Your basal metabolic rate (BMR) tells you how much energy your body burns to just “keep the lights on” – it’s the energy used to power the basic functions of your vital organs, to accomplish sufficient protein and cell turnover to keep your tissues functioning properly, and more. If you didn’t leave your bed all day and didn’t move a muscle, your basal metabolic rate is the amount of energy you’d still burn in a day.
There’s a general belief that “your metabolism slows down as you age.” In this article, we’ll dig into the research around that topic to determine how true it is. As you’ll see, your metabolism does slow down as you age, but probably not in quite the way (or at quite the rate) most people would suspect. Then, we’ll briefly discuss where the perception of cratering metabolisms with aging comes from.
The First Metabolic Slowdown
I’m sure you’ve heard people say, “I could eat everything in sight as a kid (or teenager) and not gain weight” or “my metabolism tanked when I entered my 20s.” And, as it turns out, research actually supports those observations … at least to some degree.
As covered in a previous article, smaller people tend to have a higher BMR per unit of fat-free mass, because more of their fat-free mass is composed of organs with extremely high metabolic rates. For instance, muscles have a BMR of about 13 Calories per kilogram, but the heart and the kidneys have a BMR of about 440 Calories per kilogram, the brain has a BMR of about 240 Calories per kilogram, and the liver has a BMR of about 200 Calories per kilogram. The size of these high-metabolic-rate organs doesn’t scale 1:1 with body size – someone with 50% more total fat-free mass may only have 30% more high-metabolic-rate tissue.
So, children and teenagers have considerably higher BMRs for their size than adults do, in part because their high-metabolic-rate organs are larger relative to their body size. This is pretty intuitive if you just look at the size of a baby’s head. A baby’s brain is around 25-30% as large as an adult’s brain, even if the baby itself is only about 5% as large as an adult. So, relative to its body size, its brain is 5-6 times larger than an adult’s brain. As you grow, the rest of your body grows a bit faster than your high-metabolic-rate organs do, but children and adolescents continue to have a relatively greater proportion of high-metabolic-rate tissue mass until adulthood. Nearly 20% of an infant’s total body mass is comprised of high-metabolic rate tissue – the corresponding figure is usually around 5-7% for adults.
Due to relative differences in organ sizes, you’d expect infants to burn about 85% more energy than adults per unit of fat-free mass, with the difference gradually shrinking throughout childhood and adolescence.
But, relative organ size is only half of the equation. Children and adolescents also have another major metabolic advantage: their bodies are growing. Physical growth is an energetically costly process. Adults’ BMRs reflect the energy cost required to maintain their body’s current tissues. For children and adolescents, BMR reflects the energy cost required to maintain their body’s current tissues, and the energy cost of synthesizing a lot of new tissue, quite quickly.
As a result, children and adolescents have BMRs that are up to 50% higher than you’d expect based on the quantity and composition of their fat-free mass. Consequently, instead of burning about 85% more energy than adults per unit of fat-free mass, infants burn about 135% more energy per unit of fat-free mass than adults do! This difference gradually decreases throughout adolescence, but teenagers still burn about 25% more energy per unit of fat-free mass than adults.
So, your relative metabolic rate does slow down quite a bit once you stop growing. Your absolute metabolic rate may not change very much, but your adult BMR at 80kg or 180lb may be the same as your adolescent BMR at 70kg or 160lb, which can feel a bit jarring. Growing up, you needed to eat more and more and more as your body grew. When entering your 20s, you’re slightly larger than you were in your teens, but your BMR is about the same as it was 5-10 kilograms (or 10-20 pounds) ago. Even though your absolute BMR may not change very much, I can understand why it might feel like it was cratering.
The (Very) Gradual Slide
Another common refrain is that your metabolism slows down in middle age. There’s a shred of truth to that idea – your metabolism does slow a little bit throughout adulthood – but I suspect most people dramatically overestimate the impact.
For starters, in the absence of exercise (and strength training in particular), adults lose about 1% of their lean mass each year after the age of ~30, with losses accelerating past the age of ~40. However, most of that loss comes from low-metabolic-rate tissues (primarily muscle mass). So, this loss of lean mass does contribute to a gradual decline in BMR … but it’s a very gradual decline. The average young woman has around 21kg of muscle mass, and the average young man has about 33kg of muscle mass. Losing 200-300 grams of muscle tissue in a year may be an eventual harbinger for sarcopenia or osteoporosis, but it would only be expected to decrease your BMR by about 3-4 Calories per year, which would hardly be perceptible.
