- Aug 14, 2018
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Almost every lifter has had the experience: the scale jumps a kilo overnight after a salty restaurant meal, the bathroom scale's body-fat reading swings three points in a week, and the expensive scan disagrees with both. The measurement research explains why — and it changes how much any single number should be allowed to affect your week.
Start with the scale itself. Body weight genuinely fluctuates 1–2% day to day from glycogen and the water stored with it (roughly three grams of water per gram of glycogen), sodium, gut contents, and training inflammation. None of that is fat change. The standard research-informed fix is boring: weigh under consistent conditions and judge the 7-day average trend over weeks, not any morning's reading.
Bioelectrical impedance — the body-fat function on home scales and handheld devices — is the shakiest tool. It estimates composition from how a small current passes through tissue, which makes it hostage to hydration: the same person, same fat mass, can read several percentage points apart before and after training, drinking, or a hot shower. Validation studies against reference methods routinely find individual errors of ±3–5 percentage points or worse. It's not useless — a consistent morning protocol can track a rough trend — but treating its second decimal as information is a category error.
DEXA sits at the respectable end and still isn't a truth machine. It has a real precision error of roughly 1–2% for fat mass, it's sensitive to hydration and recent meals (a big breakfast literally shows up partly as "lean mass"), and results aren't interchangeable between different manufacturers' machines. Skinfold calipers, done by a practised hand measuring the same sites, often track change as usefully as fancier tools. The humble waist measurement at the navel, taken relaxed in the morning, correlates well with the abdominal fat people actually care about and costs nothing.
The honest protocol the evidence supports: pick two or three crude tools — weekly average scale weight, waist measurement, progress photos in consistent light, and how your main lifts are moving — and look for agreement between them over a month. Muscle gain and fat loss are slow; any tool claiming to detect them day to day is measuring water and calling it progress.
What do you track, and which tool has lied to you most memorably? Anyone here abandoned the body-fat percentage entirely and just run on weight trend plus waist?
Start with the scale itself. Body weight genuinely fluctuates 1–2% day to day from glycogen and the water stored with it (roughly three grams of water per gram of glycogen), sodium, gut contents, and training inflammation. None of that is fat change. The standard research-informed fix is boring: weigh under consistent conditions and judge the 7-day average trend over weeks, not any morning's reading.
Bioelectrical impedance — the body-fat function on home scales and handheld devices — is the shakiest tool. It estimates composition from how a small current passes through tissue, which makes it hostage to hydration: the same person, same fat mass, can read several percentage points apart before and after training, drinking, or a hot shower. Validation studies against reference methods routinely find individual errors of ±3–5 percentage points or worse. It's not useless — a consistent morning protocol can track a rough trend — but treating its second decimal as information is a category error.
DEXA sits at the respectable end and still isn't a truth machine. It has a real precision error of roughly 1–2% for fat mass, it's sensitive to hydration and recent meals (a big breakfast literally shows up partly as "lean mass"), and results aren't interchangeable between different manufacturers' machines. Skinfold calipers, done by a practised hand measuring the same sites, often track change as usefully as fancier tools. The humble waist measurement at the navel, taken relaxed in the morning, correlates well with the abdominal fat people actually care about and costs nothing.
The honest protocol the evidence supports: pick two or three crude tools — weekly average scale weight, waist measurement, progress photos in consistent light, and how your main lifts are moving — and look for agreement between them over a month. Muscle gain and fat loss are slow; any tool claiming to detect them day to day is measuring water and calling it progress.
What do you track, and which tool has lied to you most memorably? Anyone here abandoned the body-fat percentage entirely and just run on weight trend plus waist?