Muscle mass and longevity: the organ you're not tracking

Apparently your skeletal muscle is the largest endocrine organ in your body. Which I did not know until embarrassingly recently, and now I can’t unknow it.

Most of us grew up thinking of muscle as something you build if you want to look a certain way, or lift a certain thing, or not get tired on the stairs. Turns out it’s also one of the strongest predictors of how well you’ll age — stronger than BMI, stronger than blood pressure in some analyses, and in a few studies stronger than whether you smoke. Which is a genuinely wild thing to say out loud.

This is the part of the longevity conversation that tends to get shortchanged. Everyone talks about VO2 Max — fair enough, the evidence is overwhelming. Everyone talks about HRV as a window into recovery and stress. Fewer people talk about skeletal muscle as the third leg of the stool, even though it’s arguably the one you can do the most about.

Let me explain.

Muscle is an organ, not just an aesthetic

Bit nerdy, but stay with me. Skeletal muscle accounts for roughly 40% of body mass in a healthy young adult, and it does far more than move you around. Muscle acts as an endocrine organ, releasing signalling molecules called myokines every time it contracts. These myokines influence glucose metabolism, systemic inflammation, brain health, and fat oxidation. Which is a long way of saying that muscle doesn’t just use energy — it actively shapes the metabolic environment of everything else in your body.

Muscle is also the body’s largest site of glucose disposal. When you eat carbohydrates, the majority of that glucose gets pulled into skeletal muscle for storage as glycogen. More muscle means more storage capacity, which means better blood sugar regulation. Less muscle means the glucose has to go somewhere less useful — typically the liver or fat cells — which is where the metabolic cascade of insulin resistance tends to start.

And then there’s the functional layer. Muscle is what lets you catch yourself when you stumble, carry a suitcase up three flights, or get off the floor without using your hands. These sound trivial until they’re not. The loss of muscle mass and strength is the single biggest driver of the transition from “independent older adult” to “assisted older adult” — and it’s largely preventable.

Robert Wolfe laid this out in a landmark 2006 paper in the American Journal of Clinical Nutrition titled, rather bluntly, “The underappreciated role of muscle in health and disease.” He argued that muscle should be understood as a metabolic reservoir, not just a mechanical one. It’s been cited thousands of times since, and the framing has quietly taken over longevity medicine.

The research that actually moved the needle

Right, so the evidence here is quite good.

A 2014 study by Srikanthan and Karlamangla in the American Journal of Medicine followed 3,659 older adults and measured muscle mass via bioelectrical impedance. People in the highest quartile of muscle mass had significantly lower all-cause mortality over the follow-up period, even after adjusting for body fat, waist circumference, and metabolic risk factors. The relationship was linear: more muscle, lower risk of dying, across the full range. Body mass index, by the way, showed no such relationship — which is why BMI as a standalone longevity metric is slightly falling out of favour.

The most famous study on this is probably Leong and colleagues’ 2015 paper in The Lancet, drawing on the PURE cohort of nearly 140,000 adults across 17 countries. They used grip strength as a simple proxy for overall muscular capacity and found that each 5-kilogram decrement in grip strength was associated with a 17% increase in all-cause mortality risk. Grip strength was a stronger predictor of mortality than systolic blood pressure. Which is the kind of finding that makes you quietly put your phone down and do a press-up.

A 2018 analysis in PLOS ONE by Abramowitz and colleagues used the skeletal muscle index — muscle mass divided by height squared — in over 11,000 American adults. They found that low skeletal muscle index was strongly associated with mortality, particularly in men and women under 65. The effect was independent of obesity: thin people with low muscle mass carried the same elevated risk as heavier people with low muscle mass. Being underweight, as it turns out, is not automatically protective.

A 2019 umbrella review in the Journal of Cachexia, Sarcopenia and Muscle aggregated dozens of studies and concluded that sarcopenia — the age-related loss of muscle mass and strength — roughly doubles mortality risk in older adults. Doubles. That’s the kind of effect size you don’t see with most lifestyle interventions, and it’s the reason this topic keeps creeping further up the priority list in longevity clinics.

