Zone 2 Running All Spring—Where Did Your Muscle Go?

Zone 2 training has a loyalty problem. It promises fat oxidation, aerobic efficiency, and longevity benefits—and it delivers on all three. But what most Zone 2 devotees discover after a dedicated spring of low-intensity running is a number on their DEXA scan they weren't expecting: lean mass is down. Not dramatically. Not catastrophically. But enough to matter. Enough that the physique they expected after months of disciplined effort looks softer, not leaner. The scale might be flat or even slightly lower, but the composition underneath has quietly shifted. If you've been running four to six days a week since February and you're now standing in July wondering where your muscle went, this article is for you. What Zone 2 Actually Does—and Doesn't Do Zone 2 cardio is typically defined as aerobic exercise performed at roughly 60–70% of your maximum heart rate. At this intensity, your body preferentially burns fat for fuel, your mitochondria proliferate, and your cardiovascular system adapts in meaningful ways. Peter Attia, Andrew Huberman, and most longevity-focused researchers consider it foundational to a healthy exercise protocol. None of that is wrong. Zone 2 is genuinely valuable. But here's what often gets omitted from the conversation: Zone 2 running is catabolic toward muscle when it's performed in sufficient volume without an adequate anabolic counterweight. Your body has no compelling reason to maintain expensive muscle tissue when the dominant training signal it receives is "sustain movement over a long duration at a moderate intensity." Muscle is metabolically costly. If you're not giving your body a reason to hold onto it—through resistance training, sufficient protein, or strength-based programming—it will quietly trade it away. This is not a design flaw in Zone 2. It's a design flaw in how most people build their training programs around it. The Spring Muscle Loss Pattern At Kalos, we've completed over 3,000 DEXA scans across the Bay Area, and the spring-to-summer scan comparison tells a consistent story. People who commit heavily to Zone 2 running through spring—often motivated by a half marathon, a commitment to longevity, or a desire to "burn more fat"—frequently arrive for their summer scan expecting to see fat loss. Some do see fat reduction. But a significant portion also show a drop in lean mass that wasn't part of the plan. What's happening at the tissue level? When you run at low intensity for extended periods, your body adapts to become more efficient at that task. Muscle fibers shift in composition. Fast-twitch fibers, which are the ones responsible for strength and size, receive far less stimulus than slow-twitch fibers. If your resistance training has dropped off—or was never substantial to begin with—those fast-twitch fibers begin to atrophy through disuse. Add a caloric deficit (common among spring runners trying to "lean out before summer") and the muscle loss accelerates. Your body will preferentially cannibalize muscle tissue when both the training stimulus and the caloric supply fail to signal that muscle retention matters. The fat stores you were hoping to deplete? They're protected by hormonal mechanisms that make them relatively stubborn compared to lean mass. Cardio keeps climbing but belly fat refuses to budge is a pattern we see regularly on scan results. How to Read DEXA Scan Results After a Cardio-Heavy Season If you're asking yourself "how to read DEXA scan results" after getting your numbers back, here's what to focus on specifically after a Zone 2-heavy training block. First, look at your total lean mass and your regional lean mass. A DEXA scan segments your body into arms, legs, and trunk. If you've been running heavily without resistance training, you'll often see a reduction in leg lean mass (your quads, hamstrings, and glutes), because those muscles have been trained for endurance efficiency—not size or strength. You may also see upper body lean mass flat or declining, because running does very little to stimulate the upper body. Second, look at your fat mass in context. If your total body weight held steady but your lean mass dropped, your fat mass increased by mathematical necessity—even if the scale didn't move. This is one of the most common and most surprising findings for endurance athletes. Stepping on the scale daily actually hides fat accumulation because it can't distinguish between what's tissue you want and tissue you don't. Third, check your visceral adipose tissue score (VAT). Zone 2 is often promoted as particularly effective at reducing visceral fat, and there's research supporting this. But if your overall protein intake has been low and your stress has been high—both common among spring runners who are training hard and eating in a deficit—VAT can remain elevated or even increase due to cortisol response. Fourth, pay attention to your appendicular lean mass index (ALMI). This is the ratio of your lean mass in your arms and legs relative to your height. It's one of the key longevity metrics Kalos tracks because it predicts sarcopenia risk and functional independence over time. A spring of heavy cardio without resistance training can nudge this number in the wrong direction even for relatively young, fit people. Can I Exercise Before a DEXA Scan? If you're planning to get scanned after your spring training block, one of the most common questions is: can I exercise before a DEXA scan? The short answer: yes, but with caveats. Intense exercise within 24 hours of a scan can cause transient fluid shifts in your muscle tissue. When you do heavy resistance training or a long run, your muscles experience micro-damage and draw in fluid as part of the recovery process. This can temporarily inflate your lean mass reading—making you