“My metabolism has slowed down” is one of the most common explanations people give for weight creeping on in their 40s and 50s – and there’s real physiology behind it. BMR does change meaningfully with age and differs by gender, but the reasons are more specific, and more within your control, than the phrase suggests.
How BMR Changes With Age
Basal Metabolic Rate tends to decline gradually across adulthood, with the rate of decline often accelerating somewhat after age 40–50. This isn’t a mysterious slowdown – it’s driven largely by a measurable, gradual loss of lean muscle mass that tends to occur with age (sometimes called sarcopenia), particularly without regular resistance training. Since muscle tissue is more metabolically active than fat, losing muscle directly lowers resting energy needs.
Why Muscle Mass Is the Real Driver
Two people of the same age, weight and height can have different BMRs depending on their body composition – someone who has maintained more muscle through strength training will typically have a higher resting metabolic rate than someone with the same weight made up of more fat tissue. This is why our guide on increasing your BMR emphasises resistance training as the most effective long-term strategy, at any age.
How BMR Differs by Gender
Men typically have a higher BMR than women at the same weight, height and age – this is reflected directly in the Mifflin–St Jeor formula, which adds 5 for men and subtracts 161 for women. The main driver is average differences in body composition: men typically carry a higher proportion of muscle mass and a lower proportion of body fat than women at the same total weight, and muscle costs more calories to maintain than fat tissue does.
| Life Stage | Typical BMR Trend | Main Driver |
|---|---|---|
| 20s-30s | Relatively stable, often near lifetime peak | Muscle mass typically at or near its highest |
| 40s-50s | Gradual decline begins to accelerate | Age-related muscle loss, hormonal changes |
| 60+ | Continued gradual decline | Further muscle loss, reduced activity in some cases |
Our guide on calorie needs over 50 covers this life stage specifically, including practical steps for adjusting your intake as your BMR shifts.
What This Means Practically
If your calorie intake has stayed roughly the same for years but your weight has slowly increased, a gradually declining BMR – often compounded by a slightly less active lifestyle – is a genuinely common and well-supported explanation, not an excuse. The practical response isn’t to accept a slower metabolism as inevitable, but to recalculate your BMR and TDEE periodically as you age, and to prioritise resistance training to help preserve the muscle mass that keeps resting energy needs higher.
A Worked Comparison Across Two Ages
Take the same woman – 165cm, 70kg – at two different ages, all else being equal:
| Age | BMR (Mifflin-St Jeor) |
|---|---|
| 25 | (10×70)+(6.25×165)−(5×25)−161 = 1,476 kcal |
| 55 | (10×70)+(6.25×165)−(5×55)−161 = 1,326 kcal |
A 150 kcal difference in resting energy needs alone, purely from the age variable in the formula – before accounting for any accompanying changes in muscle mass or activity level, which in practice would likely widen the gap further. This is why a calorie intake that maintained weight comfortably in someone’s 20s can gradually become a slight surplus by their 50s if nothing else changes.
What You Can and Can’t Control
Some of the age-related decline in BMR is a natural part of ageing and isn’t fully within your control. But a meaningful portion of it – the part driven by muscle loss – is influenced by lifestyle factors that are within your control, particularly resistance training and adequate protein intake, covered in our guide on increasing your BMR. Recognising the difference helps set realistic expectations: you likely can’t stop your metabolism from changing with age entirely, but you have more influence over the pace and extent of that change than the phrase “slowing metabolism” tends to suggest.
How This Interacts With Falling Activity Levels
Age-related BMR decline rarely happens in isolation – it’s often compounded by a gradual, sometimes unnoticed drop in overall activity level too. Jobs can become less physically demanding over a career, injuries and joint discomfort can reduce willingness to exercise, and daily routines can settle into less movement overall. Because TDEE combines both BMR and activity level, a small decline in each compounds into a more noticeable drop in total daily calorie needs than either factor would explain on its own – which is exactly why recalculating both BMR and TDEE periodically, rather than relying on a number calculated years ago, matters more as you get older, not less.
The encouraging counterpoint is that activity level is far more within your control than age itself. Deliberately maintaining or even increasing daily movement – walking more, taking up a new hobby, standing rather than sitting where practical – can meaningfully offset part of the age-related decline in total calorie needs, even where the BMR component continues its gradual, natural decrease.
Adjusting Expectations, Not Giving Up
Understanding that BMR gradually declines with age isn’t a reason to abandon weight management goals as you get older – it’s a reason to periodically adjust the numbers you’re working from. Someone who successfully maintained their weight for years on a certain calorie intake in their 30s may need to genuinely eat somewhat less, move somewhat more, or both, to maintain that same weight in their 50s, purely as a function of a lower TDEE – not because anything has gone wrong. Treating this as a normal, expected recalibration, rather than a frustrating failure of an old plan, tends to make the adjustment far easier to accept and act on.
The most useful habit to build isn’t a single dramatic response to ageing, but a routine of periodically revisiting your numbers – recalculating BMR and TDEE every year or two, or whenever your weight, activity or training routine shifts meaningfully – so your calorie targets stay grounded in where your body actually is now, rather than where it was a decade ago.
It’s worth remembering, too, that ageing affects everyone differently – genetics, historical activity levels, and how much muscle mass someone has maintained over the years all influence how noticeably their BMR shifts. Two people the same age can experience quite different rates of change, which is exactly why a personal recalculation, rather than a generic “expect your metabolism to drop by X% per decade” rule of thumb, remains the more reliable way to track your own situation.
Frequently Asked Questions
At what age does BMR start declining?
Some gradual decline can begin as early as the 20s-30s, but the drop tends to become more noticeable from the 40s onwards, particularly without resistance training to offset natural muscle loss.
Can menopause affect BMR?
Hormonal changes around menopause are associated with shifts in body composition for many women, which can contribute to changes in resting metabolic rate around this life stage, on top of the general age-related decline.
Is it possible to have the same BMR at 50 as at 25?
It’s uncommon but not impossible – individuals who maintain significant muscle mass through consistent strength training throughout their life can offset much of the typical age-related decline, though some reduction is still common.
Does a lower BMR mean I have to eat less forever?
It means your maintenance calorie number is likely lower than it was at a younger age, so recalculating your TDEE periodically helps keep your calorie target accurate, rather than continuing to eat based on an outdated number.
Do children and teenagers have a higher BMR than adults?
Relative to body size, yes – growth and development require significant energy, which is one reason calorie needs for teenagers, covered in our teen nutrition guide, differ meaningfully from adult calculations.
Is it worth getting a lab-based metabolic test instead of using a formula?
For most people, a formula like Mifflin–St Jeor provides a sufficiently accurate estimate for everyday use. Lab-based indirect calorimetry can offer more precision but is generally only necessary for specific clinical or elite athletic contexts.