Furthermore, when accounting for the granular composition of people’s tissues, BMR experiences a further very gradual decline throughout adulthood. In other words, it appears that all of your organs “slow down” a little bit as you age. Research by Wong and colleagues suggests that most of your tissues burn about 1-2% less energy per unit of mass in middle age (31-50 years old) compared to young adulthood (21-30 years old).
| Average Organ-Specific BMRs for Young and Middle-Aged Adults | |||
| Organ/Tissue | Young (21-30 years old) | Middle-Aged (31-50 years old) | Percent change |
| Liver | 202 | 199 | -1.49% |
| Brain | 242 | 239 | -1.24% |
| Heart | 443 | 438 | -1.13% |
| Kidneys | 443 | 438 | -1.13% |
| Skeletal Muscle | 13.1 | 12.9 | -1.53% |
| Adipose Tissue | 4.54 | 4.48 | -1.32% |
| Residual (everything else) | 12.1 | 12 | -0.83% |
Research by Geisler and colleagues paints a similar picture. It suggests that, after adjusting for both the amount and the granular composition of subjects’ tissues, BMR slightly decreased year over year throughout adulthood. Now, the decrease was tiny – about 2 Calories per year on average, but it was there. So, adults between 18-39 years old had BMRs that were (on average) about 25 calories higher than would be predicted based on their fat-free mass, while adults between 40-59 years old had BMRs that were about 10-15 calories lower than would be predicted based on FFM.
Pontzer and colleagues performed a similar analysis with an even larger sample size. Even after accounting for age-related changes in the composition of low- versus high-metabolic-rate tissues, they found that BMR per unit of fat-free mass slowly decreased throughout adulthood. Overall, it was about 8-9% lower at age 60 than at age 20.
So, zooming out, the idea that your metabolism slows down through middle age is technically true, both because you tend to lose some muscle mass, and because all of your organs gradually “slow down.” But, when people talk about their metabolism slowing down when they reach middle age, I think they expect there to be a more dramatic change. You’ll commonly hear statements suggesting that your metabolism falls off a cliff when you enter your 40s, but the data doesn’t support that notion.
If you compared your BMR at age 20 to your BMR at age 60, there might be a pretty notable difference. If your BMR decreases by 3-4 calories per year due to a loss of muscle mass after age 30, and your BMR adjusted for fat-free mass decreases by an additional ~2 Calories per year, your BMR at age 60 might be 150-200 Calories per day slower than your BMR at age 20.
But, it’s worth noting that this is a very gradual process. Year to year, its effects should be imperceptible – you’re not going to notice if your daily BMR is 5 Calories lower this year than last year. Furthermore, exercise (and, in particular, resistance training) should be able to stave off most of this decrease by mitigating (or reversing) age-related losses in muscle mass.
The Not-So-Groovy 60s (and Beyond)
Between the ages of 20 and 60, your BMR primarily decreases due to losses in muscle mass (particularly if you’re not exercising). Research suggests that the metabolic rates of each of your tissues also slows down very gradually, but that effect is miniscule. However, past age 60, BMRs begin slowing down more rapidly.
To be clear, there’s nothing “magic” about turning 60 years old. You can find studies identifying a metabolic inflection point closer to 55 years old, and others identifying an inflection point closer to 65 years old, but it’s always around 60 years old. I’m sure it varies person to person, due to the fact that biological aging and chronological aging aren’t identical processes. On a biological level, a 60-year-old might have signs of aging that are closer to what is typically observed in a 50-year-old or a 70-year-old. But, on average, we tend to see metabolic characteristics begin to shift as people enter their seventh decade of life.
This effect isn’t dramatic and immediate. Rather, BMR beyond 60 years old continues decreasing in much the same manner as it decreased between 20 and 60 years old, but the rate of decrease begins accelerating.
For starters, the loss of muscle mass continues at about the same absolute rate (or potentially at a slightly faster rate), but each yearly loss of 200-300g of muscle tissue has a larger relative effect. If you had 30kg of muscle tissue at 30 years old, losing 250g of muscle tissue would represent a loss of 0.83% of your total muscle mass. Once you’ve lost 5kg of muscle due to these gradual decreases, losing a further 250g of muscle tissue would represent a loss of 1% of your remaining muscle tissue. Once you’re down to 20kg of muscle mass, losing an additional 250g would represent a loss of 1.25% of your remaining muscle tissue.
But, much more importantly, the BMRs of your body’s tissues begin to slow down considerably faster after about 60 years old.