The picture across these studies is remarkably consistent. Muscle mass and muscular strength are independent, strong, dose-dependent predictors of how long and how well you live. Not a little bit. A lot.

Sarcopenia — the slow-motion problem

Sarcopenia is the clinical term for age-related muscle loss. It sounds like something that happens to other people when they’re old, but the biology starts much earlier than anyone would like.

From around age 30, adults lose roughly 3-8% of muscle mass per decade without intervention. The rate accelerates after 60, and — slightly worse — the loss of strength accelerates faster than the loss of mass. You can lose functional capacity even when your body weight on the scale hasn’t moved. This is sometimes called the “sarcopenia paradox”: someone can look the same and weigh the same at 55 as they did at 35, but underneath the skin, a chunk of their muscle has quietly been replaced by fat. Body weight is a famously unreliable narrator.

Morley’s 2016 review in Calcified Tissue International summarised what makes sarcopenia so insidious: it’s gradual, it’s asymptomatic until it isn’t, and it compounds with every year you don’t push back against it. The good news — and it is genuinely good news — is that the primary driver is disuse, not aging itself. Studies of master athletes consistently show that with continued resistance training, muscle loss slows dramatically and in some cases is nearly absent into the 70s and 80s.

This is why strength training shows up as one of the four pillars of longevity training. It isn’t decoration. It’s the one intervention with clear evidence for halting and partially reversing sarcopenia at essentially any age.

How muscle mass actually gets measured

Muscle mass is trickier to track than VO2 Max or HRV because there’s no single gold standard that’s also practical. Here’s the landscape:

• DEXA scan (dual-energy X-ray absorptiometry) — the research standard for body composition. Splits your body into fat mass, lean mass, and bone. Accurate, reproducible, requires a clinic visit plus a very small radiation dose. Useful once a year for a baseline; overkill for monthly tracking.
• Bioelectrical impedance (BIA) — consumer smart scales and handheld devices. They pass a small current through the body and estimate composition from resistance. Less accurate than DEXA in absolute terms, but surprisingly good at tracking change over time on the same device at the same time of day.
• Skinfold calipers — cheap, operator-dependent, still valid if the same person measures you each time.
• Circumference-based estimates — the “Navy formula” and similar. Rough but free. Fine for crude trends.

For most people the right answer is: a DEXA every 12-18 months for the absolute reference, plus a consumer smart scale or phone-based estimate for the week-to-week trend. Single readings don’t mean much. The slope over months does.

Sarvita pulls body composition data from Apple Health — which can ingest smart scale data, Apple Watch estimates, and DEXA reports — and converts it into a Body Composition Age. Body Composition Age sits alongside VO2 Max Age and HRV Age as the third component of your biological age. The point isn’t the absolute number on any given morning. It’s whether the trend is drifting in the direction you want.

How to actually build and preserve muscle

Genuinely, this is the most hopeful part of the whole topic: the intervention works at any age. A 2019 position statement from the National Strength and Conditioning Association (Fragala and colleagues, Journal of Strength and Conditioning Research) reviewed the evidence on resistance training in older adults and concluded that progressive strength training produces meaningful gains in muscle mass and strength even in people in their 80s. There is no age at which you are “too old to start.” The effect sizes are often larger, not smaller, in people who start later, because they’re further from their potential.

The protocol itself isn’t complicated:

• Frequency: 2-3 resistance training sessions per week. More is fine. Fewer than two and the stimulus tends to be insufficient.
• Movements: Compound lifts that train multiple muscle groups at once — squats, hinges, presses, rows, carries. Machines are fine. Free weights are fine. Bodyweight is fine to start. The movements matter more than the equipment.
• Intensity: Work to within 1-3 reps of failure on most sets. “Hard enough that you couldn’t do many more” is the honest version. Easy sets don’t drive adaptation.
• Progression: Add weight, reps, or sets over time. This is the non-negotiable part. A routine that never gets harder stops working within a few weeks.
• Consistency over time: Muscle is built in months and years, not weeks. The people who succeed at this are the ones who can still be bothered to show up in month 11.

