Why the Scanner Says 8% Body Fat and the Mirror Disagrees

The DEXA scan reports a body-fat number with a confidence the mirror refuses to confirm, and after a hundred-pound loss the gap between the two becomes the most confusing part of the journey. The journey in question ran on retatrutide, an investigational research peptide, but the measurement lessons apply to any large loss. This article is community-sourced measurement literacy for research-use contexts, and nothing in it is medical advice.

I · The scanner said 10% and the mirror disagreed

The anonymous power-user who anchors the r/Retatrutide corpus started at 350 lbs with a body-fat reading near 46%, lost roughly 150 lbs in a year on retatrutide with resistance training and a high-protein diet, and ended up with DEXA, hydrostatic weighing, and an InBody device all reporting between 7-10% body fat ¹. The mirror disagreed, because a loss that size leaves behind loose skin and changes in posture, along with pockets of stubborn tissue that no scanner printout renders the way a bathroom mirror does. The community’s own framing of the problem became a content angle that wrote itself: “Every body-composition tool says I’m 8% body fat. The mirror disagrees” [anecdotal] ¹.

The underlying tension is that people treat the DEXA number as a verdict even though it is a measurement, because the machine looks authoritative and the printout arrives with three decimal places of certainty. The mirror is the opposite kind of instrument, since it shows the whole history of the loss in skin and shadow and shape, which is exactly the information the scanner reduces to a compartment total. Both instruments read something real, and the literacy gap is in knowing which one answers which question.

The question this article works through is what each tool can claim in good faith, because the answer changes what a person should do with the number on the printout. The sections that follow walk the measurement stack from the X-ray machine down to the bathroom scale, and the standing rule that emerges is that every number needs a method attached to it.

II · What a DEXA scan measures

DEXA stands for dual-energy X-ray absorptiometry, and the machine works by sending two X-ray beams of different energies through the body and reading how much each beam weakens on the way through ². Bone and fat attenuate the two energies differently, which is how the software separates them, and everything that remains gets poured into a third compartment called lean mass. That third compartment is the source of most of the confusion, because lean mass is a leftover category: it holds water, glycogen, organs, skin, connective tissue, and intramuscular fat alongside actual muscle ³.

The precision of the machine adds to the problem, because DEXA is repeatable enough to track a trend, which is why the community runs monthly scans and compares them, while still being systematically off in ways that matter for a single reading ². Precision describes repeatability, whereas accuracy describes truth, and the community’s blunt summary of the tool is that it is the best budget option without being a good one ².

“DEXA scans are notoriously inaccurate at measuring body fat (perhaps the best budget option but that ain’t saying much).”

Community source, r/Retatrutide corpus, 2025

The most telling artifact in the corpus is the case the community calls the trap man, where a DEXA scan attributed 28.3 lbs of lean mass to the torso and only 3 lbs to the limbs, a distribution the community read as a scanner artifact ⁴. When the machine can swing roughly 25 lbs of lean mass between body regions on a single scan, the absolute lean-mass number deserves suspicion, and the same caution applies to the fat number printed beneath it.

III · The lean-mass bucket holds more than muscle

The muscle-loss panic that runs through every GLP-1 community rests on DEXA printouts, and the correction starts with a single phrase the corpus repeats: “they don’t actually measure muscle” [anecdotal] ³. Water and glycogen move in and out of the lean compartment day to day, dragging inflammation with them, so a month of rapid loss can produce a lean-mass gain that is pure measurement artifact, since “Most of the time when lean mass increases while losing 30 pounds in a month it’s because the measuring system is inaccurate” [anecdotal] ³.

The contrast that matters comes from MRI-based analyses, because magnetic resonance imaging can resolve muscle as tissue ⁵. The community source points to the body-composition analyses of the pemvidutide and survodutide trials, which put the muscle share of weight lost at about 18% and 10% respectively, far below the lean-mass shares that DEXA-based reporting produces for the same class of drugs ⁵. The gap between those numbers is the measurement itself, because a DEXA lean bucket and an MRI muscle count answer different questions about the same loss, and the community’s reading is that the DEXA version overstates the muscle damage ⁵.

Skin is the other silent occupant of the lean bucket, because after a hundred-pound loss the extra skin is real mass and DEXA counts it as lean ⁶. One community voice put the number at roughly 20 lbs of extra skin, which is lean mass by the scanner’s accounting and loose tissue by every other standard ⁶. The single-case claim that best captures the paradox: “How many people have you seen cut 90% of their body fat (from 150 lbs to 15 lbs) while gaining 11 lbs of lean mass in a year?” [anecdotal] ¹.

IV · Water weight makes the scale lie

The first week of any GLP-1 protocol is the loudest demonstration of the problem, because the source who tracked everything reports losing over 14 lbs in week one and states plainly that it was not fat ⁷. Glycogen and the water bound to it drain early, so the opening weeks of a loss overstate fat loss, and the later weeks understate it when the body refills ⁷. The same mechanism runs in reverse for anyone stacking growth hormone or tesamorelin, since “I will easily put on 10 pounds over a few weeks after starting tesa and that’s enough to completely hide a 1000 calorie per day deficit on the scale” [anecdotal] ⁷.

The stalls that follow are the same story with the sign flipped, because the scale can sit flat while body fat keeps falling. The corpus field guide to this question is direct: the scale stall and the post-illness gain are water and glycogen along with dehydration recovery, and “you might still be successfully losing body fat even if it’s not reflected on the scale” [anecdotal] ⁸. The most extreme recorded bounce in the corpus came from the source himself, who hit a low of 152 lbs and watched the number jump to 213 within days of eating more, which he attributed entirely to water weight ⁹.

