Body Composition, Muscle Mass, and Why the Number on Your Scale Is the Least Interesting Metric About Your Body

longevity Jun 12, 2026

Part of the Longevity Metrics Series: Auditing My Health as I Turn 55

Metric 6 of my birthday longevity audit: body composition.

This one is the most personally complicated number in the series, and probably the most relatable.

I gained about ten pounds as I transitioned into menopause, and still have about five more to go til I'm back at a weight I could easily maintain. I don't like it. Aging is genuinely challenging on the ego - skin changes and body composition has shifted such that sometimes I don't recognizable myself when I look in the mirror.

Over the last month I've lost some weight simply by eating less. That realization was its own lesson: with less muscle than I had at 45, I can't eat what I used to eat and maintain the same weight. My body has changed. I'm finding my new normal, and part of that is updating what "normal" looks like.

What I do have is perspective. My body is, frankly, a total badass. It can do almost anything I want to do. It has bounced back from every bout of Lyme and other illnesses with even more vigor. So to get bogged down over some extra pounds of fat would be to have a total lack of perspective. I wrote about this dynamic a few years ago in Stop Looking at the Scale , which is worth reading alongside this post. The metrics that matter aren't the ones most people are tracking.

The body composition metrics I looked at for this audit tell a better story than my scale and pant size do.

I have a strong baseline to work from, and I know exactly what I need to do to get closer to where I want to be. Exercise consistency - especially of a certain kind - is the lever. It's also the thing I'm most struggling with, which I'll get to.

Why the Standard Scale Is the Wrong Tool

Weight tells you how heavy you are. It doesn't tell you what you're made of. Two people can weigh exactly the same and have completely different metabolic trajectories, disease risk profiles, and functional futures, depending on how much of that weight is muscle versus fat and where the fat is carried.

BMI (Body Mass Index), the number most doctors still use as a health proxy, makes this problem worse. BMI divides weight by the square of height. It cannot distinguish muscle from fat. A highly muscular woman and a sedentary woman of the same height can share an identical BMI. A person who has been losing muscle and gaining fat for a decade (a pattern that accelerates in midlife, especially after menopause) can have a perfectly "normal" BMI while carrying metabolically dangerous visceral fat (fat packed around the internal organs in the abdominal cavity) and dangerously insufficient lean tissue.

BMI was designed as a population-level epidemiological tool, not a clinical measure of individual health. Using it as the primary indicator of how someone is aging metabolically is one of the most consequential gaps in conventional medicine.

What actually matters: how much muscle you have, how much visceral fat you're carrying, and where your fat is distributed. These three things predict longevity in ways that weight alone simply cannot.

Muscle Is Not Optional

Skeletal muscle is the body's largest metabolic organ. It accounts for more than 80% of insulin-stimulated glucose uptake, meaning that muscle mass is one of the primary factors controlling your blood sugar. The more muscle you have, the more efficiently your body handles glucose. The less muscle you have, the more glucose accumulates in the bloodstream, the more insulin your pancreas has to produce, and the greater the downstream risk of insulin resistance, metabolic syndrome, and type 2 diabetes.

That connection is exactly why body composition comes before blood glucose in this audit. They're not separate topics.

A 2023 meta-analysis of sixteen prospective studies involving 81,358 participants found that individuals in the lowest muscle mass category had a 57% higher risk of all-cause mortality compared to those with normal muscle mass. Not cardiovascular mortality. Not cancer mortality. All-cause mortality. Muscle mass predicts how long you live across the board.

Sarcopenia (the clinical term for low muscle mass combined with reduced strength and physical performance) is associated with a 79% higher odds of all-cause mortality in a meta-analysis of 29 studies . Among community-dwelling older adults, median survival without sarcopenia is 16.3 years. With sarcopenia, it's 10.3 years.

Six years of expected life, determined in significant part by how much muscle you're carrying.

The Leg Strength Argument, and the Thigh Flip

I was nicknamed "Caiti Thunder-Thighs" the day I out-paddled the boys on the swim team. I was mortified. But here's a finding that gave me a fun reframe, and I hope will support how you think about your own body.

A prospective cohort study found that thigh circumference below 60 centimeters (roughly 23.5 inches) was independently associated with significantly higher risk of cardiovascular disease and all-cause mortality in both men and women, with a clear threshold effect right at that mark. The association held after controlling for abdominal obesity, blood pressure, lipids, lifestyle, and general body size.

Read that again. Having larger thighs is protective. The research runs directly counter to decades of messaging that celebrated the "thigh gap," and it is unambiguous.