Honestly, the boring part is the hard part. I train three mornings a week at a small gym in Glockenbach — not a chain, exposed brick, no mirrors wall-to-wall, more powerlifting than boutique fitness — and the thing that’s actually working for me isn’t the program, it’s the fact that I keep going.

The protein conversation

You can’t build or preserve muscle without enough protein, and most people eat nowhere near enough.

The official RDA of 0.8 g/kg/day is widely considered outdated by researchers working in this space. Stuart Phillips and colleagues argued in a 2016 paper in Applied Physiology, Nutrition, and Metabolism that the RDA reflects the minimum needed to avoid deficiency, not the amount needed for optimal muscle maintenance. The working recommendation from most of the modern literature is somewhere between 1.2 and 1.6 g/kg/day for healthy adults, and 1.2-2.0 g/kg/day for older adults actively resistance training. Bauer and colleagues’ 2013 consensus paper in JAMDA specifically recommended 1.0-1.2 g/kg/day as a floor for healthy older adults, with higher needs during illness or intensive training.

In real food terms, for a 70 kg adult, that’s roughly 100-140 g of protein per day. Which, if you track it for a week, is almost always a shock. Most people land closer to 50-70. Apparently I’ve been eating like a Victorian orphan, to borrow an earlier phrase of mine.

Distribution matters too. Several studies suggest spreading protein across 3-4 meals of roughly 25-40 g each produces better muscle protein synthesis than one enormous evening dose. You don’t need to optimise this obsessively — just have protein at breakfast and lunch, not only at dinner, and you’ll be most of the way there.

Common misconceptions

A few things I wish more people knew:

• “Lifting will make me bulky.” It won’t. Hypertrophy at the level most people worry about requires years of deliberate training plus eating intentionally for size. Normal resistance training produces firmer, stronger, more metabolically active muscle — not the physique of a competitive bodybuilder.
• “Cardio is enough.” It isn’t. Zone 2 cardio is brilliant for cardiovascular health and mitochondrial density, but it doesn’t build or maintain muscle mass in any meaningful way. The two training types address different problems.
• “I’ll start when I’m older and it actually matters.” The evidence is clear that the muscle you build in your 30s and 40s is directly protective in your 60s and 70s. You’re not preparing to be fit — you’re banking capacity that decays slowly.
• “My BMI is normal, so I’m fine.” BMI tells you nothing about muscle. Someone with a “normal” BMI and very little muscle mass — the condition sometimes called sarcopenic obesity — is at higher metabolic and mortality risk than someone with a higher BMI and more muscle.

The practical bit

If I were handing someone a one-page summary of this entire post, it would read like this:

1. Muscle is one of the three or four strongest predictors of how well you age. Treat it accordingly.
2. Lift something heavy 2-3 times a week. Progress over time. Compound movements, honest effort, boring consistency.
3. Eat roughly 1.2-1.6 g/kg of protein per day, distributed across meals.
4. Measure body composition somehow — DEXA annually, smart scale weekly, or both — and watch the trend, not any single reading.
5. Don’t wait. Every year of resistance training compounds.

That’s genuinely most of the value. The rest is detail.

The thing I find quietly reassuring about muscle as a longevity marker is that it’s the one your behaviour changes the fastest. VO2 Max moves in weeks-to-months. HRV moves in weeks-to-months. Muscle mass, measured honestly, moves in the direction you train it within months — and keeps moving for years. You’re rarely stuck with the body composition you have now. You’re stuck with the one you’re currently training for.

Anyway. Worth thinking about. Link’s there if you want Sarvita to track the Body Composition Age part for you alongside VO2 Max and HRV. No pressure.