Trends over absolutes is the rule that falls out of all of it, since a 90-day trend line absorbs the weekly noise that a single weigh-in cannot. The metabolic-ward data reinforce the point, because Ravussin’s 2025 analysis in Cell Metabolism, which the corpus cites repeatedly, showed energy expenditure adapting downward as weight fell, which means a plateau on the scale is often the body spending less while the plan continues to work ¹⁰.

V · Impedance scales are the weakest of the three

The same physics that makes water the enemy of the bathroom scale makes it the enemy of impedance devices, since a bioimpedance analyzer estimates fat by measuring how easily a small electrical current travels through tissue, and water conducts while fat resists ¹¹. Muscle and water both conduct well, so the estimate shifts with hydration and food timing, which is why the community’s field guide to the tools carries the one-line verdict: “Impedance scales lie during transition” [anecdotal] ¹¹.

The consumer versions are the weakest link in the chain, because a bathroom scale with metal foot pads is a single-frequency estimate that assumes a constant hydration state in every body. The multi-frequency units used in gyms and clinics, such as the InBody devices, do better, and the corpus’s anchor source used an InBody alongside DEXA and hydrostatic weighing and got the same 7-10% reading from all three ¹. That agreement matters, because three independent methods converging on the same number is the closest thing the community has to a gold standard, whereas a single method agreeing only with itself proves nothing.

The practical takeaway is that impedance devices are trend instruments at best, since they can track direction over months when measured under identical conditions, but a single reading is close to meaningless. The strongest rule from the corpus is to measure the same way every time, at the same time of day, under the same hydration and food state, and then compare month to month, letting the trend carry the meaning ¹¹.

VI · The hydrostatic cross-check

The cornerstone anecdote of the corpus is a man who did not believe his own DEXA scan. He had watched the number fall from 46.5% at the start to 10% at the end of a 150-lb loss, and he wanted a second opinion from a method with different assumptions, so he found a facility that does hydrostatic weighing ¹². His account of the decision is understated: “I kind of doubted it myself which is why I did the hydrostatic weighing” [anecdotal] ¹², and the result was that the tank agreed: “I doubted the DEXA scan saying I was at 10% so I went and found a place to do hydrostatic weighing. That also came back at 10%.” [anecdotal] ¹².

Hydrostatic weighing works on Archimedes’ principle, since a body submerged in water displaces its own volume, and the ratio of weight in air to weight in water yields a body density from which fat and lean fractions are estimated ¹². The method is old and uncomfortable, and most clinics have replaced it with DEXA, but it measures something different from the X-ray machine, which is exactly why it works as a cross-check. Two instruments with different error structures agreeing on the same number is stronger evidence than either instrument alone, and the corpus’s anchor case is the cleanest example of that logic in the community record ¹².

The honest limits of the cross-check deserve equal billing, because hydrostatic weighing still depends on assumptions about the density of fat-free mass, and residual lung volume has to be measured or estimated ¹². The tank settles on a convergence, and three methods reporting the same single-digit figure shift the burden of proof from the scanners to the mirror.

VII · The trend is the signal

The measurement protocol that falls out of the corpus is boring on purpose: one device, used at the same time of day under the same hydration and food state, on a monthly cadence. The source who logged the corpus’s best data ran DEXA roughly every four weeks through the entire loss, which is what made his numbers readable as a single continuous trend ¹. The community guidance on the lean side of the printout is to focus on fat mass, since the lean bucket is the noisy one, and to ignore any lean-mass gain that appears during a month of rapid loss ³.

The mirror gets a vote in this protocol, because the corpus’s answer to the question of who is right when the scanner and the mirror disagree is that both are reading different things. The scanner reads compartments, whereas the mirror reads history, since loose skin and posture, along with the way clothes fit, are real data even if no device prints them ⁶. One community voice tried to settle the argument about residual tissue by asking a plastic surgeon who operates on massive weight loss patients, reasoning that a surgeon who cuts that population open all day would know whether the remaining tissue is fat or skin [anecdotal] ¹³.

Strength is the final co-arbiter, because the community’s line on muscle loss is that losing 20, 50, or 100 lbs normally costs some muscle while relative strength improves, since “it’s a lot easier to do a pushup at 20% body fat than at 45% body fat” [anecdotal] ¹⁴. The gym numbers and the waist measurement, with the mirror as the third vote, form the ground truth that no single scan provides, and the scan is most useful when it is read as one witness among several, each with an equal vote ¹⁴.

VIII · The same skepticism applies to the claims

The community that produced this corpus has a public record of correcting its own authorities, and the pattern matters because it is the same discipline that measurement literacy demands. When a commenter misattributed the retatrutide phase 2 obesity trial, the source corrected it in public: the obesity trial is Jastreboff et al, whereas Sanyal et al is the liver-fat subtrial from the same program ¹⁵. That correction matters because a trial’s body-composition numbers are only as good as the paper they are pulled from, and a name attached to the wrong study is a measurement error in exactly the same sense as a scanner artifact.

The negative example is the influencer case, since the corpus documents Trevor Bachmeyer, a popular peptide influencer, making confident claims that retatrutide requires carbohydrates, and the community’s fact-checkers found his citations were fabricated ¹⁶. The same trust that makes people believe a DEXA printout makes them believe a confident video, and the community’s data-integrity norm is the antidote to both: check the source and verify the quote, treating unverifiable numbers as unverified ¹⁶.

The closing position of the corpus is that a number is only as good as the method that produced it, and the method is only as good as the conditions it was taken under. Track fat-mass trends from a consistent device, cross-check the big claims with a second method, let the mirror and the gym vote, and apply the same suspicion to any single number that you would apply to a citation you cannot find ¹⁶. This article is community-sourced measurement literacy, not medical advice, and every compound mentioned here is research use only.