Did My Metabolism Just Change?
I’m Old, and My Metabolism Changed…
I just turned 35. Along with a recent Hashimoto’s diagnosis, I have started to notice that things in my body are just not quite the same. It’s hard to tell sometimes what is within my control and what is not. Now, I know that I don’t speak for everyone. There are a (seemingly) blessed few who manage to remain the same size throughout their lives with seemingly little effort. I am not one of them. Like many, my weight has fluctuated over the years. I hear and have heard quite frequently -
“my metabolism slowed down!”
“I just can’t eat like I used to.”
“I am eating the same as I always have, but I can’t seem to lose weight!”
So…what’s actually going on? Is there a certain point, mid 30s to 40s where metabolism just crashes? Is this a fact of life?
What the Research Really Says About Metabolism and Aging
In 2021, researchers conducted a large study measuring total and basal metabolic expenditure in over 6,000 participants—from 8 days to 95 years old. The group included 64% women and spanned 29 countries.
This kind of broad, diverse sample is important when looking at general human metabolism.
Their goal was to understand how metabolism changes over the human lifespan.
The results?
Your metabolism doesn’t mysteriously slow down in your 30s…or 40s…or 50s…—at least not the part most people are worried about.
Total Daily Energy Expenditure (TDEE)...
Before diving into the study findings, let’s quickly break down how the body expends energy. This context is important for understanding the study and your own metabolism.
Basal Metabolic Rate - Think of BMR as non-negotiable energy your body needs to survive. Heart beat, breathing, brain activity, and temperature regulation as well as other organ functions. As we see by the chart, this is by far the largest component of our daily expenditure at 60-70%. When people say that their metabolism has slowed down with age, this is the portion of the pie that they are referring to.
Thermic Effect of Food - TEF is how much energy you use to digest, absorb, and metabolize your food. Protein has a higher TEF than carbs or fat, but as you can see this is the smallest portion of our energy pie chart at about 10%. Protein is important, but even if you changed to an all protein diet, it wouldn’t meaningfully affect your metabolism.
Exercise Activity Thermogenesis - EAT is the calories that you spend when you are exercising intentionally. This is pretty variable. I am showing this on the lower end but it can be from about 15-30% depending on your exercise habits. This is where we have a little bit of control, however as we will discuss, it is very hard to “out-exercise” a surplus diet.
Non-Exercise Activity Thermogenesis - NEAT is the energy that is burned doing unintentional movements - walking around, fidgeting, standing, etc. NEAT varies widely (15-30%) from person to person and plays a surprisingly large role in our overall metabolism.
So…Does Metabolism Really Slow Down in Your 30s and 40s?
All of these areas of expenditure vary person to person, but according to Pontzer et al. (2021), total energy expenditure - adjusted for body size and composition - stays largely stable from ages 20-60.
“Adult expenditures, adjusted for body size and composition, are remarkably stable, even during pregnancy and postpartum.”
BMR does not significantly decline in 30s and 40s.
So why do so many of us feel like our metabolism slowed down?
Aging is associated with:
An increase in fat mass
A decrease in muscle mass
This matters. But it’s likely more about lifestyle changes than some invisible metabolic shift. In our younger years, we:
Move more
Have fewer responsibilities
Spend less time sitting
Often have more social and physical activity built into our routines
As life changes—desk jobs, parenthood, longer work hours—our activity levels (EAT and NEAT) tend to drop. Less movement > less muscle stimulation > more fat storage.
Muscle is metabolically active, meaning it burns more calories even at rest. If you lose muscle and continue eating the same amount, your body stores the excess energy as fat.
So it’s not that your metabolism is betraying you…
It’s that your lifestyle may no longer match your energy intake.
Can we do something about it?...
The bad news?
We are not innocent bystanders whose bodies turn against us randomly in our 30s, 40s, and 50s.
The good news?
We’re not helpless. It would be terrible news if the research did indeed find that our BMR drops significantly with age regardless of lifestyle. If your metabolism hasn’t changed much, then you still have influence over your total energy balance—through muscle mass, activity, and nutrition.
Of course, life is messy. As we age many of us juggle more -
Jobs
Parenting
chronic pain
time constraints
financial limitations
These factors absolutely affect movement, food choices, and consistency. If we’re aware of them, we can make realistic changes that support a healthy metabolism.
Important Note: When increasing activity, be mindful of increased appetite. Many people eat more when they start working out, which can cancel out the calorie deficit they’re aiming for, that is if weight loss or maintenance is the goal.
“What About After Age 60?”
Even though BMR stays pretty stable through your 20s, 30s, 40s, 50s, things do begin shifting more noticeably after about age 60. The research shows that after 60, basal energy needs decline by about 0.7% per year. That might not seem like a lot at first, but over a couple of decades it adds up. By your 90s, you may need roughly 25‑26% fewer calories daily than you did in your “midlife” years (assuming size/composition are similar).
So turning 60 doesn’t mean your metabolism drops off a cliff — it means the gradual decline in energy needs picks up pace, especially if muscle mass isn’t maintained and physical activity drops. The good news: many lifestyle factors at this stage still matter a lot (strength training, protein intake, daily movement, etc.).
Key Takeaways
Your BMR remains stable from age 20 to 60, according to research.
Most midlife weight gain is due to reduced activity, muscle loss, and increased calorie intake—not a crashing metabolism.
Building or maintaining muscle helps support metabolism.
Movement matters—even non-exercise activity (NEAT) has a big impact.
Awareness is key: lifestyle changes play a huge role in how your body uses energy.
Want to Learn More or Work Together?
This is just the beginning of the conversation. If you're navigating body changes in your 30s or beyond, you're not alone—and you're not broken.
As a nutritionist, I’m here to help you understand your body, your habits, and what really works for you.
Sources & Further Reading:
Pontzer H. et al. (2021). Daily energy expenditure through the human life course. Science, 373, 808–812. DOI:10.1126/science.abe5017
Daily energy expenditure through the human life course | Science
Metabolic changes in aging humans: current evidence and therapeutic strategies – PMC
Metabolic changes in aging humans: current evidence and therapeutic strategies - PMC
Lifestyle factors and metabolomic aging biomarkers: Meta-analysis of cross-sectional and longitudinal associations in three prospective cohorts – PubMed