appear to have more muscle than your baseline state would show—and deflate your fat mass reading proportionally. This doesn't mean a single workout will dramatically skew your results, but it introduces noise that makes comparison across scans less precise. For best accuracy, especially if you're doing a before-and-after comparison of a training block, try to scan under consistent conditions each time. If you exercised the morning before your first scan, exercise the morning before your follow-up scan. What you're optimizing for is consistency, not perfection. The DEXA methodology at Kalos is designed to capture your true body composition, and your Kalos analyst will ask about your recent activity, hydration, and eating to contextualize your results accurately. Also: don't fast aggressively before a scan. Some people make the mistake of skipping food and water thinking it will make their results look better. Dehydration can actually cause your lean mass reading to drop artificially, because muscle is roughly 70% water. Dehydration hides fat gain in ways that scans reveal once you're properly hydrated for a follow-up scan. The Spring Muscle Loss Is Fixable—But Only If You Can See It Here's the piece that frustrates most people when they first hear it: they feel fit. After months of Zone 2 running, their cardiovascular fitness has genuinely improved. Their resting heart rate is lower. They can run farther with less effort. VO2 max is up. These are real adaptations with real health value. But improved cardiovascular fitness does not protect lean mass. It doesn't prevent muscle loss. And it doesn't translate to the body composition outcome most people are actually after when they lace up their shoes in February and start running. This is the problem Kalos was built to solve: the gap between training effort and body composition outcome. Marathon season muscle loss is something scans catch that training logs completely miss. The same is true for a spring of Zone 2 running. You can have a full Strava history documenting every mile and every heart rate zone—and still not know what that training did to your body composition. The data you need isn't in your GPS watch. It's in your tissue. What Should Have Been Happening All Spring At Kalos, we use a data-driven framework to evaluate what's actually driving body composition outcomes. When it comes to exercise, the dominant variable—accounting for roughly 80% of results—is consistency. Whether you're going to the gym. Whether you're showing up. Zone 2 runners typically score extremely well here. They are consistent. But the next tier—programming—matters enormously for body composition. And programming Zone 2 running without resistance training is an incomplete stimulus for the outcome most people are seeking. A well-structured spring training protocol for someone prioritizing both cardiovascular health and muscle retention would look something like this: two to three Zone 2 sessions per week for aerobic adaptation, paired with two to three resistance training sessions that prioritize compound movements, progressive overload, and sufficient volume to maintain fast-twitch fiber recruitment. Protein intake needs to be calibrated to support lean mass—typically around 0.7 to 1 gram per pound of bodyweight—because running increases protein turnover and a deficit amplifies the risk of muscle loss. Protein targets mean nothing without measuring actual muscle gains—and that's precisely what DEXA scanning allows you to do. You can eat what you believe is sufficient protein and still be losing lean mass. The scan tells you whether your approach is working. Why This Matters More Than Aesthetics Muscle loss matters beyond how you look at the pool. Lean mass is a primary predictor of metabolic rate, insulin sensitivity, injury resilience, and long-term functional independence. The Appendicular Lean Mass Index, which your DEXA scan reports, is used clinically to assess sarcopenia risk—the progressive loss of muscle mass that accelerates after 35 and becomes a major health liability after 60. When younger runners in their 30s and 40s consistently prioritize cardio without resistance training, they're not just missing an aesthetic opportunity. They're potentially accelerating the timeline on muscle loss that will compound over decades. Tracking sarcopenia risk before symptoms appear changes everything about how proactively you can intervene. This is the core of Kalos's triangle framework: Aesthetics, Performance, and Longevity are all served by maintaining and building lean mass. Zone 2 training alone addresses the longevity vertex—specifically cardiovascular longevity—but leaves the muscle-related longevity and performance metrics largely unaddressed. Getting a Scan at Kalos After a Training Block If you've been running Zone 2 all spring and you want to know exactly what it did to your body, Kalos scans at three Bay Area locations: San Francisco, Palo Alto, and San Jose. Every scan comes with an in-person analysis session with a NASM-certified performance analyst who will walk you through your results in plain language and connect what the numbers mean to what you should actually do next. The scan is the entry point. The conversation is where it becomes actionable. You'll leave knowing your exact lean mass by region, your visceral fat score, your bone mineral density, and your body fat percentage measured at clinical-grade accuracy—not estimated by an algorithm or inferred from bioelectrical impedance. And you'll have a clear picture of whether the last few months of training served your body composition goals, or worked against them. All Kalos scans and coaching memberships are HSA/FSA eligible. If you've spent a season running and you're ready to find out what it actually built—or cost—the data is waiting.