Quadriceps strength (the large muscles at the front of your thighs) tells a similar story. A study in the American Journal of Medicine found that each 10% increase in maximal quadriceps strength relative to body weight was associated with a 23% reduction in all-cause mortality risk and a 34% reduction in cardiovascular mortality risk, after adjusting for age, activity level, inflammation, and body size. The muscle-longevity relationship holds from midlife through extreme old age.

Why Muscle Mass Is Your Best Insurance Against Hospitalization

The most compelling argument for building muscle now has nothing to do with how you'll look at 55. It's about walking out of the hospital at 85.

Professor Luc van Loon, a Dutch exercise physiologist, has spent his career studying muscle loss in hospitalized and immobilized elderly patients. His finding: just 5 to 7 days of hospital immobilization causes approximately 1.4 kilograms (over 3 pounds) of muscle loss. The muscle is gone in days. Rebuilding it takes months, and that's assuming the person is healthy enough to rehabilitate at all.

You lose muscle quickly and rebuild it slowly.

Going into surgery or a serious illness with low muscle mass means going in already at the bottom of the reserve tank. Research confirms that skeletal muscle mass predicts ventilator-free days, ICU-free days, and mortality in elderly ICU patients.

"Prehabilitation" - building muscle specifically before a planned surgery to improve recovery outcomes - is now an emerging clinical practice for exactly this reason. Pre- and post-surgery care is something I do regularly with my clients, and these studies are why mobility and strength are such a huge part of the protocol.

If you're looking for a reason to take resistance training seriously that has nothing to do with aesthetics, this is it. It's about what reserves you'll have when your body faces a serious challenge.

Van Loon has appeared on the Peter Attia Drive podcast (episode 299) and Found My Fitness with Rhonda Patrick . Both are worth your time.

Visceral Fat: Where Fat Lives Matters More Than How Much You Have

Not all fat carries the same risk. Subcutaneous fat (under the skin at the hips, thighs, and arms) is metabolically relatively benign. Visceral fat is a different substance entirely. It's metabolically active in damaging ways: it secretes pro-inflammatory molecules, drives insulin resistance, disrupts lipid metabolism, and is an independent predictor of cardiovascular disease, type 2 diabetes, and all-cause mortality, regardless of total body weight.

A Mendelian randomization study confirmed a causal relationship between visceral fat accumulation and reduced longevity. This isn't just correlation: visceral fat causes the downstream risk.

Every 4-inch increase in waist circumference is associated with an 8% higher risk of premature all-cause mortality in men, and 12% in women. Women carry disproportionately greater mortality risk per unit of visceral fat gain.

What Menopause Does to Body Composition

This is the section that makes body composition a women's health issue in a way that most general fitness content never addresses.

Estrogen is deeply involved in muscle physiology. Skeletal muscle contains estrogen receptors that regulate the activation and function of satellite cells (the stem cells of muscle tissue, responsible for repair and regeneration). When estrogen declines at menopause, this repair system is impaired. At the same time, reduced estrogen and elevated FSH are associated with increased pro-inflammatory cytokines that promote protein breakdown and suppress anabolic signaling. You start losing muscle faster.

Simultaneously, estrogen loss removes one of the key signals that keeps fat distributed at the hips and thighs. Without estrogen, fat migrates to the internal organs. This is why so many women in perimenopause - including myself - notice abdominal fat appearing without meaningful changes in diet or activity. It's not willpower. It's hormone-driven fat redistribution.

Research documents that the menopausal transition involves an average weight gain of approximately 3.5 lbs, muscle loss of around 0.4 lbs, and significant fat redistribution from the periphery to the abdomen over roughly 3.5 years. That's just the transition window. The broader age-related muscle loss rate of 1 to 2% per year without intervention continues independently alongside it.

The good news: the body composition changes from the menopause transition itself essentially stabilize approximately two years after the final menstrual period. The trajectory isn't perpetually downward. What you do in perimenopause and early postmenopause to protect and build muscle has an outsized impact on where you land.

The intervention is resistance training. A 2025 meta-analysis of 126 studies involving more than 4,000 women found resistance training significantly improves muscular strength in both pre- and postmenopausal women. 2026 systematic review confirmed that resistance training meaningfully improves lean body mass in postmenopausal women across the lifespan.

For women over 50, resistance training isn't one option among many. It's the intervention with the clearest evidence base for the specific challenge menopause creates.

How to Actually Measure Body Composition

The gold standard is a DEXA scan (dual-energy X-ray absorptiometry). Originally developed for bone density measurement, it's the most precise body composition tool available, with accuracy of plus or minus 1 to 2% for body fat percentage. It measures bone, lean mass, and fat mass in specific body regions and can measure visceral fat directly. It requires a medical or clinical setting, involves minimal radiation, and costs roughly $100 to $150 at most longevity clinics. An annual or twice-yearly DEXA is worth doing if you can access one.

