The Measurement Problem — and the Breakthrough
For a long time, sarcopenia research was stalled by a deceptively simple problem: we didn’t have a reliable way to measure actual muscle mass.
Most studies relied on “fat-free mass” or “lean body mass,” measurements typically produced by DEXA scans. On the surface, it sounds logical: if it’s not fat, it must be muscle. But the reality is more complicated. Among older adults, muscle accounts for only about half of fat-free mass. The rest is connective tissue, water, bone, and other components. As a result, fat-free mass measurements often don’t correlate with real-world outcomes like fall risk, mobility, or disability.
“It’s unrelated to the risk of disability. It’s unrelated to the risk of falling,” Dr. Evans explained. This led many researchers to wrongly conclude that muscle quantity wasn’t the key issue — that maybe it was muscle quality, or strength, that mattered most. Newer definitions of sarcopenia even dropped muscle mass measurements entirely, relying only on strength metrics.
Dr. Evans disagreed, and set out to build a better tool.
The D3 Creatine Dilution Method
The breakthrough came in the form of an elegant, non-invasive test based on a simple biological fact: about 98% of all the creatine in your body lives in your muscle cells. That’s because creatine is converted into creatine phosphate — a high-energy compound muscle used for immediate bursts of activity, like jumping out of the way of a speeding car.
Muscle doesn’t produce its own creatine. The liver and kidneys make it and pump it into muscle cells. So if you can track creatine throughout the body, you can calculate how much muscle is there.
Here’s how the test works:
- You swallow a small capsule containing deuterated creatine — creatine labeled with a heavy hydrogen atom. The dose is roughly 30 milligrams, or less than 3% of the creatine in a hamburger.
- That labeled creatine travels to your muscles, distributes evenly, and eventually converts to creatinine, which is excreted in your urine.
- A urine sample collected on filter paper goes to a lab, where the ratio of labeled to unlabeled creatinine reveals your total muscle creatine — and from that, your actual muscle mass.
The result is a direct, accurate measurement of functional muscle mass. Dr. Evans and his team have now validated this method in thousands of individuals, from premature infants weighing as little as two kilograms to adults in their 80s and 90s. It’s been incorporated into landmark studies including the Framingham Heart Study and the Women’s Health Initiative.
The Numbers That Matter
So what does a good or bad muscle mass number actually look like?
Dr. Evans’s research points to clear percentage thresholds. When muscle mass drops below a certain point, mobility disability becomes dramatically more likely:
- Men: 25% muscle mass or below is associated with significantly increased risk of mobility disability
- Women: The threshold is around 20%
Fascinatingly, similar thresholds have appeared in a completely different context: boys with Duchenne muscular dystrophy. In a recent study of approximately 100 patients aged 4 to 24, Dr. Evans found a clear split at the 20% muscle threshold between those who could walk and those who couldn’t, even when controlling for age.
The key insight is that it’s not just how much muscle you have in absolute terms — it’s muscle as a percentage of body weight. That’s why maintaining a healthy weight matters so much for mobility, particularly for people who can’t exercise intensively.
Why We Lose Muscle + What to Do About It
Several interconnected factors drive muscle loss with age:
Hormonal shifts. Declining testosterone, growth hormone, and estrogen all reduce the body’s ability to build and maintain muscle tissue.
Insulin resistance in muscle. As muscle becomes less sensitive to insulin, it struggles to take up glucose efficiently, which impairs both metabolism and muscle repair.
Inadequate protein. The standard recommended daily allowance for protein (0.8 g/kg/day) was established in young, healthy adults and was assumed to apply to older people too. It doesn’t. Dr. Evans’s research shows older adults need closer to 1.6 g/kg/day to maintain muscle — nearly double the RDA.
Inactivity. Even small reductions in daily movement accelerate muscle loss.
The Most Powerful Tool: Resistance Training
If you take away one thing from this research, it’s this: resistance exercise works at any age. In one landmark study published in the New England Journal of Medicine, Dr. Evans and his team put adults in their 90s through a 10-week resistance training program. The result? On average, they tripled their strength.
“It’s never too late to start,” Dr. Evans emphasized. While resistance training builds strength, it also makes muscle more efficient at using dietary protein, meaning even modest protein intake goes further when you’re exercising regularly.
Simple interventions matter too. Walking up and down stairs daily, doing chair stands, and maintaining consistent daily activity all contribute to preserving functional muscle mass over time.
Beyond Strength: The Wider Impact of Muscle
Muscle loss isn’t just a physical problem. It touches nearly every dimension of health:
Cognitive function. Muscle mass and physical activity are both strongly associated with preserved cognitive function and reduced dementia risk. Dr. Evans noted that its effects extend to “instrumental activities of daily living,” including the ability to manage finances.
Metabolic health. Muscle is the body’s primary site of glucose uptake. Low muscle mass means lower insulin sensitivity, which raises the risk of type 2 diabetes and metabolic syndrome.
Fall and fracture prevention. Falls are a leading cause of disability and death in older adults. Adequate muscle mass is one of the strongest protective factors.
All-cause mortality. Across multiple large studies, higher muscle mass is associated with lower risk of death from all causes — outperforming fat percentage, BMI, and even the concept of “sarcopenic obesity.”
On that last point, Dr. Evans challenged a popular framework: “Even among obese people, it’s the amount of muscle that they have that ultimately determines how functional they are.” Fat matters less than you might think. Muscle matters more than you realize.
The Connection to GLP-1 and Weight Loss
For Thrive members managing weight with GLP-1 medications, muscle preservation is especially relevant. Any significant weight loss — whether from medication, diet, or surgery — carries some risk of muscle loss alongside fat loss. This is a known concern, and pharmaceutical companies are actively researching agents that could preserve muscle mass during weight loss.
The takeaway for anyone using GLP-1 therapy: pairing it with resistance training and adequate protein intake isn’t optional — it’s essential for protecting the muscle you have.
What You Can Do Starting Now
The science is clear, and the interventions are accessible. Here’s where to focus:
Lift weights. Resistance training is the single most powerful tool for building and maintaining muscle at any age. You don’t need a gym membership — bodyweight exercises, resistance bands, and even stair climbing all count.
Eat enough protein. Aim for approximately 1.6 g/kg of body weight per day. For a 150-pound (68 kg) person, that’s around 109 grams of protein daily. Spread it across meals rather than consuming it all at once.
Stay active daily. Walking, yard work, and simply moving throughout the day all help preserve muscle function.
Ask your doctor about functional assessments. Request a simple walking speed test or chair stand test at your next appointment. These give you a meaningful baseline to track over time, and small declines are much easier to address early than late.
The Bottom Line
Sarcopenia isn’t just a niche concern for seniors. It’s one of the most consequential and under-recognized threats to healthy aging, but it’s also one of the most actionable. Unlike many chronic conditions, muscle loss can be slowed, halted, and even reversed with the right interventions.
Dr. Evans’s decades of research converge on a single conclusion: if you want to stay mobile, sharp, and metabolically healthy as you age, protect your muscle. Measure it when tools become available, train it consistently, feed it well.
Muscle is the currency of aging. Spend it wisely.
This article is for informational purposes only and does not constitute medical advice. The information contained herein is not a substitute for and should never be relied upon for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.