Going back to the study by Wong and colleagues, we saw that organ-specific BMRs decreased by about 1-2% between young adulthood and middle age. But, the same study also examined organ-specific BMRs in a 51-73-year-old cohort. It found that the older adults had organ-specific BMRs that were about 2-3% lower than the middle-aged cohort. This is approximately twice as large as the difference between young adults and middle-aged adults.
| Average Organ-Specific BMRs for Middle-Aged and Older Adults | |||
| Organ/Tissue | Middle-Aged (31-50 years old) | Older adults (51-73 years old) | Percent change |
| Liver | 199 | 194 | -2.51% |
| Brain | 239 | 233 | -2.51% |
| Heart | 438 | 426 | -2.74% |
| Kidneys | 438 | 426 | -2.74% |
| Skeletal Muscle | 12.9 | 12.6 | -2.33% |
| Adipose Tissue | 4.48 | 4.36 | -2.68% |
| Residual (everything else) | 12 | 11.6 | -3.33% |
The study by Geisler and colleagues had similar findings. After adjusting for differences in fat-free mass, 40-59-year old adults had BMRs that were only 10-15 calories lower than would be predicted. In contrast, 60-69-year old adults had BMRs that were about 50 Calories lower than would be predicted, and above age 70, BMRs were about 120 Calories lower than would be predicted. So, from 20-60 years old, BMR per unit of fat-free mass decreased by about 2 Calories per year, but above 60 years old, the decrease is closer to 4-5 Calories per year.
Finally, the study by Pontzer found that, after adjusting for the composition of low- versus high-metabolic-rate tissues, BMR per unit of fat-free mass decreased by about 13% between 60 and 80 years old. So, the relative rate of decline was about 0.2% per year from 20-60 years old, and about 0.65% per year beyond age 60.
Add it all up, and your BMR decreases about 3 times faster above age 60 than it decreased throughout young adulthood and middle age. Again, this isn’t an immediate night-and-day difference. Your metabolism isn’t going to fall off a cliff between your 60th and 61st birthdays. But, it does mean that the metabolic slowdown you can expect between age 60 and 70 should be similar to the metabolic slowdown you experienced between age 30 and age 60.
What explains the perception that BMRs slow down dramatically faster, and dramatically sooner?
So, just to recap: your BMR does tend to decrease throughout adulthood, but the decrease is very gradual until approximately 60 years old. Your BMR doesn’t fall off a cliff when you reach your 20s, or your 30s, or your 40s, or even your 50s. Even past 60 years old, the year-over-year decrease isn’t particularly dramatic – you’d probably notice a 100-150 Calories decrease over the course of a decade, but you probably wouldn’t notice a 10-15 Calorie decrease over the course of a year. So, why do so many people believe that their metabolism dramatically slowed down when they entered their 20s or 30s or 40s?
This isn’t a particularly sexy answer, but it mostly comes down to lifestyle and physical activity levels.
As we get older, our activity levels tend to decrease. The change isn’t particularly large for light physical activity until your 50s or 60s, but levels of moderate-to-vigorous physical activity decrease dramatically from childhood to adolescence to young adulthood, and tend to further decline as we age. Furthermore, total sedentary time tends to increase as we age.
Thinking back on my own life, I found that I had a much harder time controlling my weight once I graduated college. I maintained a consistent exercise habit, but my lifestyle significantly shifted. In college, I’d spend at least an hour each day traversing a reasonably large campus on foot. After graduation, I moved to an apartment that didn’t have anything within walking distance, so I drove everywhere, and my daily step count probably decreased by at least 75%. So, my personal experience didn’t perfectly match the population-level trends – my light activity took a nosedive while I maintained my previous levels of moderate-to-vigorous physical activity – but my total activity levels certainly decreased. If I didn’t know any better, I might have thought my increased struggles with weight management reflected a precipitous drop in my BMR. But, realistically, I was just moving around less.
I suspect this dynamic explains most (if not all) of the perception that your metabolism dramatically slows down at a certain stage in your life. Even though shifts in lifestyle, activity levels, and sedentary time look like they occur gradually at the population level, those changes can be swift and dramatic for individuals. Here are some examples of how physical activity levels can rapidly change in adulthood:
- You may experience a precipitous drop in light physical activity after graduating college, since you don’t need to walk between classes anymore.
- Your moderate-to-vigorous physical activity may rapidly decrease because you aged out of competitive sports, or an injury sidelined you from participation in some intense physical activity.
- Activity levels can dramatically change because you get a promotion from an entry-level job that keeps you on your feet all day, to a management position that puts you behind a desk.
- Time constraints and exhaustion after having your first child might seriously curtail your ability or desire to work out as hard or for as long as you used to.
- Your activity levels may have take a nosedive when you move from a walkable city to suburbia.
In short, most perceptions of rapid metabolic slowdowns are likely the result of increases in sedentary time, or decreases in activity levels. These shifts can certainly put a big dent in your total energy expenditure, even if they don’t actually affect your metabolism very much.
Wrapping Up
This article may have been a bit of a downer, but you should stick around for the next article in this series. It will discuss BMR in athletes, but, even if you wouldn’t consider yourself an athlete, it will also discuss how we might be able to mitigate a pretty hefty chunk of the age-related decrease in BMR.
After that, future articles will discuss how weight gain and weight loss affect your BMR, and how we can use all of the information covered in this series to improve on the (current) best BMR prediction equations.
