.
Why Scientists Had So Much Trouble Answering This Question
Given how often we hear about metabolism slowing with age, you might assume scientists have understood this process for decades. Surprisingly, there have been major gaps in our knowledge.
One problem is that many large studies measured only resting metabolic rate. But RMR is only part of total daily energy expenditure.
Another problem is that accurately measuring calorie expenditure in people going about their normal lives is technically difficult.
And aging complicates things even further. As people get older, several factors may change at the same time:
- Body size,
- Muscle and other fat-free mass
- Physical activity
- Body fat
- The metabolic activity of the body’s organs.
If energy expenditure falls, researchers therefore need to determine whether aging itself is responsible for that metabolic slowdown whether the decline is explained by one or more of these other changes. That is what made this particular study so interesting.
The Metabolism And Aging Study
An international group of researchers analyzed one of the largest datasets of human energy expenditure ever assembled.
The data came from 6,421 people in 29 countries, ranging in age from eight days to 95 years. Women made up 64% of the participants and men 36%.
Total daily energy expenditure data was available for all subjects, and resting metabolic rate data was available for more than 2,000 of them.
One of the strengths of the research was the method used to measure energy expenditure: doubly labelled water. This technique allows scientists to estimate how much energy people expend while they are living normally rather than being confined to a laboratory.
That’s important because calorie numbers from fitness trackers, cardio machines and online calculators are estimates. Doubly labelled water gives researchers a much stronger way to study real-world energy expenditure.
The large dataset also allowed the scientists to adjust for differences in body size and fat-free mass so they could better isolate the effects of age and sex.
How Does Metabolism Change With Age?
The researchers found four broad phases of energy expenditure across the human lifespan.
1.Infancy: Energy expenditure rises extremely rapidly. Relative to body size, metabolism during the first year of life is exceptionally high.
2.Childhood and adolescence: Total energy expenditure continues to rise as the body grows, but size-adjusted expenditure gradually declines toward adult levels.
3.Age 20 to 60: Energy expenditure enters a remarkably stable period.
4.After about age 60: Total and resting energy expenditure begin to decline.
For people who thought that metabolism slows down at 40 or at least by 50, what happens in the third phase is the most surprising finding of this study.
Your Metabolism Is Surprisingly Stable From Age 20 To 60
One of the findings that attracted the most attention was that total daily energy expenditure and resting metabolic rate were relatively stable from approximately age 20 to age 60.
This directly contradicts one of the most common beliefs about aging and metabolism.
You’ve probably heard some version of:
- “Metabolism can even slow down in your 30s.”
- “Your metabolism slows down after 40.”
- “Once you hit 50, you can’t eat the way you used to.”
- “It’s all downhill from here.”
But this study didn’t find evidence that metabolism automatically slows down in middle age simply because another birthday has passed.
But there was an important qualification. The researchers adjusted for factors such as body size and fat-free mass. This does not mean every person burns the same number of calories at 50 that they burned at 25.
What happens when a person has a slower metabolism (burns fewer calories) in their 30s, 40s, or 50s, it’s because they lost muscle and became less active than when they were younger.
The main point is that chronological aging itself does not appear to independently suppress energy expenditure between age 20 and 60 in the way most people assume.
That raises an obvious question. If metabolism stays relatively stable, why do so many people find it harder to stay lean after 40?
Why Do So Many People Have Trouble Losing Weight Or Even Gain Weight After 40?
When someone finds that losing weight is harder after 40, their experience is real. It’s the explanation that’s often wrong.
Many people do lose muscle and gain body fat during middle age. But when this happens, it’s easy to confuse the result with the cause.
Someone notices that they have gained weight, their waist is larger, and they don’t look as muscular as they did in their 20s or 30s.
The natural conclusion is: “My metabolism slowed down.” But several other things may have changed.
People usually become less physically active as they get older. They may spend more time sitting at work, have fewer physically active hobbies, walk less, or stop participating in sports.
Think about it: Honestly, how many 50-year olds are as active as they were at 20? How many are playing the same sports?
Resistance training may also disappear from their routine. Without lifting, you can start losing muscle even in your 30s and it only accelerates with each passing decade. Use it or lose it.
If muscle and other fat-free mass decrease, BMR decreases with it. And if overall daily activity falls, total daily calorie expenditure declines as well even if resting metabolism has not been independently damaged by age.
A more accurate way to describe what often happens in middle age is therefore: Your lifestyle may have changed more than your metabolism did.
That is an important distinction because lifestyle is something we can influence.
A “Fast Metabolism” May Sometimes Be A Very Active Lifestyle
Another fascinating result from the study was the enormous variation in total daily energy expenditure between individuals.
Among adults, TDEE ranged from roughly 1,900 calories per day to as high as 7,000 calories per day in the most extreme cases. That’s more than a threefold difference.
It would be tempting to explain that by saying some people are simply blessed with incredibly fast metabolisms.
But the biggest reason for these enormous differences was physical activity, not equally enormous differences in resting metabolic rate.
An extremely active endurance athlete may burn a huge number of calories per day. A sedentary office worker of similar age may burn dramatically less. Most people fall somewhere between those extremes, of course.
But this finding illustrates something important: What looks like a “fast metabolism” may sometimes be a very active lifestyle.
That also explains why calorie recommendations based on population averages can be misleading for people at either extreme.
