If a scale ever told you that you lost a pound and a half of muscle in four weeks while you were lifting weights three to four days a week and eating more protein than before, would you believe it? One of Rob's clients did — and it nearly tanked their motivation. Understanding the true rate of tissue change for fat and muscle is one of the most important (and most misunderstood) concepts in body composition, and it's the difference between staying the course and quitting over data that was never accurate in the first place.
Rob Polenik has spent over seven years teaching exercise physiology at the university level, running body composition labs using everything from bioelectrical impedance devices to skinfold calipers, Bod Pods, hydrostatic weighing, and DEXA scans. He holds a Master's in Human Nutrition, is ACSM-certified, and has coached over 3,000 clients. His master's thesis alone involved reviewing 200 peer-reviewed studies on nutrient timing — so when he tells you a device is lying to you, it's coming from someone who has read the research and run the tests himself.
In this post, you'll learn exactly how fast fat and muscle tissue can realistically change, why your bioelectrical impedance scale (like an InBody) is misleading you, what's actually happening when your muscles look "flat" or "watery" during a diet, and which body composition testing method is actually worth your money.
Bioelectrical impedance devices — the scales with silver foot pads or hand grips — work by sending a small electrical current through your body and measuring how fast it travels. That speed gets run through equations to estimate your body fat percentage, muscle mass, and fat mass. Sounds sophisticated. It isn't reliable.
Rob taught with these exact devices in university labs for years and calls them "damn near useless" outside of extremely controlled conditions. Here's why:
These devices carry a standard of error of 3% or more. If your actual body fat is 20%, the device might read anywhere from 17% to 23% and still be considered "accurate" by its own margin. That's not a rounding error — that's the difference between looking and feeling like a completely different person, both physically and emotionally.
Hydration level alone can shift your reading significantly. Weighing yourself in the morning versus midday versus after dinner will produce wildly different results because your water content changes throughout the day. If you want to use one of these devices at all, you must test under identical conditions every single time — same time of day, same hydration status, same clothing, ideally first thing in the morning after using the bathroom. Even then, expect noise in the data.
The client data that inspired this episode showed daily fluctuations — muscle mass bouncing up and down, an apparent loss of over a pound of muscle in four weeks, and an apparent gain in body fat. The client was devastated. Rob's response: none of that happened.
Here's what actually occurred. The client had been traveling, still learning a new nutrition tracking approach, and testing at inconsistent times of day. That alone was enough to produce nonsensical readings. But more importantly — the client had been weight training three to four days per week and eating more consistent, higher protein meals than before. Under those conditions, losing 1.5 pounds of actual muscle tissue in a month is physiologically implausible.
Rob points to research from his master's program on complete bed rest patients — people who did not move at all, not even fidgeting — which found it took four full weeks of zero movement before statistically significant muscle loss occurred. If total immobility takes a month to produce measurable muscle loss, an active person lifting weights and eating a reasonable diet is not losing a pound and a half of muscle from a few inconsistent weeks of travel.
So if the muscle wasn't lost, what explains the visual change and the device's negative reading? Glycogen and water retention.
When you eat dietary carbohydrates along with sodium and water, your body stores glycogen inside your muscle tissue. Rob compares this to a water balloon: more water makes the balloon bigger, but you haven't added more rubber — you've just stretched what's already there. The same applies to your muscles. More glycogen means more fullness; less glycogen means the muscle looks flatter, softer, or what Rob's wife calls "squishy" (or, in more extreme diet phases, "pudding skin").
When you reduce carbohydrates — either by eating less or moving more — you deplete glycogen stores, and your muscles lose that fullness. Combine that with the cortisol response to being in a calorie deficit, which increases water retention and inflammation, and you get a "watery," less-detailed physique even while you're doing everything right.
This is a purely visual and hydration-based effect — not tissue loss. As you lean out further, you'll have less fat covering the muscle, and once you normalize your carbohydrate and sodium intake, water retention drops and that fullness returns.
If you're diet-fatigued and feel like your hard work isn't showing, Rob recommends what contest-prep athletes use: a planned refeed or higher-carbohydrate day. Loading carbohydrates for a day can restore muscle fullness, reduce the "flat" look, and give you both a physical and psychological boost before returning to your plan.
This is the core data point of the episode, and it's worth memorizing:
Notice the massive difference in timescales: fat can shift by a pound in a week, but muscle moves at a fraction of that pace per month. Any device or scale telling you otherwise over a few days or weeks is giving you noise, not data.
Given all of this, what should you use to track progress? Rob ranks the common options:
For most clients, a DEXA scan every three months hits the sweet spot between accuracy, cost, and convenience.
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Q: How fast can you realistically build muscle?
Under excellent training and nutrition conditions, you can expect to gain roughly 0.5 to 1 pound of actual muscle tissue per month. Most people won't hit the top of that range consistently, so if you're gaining muscle at all with consistent training and adequate protein, you're on track — don't expect dramatic month-to-month jumps.
Q: How fast can you lose fat safely?
About 1 pound of fat loss per week is a strong, sustainable target, achieved through roughly a 3,500-calorie weekly deficit. Rapid drops of 5–8 pounds in the first week of a new diet are almost always water weight, not fat, so don't mistake that for your ongoing rate of loss.
Q: Are InBody scales and other bioelectrical impedance devices accurate?
No — they carry a standard of error of 3% or more, meaning a true 20% body fat reading could show anywhere from 17% to 23% and still be considered "accurate" by the device's own standards. They're also highly sensitive to hydration, time of day, and testing conditions, making short-term tracking essentially unreliable.
Q: Why do my muscles look flat or "watery" when I'm dieting?
This is due to reduced glycogen storage and increased water retention, not muscle loss. Lower carbohydrate intake means less glycogen (and therefore less fullness) in the muscle, while calorie deficit-related cortisol increases water retention and inflammation, both of which make muscles look softer or less defined.
Q: What's the best way to track body composition over time?
A DEXA scan is Rob's top recommendation, offering just a 1–2% standard of error compared to 3%+ for impedance devices, at an accessible cost of around $150 per scan. You only need to repeat it every 3 to 12 months depending on your goals, making it both accurate and practical.
Q: Can you lose a pound of muscle in a few weeks?
Not if you're active and eating a reasonably normal diet. Research on complete bed rest patients found it took a full four weeks of zero movement whatsoever before statistically significant muscle loss occurred — so someone lifting weights and eating adequate protein is not losing meaningful muscle tissue in that same timeframe.
The biggest takeaway from this episode is simple: your body doesn't change nearly as fast — or as chaotically — as your bathroom scale wants you to believe. Fat can shift by about a pound a week with consistent effort, but muscle moves at a fraction of that pace per month, not per day. When a bioelectrical impedance device tells you that you lost muscle in a matter of days, it isn't revealing a hidden truth about your physiology — it's showing you the limitations of an inaccurate tool combined with your body's normal glycogen and water fluctuations.
Don't let flawed data derail real progress. Trust the process, stick with resistance training and adequate protein, and if you want objective, trustworthy numbers, invest in a DEXA scan every few months instead of an at-home device that's guaranteed to mislead you.
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Want to hear Rob break down the full story behind this client's data, the research on muscle loss in bed rest patients, and his complete take on body composition testing methods? Listen to the full episode here: 2285 – Rate of Tissue Change - Fat & Muscle.