Most people use bioelectrical impedance analysis (BIA), the technology inside home scales and gym devices branded as InBody, Hume, Withings, and similar names. These send a low-level electrical current through the body and estimate composition from the resistance. Accuracy is plus or minus 5 to 8% for body fat, and readings shift significantly with hydration, recent exercise, and time of day. I use a Hume scale. I don't consider it highly accurate in absolute terms, but I'm interested in trends over time on the same device under consistent conditions, and for that it's useful. Honest self-assessment requires honest caveats about your tools.

Waist circumference is the lowest-tech option and consistently predictive in the research as a visceral fat proxy. You don't need a scale or a scan to start paying attention.

My Numbers

I stepped on the Hume scale about ten days ago, before leaving on a trip. Close enough to call current.

Weight: 132 lbs, about five pounds above where I was pre-menopause, and where I want to be. BMI: 23.8, within the normal range but near the upper end. Body fat: 23.5%. For women 50 to 59, the healthy/fit range is 23 to 29%, and the athletic threshold is 16 to 23%. I'm right at the low end of healthy, just above athletic. That's something, but it’s not my preferred number.

Visceral fat score: 7, within the healthy range on the Hume scale (1 to 9 is low, 10 to 14 is standard, and above that is high). Given what the research says about visceral fat as an independent driver of cardiovascular risk and all-cause mortality, this number matters more to me than the five extra pounds under my skin. My fat distribution appears to be predominantly subcutaneous, which is the less metabolically dangerous kind.

Skeletal muscle mass: 61.7 lbs, approximately 46.8% of my total body weight. Average for women tends to fall in the 35 to 40% range.

Metabolic age: 50. I'm 55 chronologically. That five-year gap reflects the work, even when the work is imperfect.

Basal metabolic rate: 1,187 calories. This is on the lower end, consistent with my short frame, and it's the context I keep in mind when thinking about supplementation doses and, now, when thinking about how much I can eat to maintain weight.

The honest summary: the scale says I'm heavier than I want to be. The composition data says my visceral fat is healthy, my muscle mass is in strong range, and my metabolic age is five years younger than my chronological age. I'm not thrilled about the five pounds of fat, but I'm also clear-eyed and optimistic about what the more meaningful numbers are telling me.

My Philosophy and My Action Plan

Resistance training is the longevity intervention I’ve most recently added to my routine, and it’s the one I'm most inconsistent about.

I travel a great deal. When I do, getting to weights is difficult. It requires planning, and the willingness to prioritize it over other things competing for the same hours: work, and enjoying the places I'm in and the people I'm visiting. Something has to give - and it's usually strength training.

When I'm in New York, I work with free weights and kettlebells about three hours a week, using functional movement and yoga poses integrated with weights rather than traditional machine-based training or classic pumping iron. On the road, I know how to train with bodyweight.

The issue isn't knowledge. It's execution.

A major goal for the year ahead is to change that, not by finding perfect conditions, because those rarely exist on the road. Bodyweight can be enough. Twenty minutes can be enough. Consistent and imperfect beats perfect and occasional every time. This connects directly to the VO2 max work I've written about: building aerobic capacity and building muscle reinforce each other, and both require showing up more consistently than I currently do when life gets busy.

On supplementation: I’ve been taking creatine for a little over a year. The evidence for creatine supporting muscle mass in postmenopausal women is among the most consistent in the supplement literature, and the risk profile is low - so far as we know now.

I also recently added testosterone to my HRT stack. Testosterone declines alongside estrogen at menopause, and that decline directly affects the body's ability to build and maintain muscle. I was frustrated with my progress building muscle, and the hormonal picture was almost certainly a significant part of why. How I think about that decision (and how I think about hormone replacement more broadly) is a longer conversation I'll have fully in a later Longevity Metric where I’ll talk all about hormones. The short version: it's a more nuanced decision than either the "HRT is dangerous" camp or the "everyone should be on it" camp suggests.

I'm not considering any pharmaceutical support for the weight itself. What I'm doing is reassessing what my body needs now versus what it needed seven years ago. Less muscle means less metabolic overhead. That's just physiology, and the answer is to build more muscle - not to eat the same way I always have and then be confused when the result changes.

What Accelerates Muscle Loss and What Reverses It

The factors that accelerate muscle loss and visceral fat accumulation:

Estrogen and testosterone decline from menopause
Sedentary periods (including travel and recovery from illness)
Poor sleep (growth hormone is secreted primarily during deep sleep. Sleep deprivation directly impairs muscle protein synthesis.)
Chronic psychological stress (cortisol is catabolic and drives visceral fat accumulation simultaneously)
Insufficient protein
Too much alcohol (which disrupts both protein synthesis and sleep quality at once).