What Really Happens To Metabolism After 60?
The story changes after approximately age 60. At this point, the long period of relative metabolic stability begins to break.
Total daily energy expenditure and resting metabolic rate start to decline, and fat-free mass tends to decrease as well.
Some of that decline is easy to explain:
If you carry less muscle and other lean tissue, you require less energy.
If you move less, you burn fewer calories through physical activity.
But the researchers found that these factors did not explain the entire decrease.
Another intriguing finding was that the metabolic activity of internal organs appears to decline in older adulthood.
Organs such as the brain and liver are highly metabolically active. The study suggested that organ-specific energy expenditure was lower during this later phase of life.
This means that after about 60, there appears to be a genuine biological component of declining metabolism associated with aging itself.
That part is not completely within our control. But it would be a mistake to conclude that nothing can be done.
We may not be able to prevent every age-related change in energy expenditure, but we can still influence our muscle mass and activity level – two major contributors to how many calories we burn.
Do Women Have Slower Metabolisms Than Men?
Another common belief is that women simply have slower metabolisms than men. At first glance, that appears to be true. Men generally burn more calories per day than women.
But men are also, on average:
- Taller,
- Heavier,
- And carry more fat-free mass.
Even when a man and woman weigh the same, the man typically carries more of that bodyweight as lean mass.
When the researchers adjusted for fat-free mass, they found no meaningful difference in energy expenditure between men and women.
In other words, men burn more calories because they have larger bodies and more metabolically active lean tissue – not because being male automatically gives someone a fundamentally faster metabolism.
The study also didn’t find evidence that women’s metabolisms begin declining earlier than men’s or decline more rapidly with age after accounting for differences in body size and fat-free mass.
That’s yet another widespread metabolic myth worth putting to rest.
What About Menopause And Middle-Age Weight Gain?
Women in their 40s and 50s may experience very real challenges around menopause.
Changes can include increased appetite, fatigue, reduced physical activity, and difficulty sleeping. There may also be changes in where body fat tends to be stored, including a greater tendency toward storing abdominal fat. (Whereas before menopause most women store more in the hips and thighs.
Those changes can genuinely make weight management more difficult. But that’s different from saying that a woman’s metabolism automatically becomes slower simply because she enters middle age.
This distinction matters because if changes in appetite, activity, muscle mass and lifestyle are contributing to weight gain, those factors offer potential points of intervention.
Muscle Matters – But So Does Movement
One of the clearest findings from the research was the relationship between fat-free mass and energy expenditure.
People with more fat-free mass generally expend more energy. That gives us another reason to make maintaining muscle a priority as we age. If you stop resistance training, become inactive and lose muscle, your energy expenditure may fall.
This is particularly important during fat loss. Dieting without resistance training can increase the risk of losing lean mass along with fat.
Maintaining muscle therefore helps preserve the metabolically active tissue you already have.
But muscle is only part of the picture. Daily activity can produce enormous differences in total calorie expenditure.
The best strategy is not simply “build muscle to boost your metabolism.”
It is to maintain as much lean mass as reasonably possible while continuing to live an active life.
What You Can Control At 40, 50, 60 And Beyond
This research gives us two different messages depending on your age.
Before 60
Many of the changes people blame on an aging metabolism may actually reflect:
- Reduced physical activity
- Less resistance training
- Loss of lean body mass
- Gradual changes in eating habits (more food quantity, less food quality)
These factors are substantially within your control.
If you continue strength training, remain physically active and avoid allowing gradual weight gain to accumulate year after year, there is no reason to assume your 40s or 50s must automatically bring a major metabolic decline.
After 60
At this stage, a genuine age-related decrease in energy expenditure begins to become measurable.
We cannot completely prevent that kind of biological change. But we can influence how much muscle we retain and how physically active we remain.
That means we can avoid adding a large lifestyle-related decline on top of the smaller biological decline that comes with older age.
And resistance training remains important even in later life.
Men and women can continue maintaining – and in many cases building — muscle and strength well beyond middle age.
These findings also fit what I have observed over decades in natural bodybuilding. Men and women well past 40, 50 and even 60 can maintain impressive levels of muscle, strength and leanness when they continue training consistently.
Aging matters, especially in the later decades of life. But it’s only one part of the equation.
The Bottom Line: Does Metabolism Slow Down With Age?
If maintaining your weight feels harder at 40 or 50 than it did when you were younger, the change you are experiencing may be very real.
But the evidence suggests that chronological aging itself is probably not the main reason your energy expenditure has fallen in middle age.
Throughout most of adulthood, metabolism is far more stable than most people believe.
Changes in physical activity, muscle mass, eating habits and lifestyle may explain much of what gets blamed on a “slow metabolism” during middle age.
After approximately age 60, age itself begins to contribute more directly to declining energy expenditure. Even then, the message should not be one of resignation.
You can’t stop aging.
But you can continue resistance training, maintain as much muscle as possible, stay physically active and avoid adding unnecessary lifestyle-related decline to the biological changes that come later.
At every age, those are factors worth controlling.
-Tom Venuto,
Author of Burn the Fat, Feed the Muscle
Founder of Burn the Fat Inner Circle
Scientific Reference:
Pontzer H, et al. Daily energy expenditure through the human life course. Science. 2021;373(6556):808–812.
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