The factors that protect and build muscle:

Resistance training, which works at every age and every menopause status, and can be done anywhere without equipment if necessary.
Adequate dietary protein, with current evidence supporting 1.2 to 1.6 grams per kilogram of body weight for muscle maintenance in older adults, and higher for those actively building.
Quality sleep .
Zone 2 aerobic training, which reduces visceral fat and supports metabolic health broadly.
Stress regulation (the cortisol-muscle connection is one reason nervous system work isn't separable from physical training, and the HRV post covers this directly).
Creatine, which has consistent evidence for both muscle mass and cognitive benefit in postmenopausal women specifically.

The Summary

Weight tells you how much you are. Body composition tells you what you're made of, and that's the number that predicts how long you live and how functional that life is.

Muscle mass is independently associated with lower all-cause mortality, better glucose metabolism, stronger surgical outcomes, and faster recovery from illness. Your thighs in particular: the research on thigh circumference and longevity is one of the clearest inversions of conventional body-image messaging in the scientific literature.

Menopause changes body composition through hormonal mechanisms, not willpower failures. Understanding that is the beginning of responding to it effectively.

And the most important variable is the consistency of the inputs over time. Even imperfect, inconsistent training compounds. The goal isn't perfection. It's not letting the imperfect become the permanent.

My numbers are solid in the places that matter most. My consistency has room to grow. My weight isn't where I want it. And I have a clear picture of what needs to change, which is more than a lot of people have when they step on the scale and feel bad about a number without understanding what it does and doesn't mean.

I work with clients to build exactly this kind of picture of their own bodies - what the numbers mean, what's driving them, and what's actually worth changing. If that's a conversation you want to have, my concierge program is where we go deep.

Frequently Asked Questions

Does muscle mass really matter more than weight for longevity?

For longevity purposes, yes - significantly. Weight is a crude proxy that conflates muscle, fat, bone, and water. Muscle mass is independently associated with lower all-cause mortality, better metabolic health, stronger surgical and recovery outcomes, and healthier aging across virtually every measure that matters. Two people at the same weight can have completely different health trajectories depending on their body composition.

What is sarcopenia and should I be worried about it?

Sarcopenia is the clinical term for low muscle mass combined with reduced strength and physical performance. It's associated with a 79% higher odds of all-cause mortality in the research, and a six-year difference in median survival in community-dwelling older adults. It becomes increasingly common after 50 and accelerates in the absence of resistance training. If you're not doing any strength training, this is the reason to start.

What's the difference between visceral fat and subcutaneous fat?

Subcutaneous fat sits under the skin at the hips, thighs, and arms. It's metabolically relatively benign. Visceral fat packs around the internal organs in the abdominal cavity and is metabolically active in damaging ways: it secretes pro-inflammatory molecules, drives insulin resistance, disrupts lipid metabolism, and is an independent predictor of cardiovascular disease and all-cause mortality. Where you carry fat matters as much as how much you carry.

Why do my thighs matter for longevity?

Research consistently finds that larger thigh circumference (above 60 cm) is associated with lower cardiovascular and all-cause mortality risk, independent of overall body size. The thighs house the body's largest muscle groups and their size and strength are proxies for total lower-body muscle mass, which is one of the strongest physical predictors of longevity and recovery capacity. Decades of messaging told women their thighs were a problem. Science says the opposite.

Can I build muscle after menopause?

Yes. Multiple meta-analyses confirm that resistance training meaningfully improves both muscle strength and lean body mass in postmenopausal women. The hormonal environment is less favorable than it was pre-menopause (estrogen and testosterone both decline, both matter for muscle protein synthesis) but the response to training remains real. Adequate protein, creatine supplementation, and consistency are the primary levers.

How often do I need to strength train to see results?

Current evidence supports two to three sessions per week as the minimum effective dose for muscle maintenance and improvement in older adults. Total volume matters more than session length. Consistent shorter sessions beat occasional long ones. And bodyweight training counts. You don't need a gym to maintain the inputs that protect muscle mass.

What is creatine and should I take it?

Creatine is a compound that helps regenerate ATP, the molecule your muscles use for energy during high-intensity effort. It's among the most well-researched supplements available and has consistent evidence for supporting muscle mass, strength, and cognitive function, particularly in postmenopausal women. The risk profile is low at standard doses of 3 to 5 grams per day. If you're a postmenopausal woman doing resistance training and not taking creatine, it's the first supplement worth discussing with your doctor.