I would like to know what to measure as much as what to do.
I would like the axes of comparison first and the recommendation second.
I have tried the first option and it works; the question is whether the second is better rather than merely different.
Is there a defensible reason to prefer one, or is this a coin flip?
6 weeks is 42 days, and the only question is whether 6 weeks accumulates more signal than the instrument's own noise. Scan-to-scan precision on lean mass is about one to one and a half per cent under standardised conditions, so on a 60 kg lean mass the least significant change is roughly 0.9 kg. At half a kilogram of total loss a week with a quarter of it lean — 0.125 kg of lean per week — 6 weeks accumulates 0.75 kg. 0.75 kg is under the 0.9 kg threshold, so a repeat every 6 weeks mostly reports positioning and hydration. On those figures you need about 7 weeks between scans before the trend outruns the error bars. Standardised means the same machine, the same time of day, the same hydration state, the same clothing. Change any of those and the precision figure this arithmetic rests on is no longer yours. Three scans on a line beat two scans and a difference at any interval: the third is what tells you whether the second was a measurement or an outlier.
More usefully, this is an instrument question rather than a physiology question, and treating it as one resolves most confusion in this tag.
Hydration is the dominant confounder. A litre of fluid is a kilogram, and it is allocated mostly to the lean compartment, so an under-hydrated scan flatters the body-fat percentage and a post-carbohydrate-loading scan does the opposite.
| Body mass | 1.2 g/kg | 1.6 g/kg | 2.0 g/kg | Per meal at 1.6 (÷3) |
|---|---|---|---|---|
| 62 kg | 74 g | 99 g | 124 g | 33 g |
| 74 kg | 89 g | 118 g | 148 g | 39 g |
| 88 kg | 106 g | 141 g | 176 g | 47 g |
| 103 kg | 124 g | 165 g | 206 g | 55 g |
| 124 kg | 149 g | 198 g | 248 g | 66 g |
At roughly 4 kcal per gram, 141 g of protein is about 564 kcal — a substantial fraction of a 900 kcal budget, which is the real constraint.
Standardise: same scanner, morning, twelve hours fasted, no exercise in the preceding twenty-four hours, no unusual carbohydrate intake in the preceding two days, and the same clothing.
Between-manufacturer differences in body-composition output on the same subject are documented in cross-calibration studies and are not small.
Nothing here is medical advice.
Precision is good, accuracy is conditional. Track change, do not compare absolutes.
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Browse resultsAnswering this needs the precision error of the specific scanner, which good facilities publish and most do not.
Different manufacturers use different edge-detection and tissue-decomposition algorithms, so cross-machine comparisons can differ by several percentage points of body fat on the same person on the same day.
Typical precision error on a good DXA is around one per cent for fat mass and one to two per cent for lean mass, which means the least significant change is roughly two and a half to three per cent — about one to one and a half kilograms of lean mass for most people.
A body-fat percentage from one machine and one from another are not the same measurement and should never be compared.
Same machine, same conditions, twelve weeks apart. Anything else is noise collection.
Regional analysis — trunk versus limbs — is more informative than the total for anyone tracking training effects.
Scanning more often than every twelve to sixteen weeks generally produces changes inside the precision error, which reads as noise and gets interpreted as signal.
Bone mineral density falls modestly during sustained weight loss, and having the baseline on the same series is the only cheap way to see it.
DXA precision error and least-significant-change calculations are standard quality metrics published by densitometry societies and by well-run facilities.
Ask the facility for its precision error. If it does not have one, that is information.
edited 4 Jun 2025 by area_percent — reworded for clarity after a comment
The short version: same machine, same time of day, fasted, rested, and then the series means something.
Radiation exposure is very low — of the order of a few microsieverts, comparable to a day or two of natural background — so scan frequency is limited by cost and by precision, not by dose.
Appendicular skeletal muscle mass index from DXA is the confirmatory measure in the consensus sarcopenia definitions.
Take the bone density seriously; it is the part of the scan nobody reads.
Start with standardising the conditions, because hydration, food, recent exercise and time of day all move the result by more than the change you are trying to detect.
Scans are usually reported with a total and with regional segments. Regional lean mass in the limbs is the appendicular skeletal muscle measure used in the sarcopenia definitions, and it is more useful than the total for training questions.
Bioimpedance is not a substitute and is far more sensitive to hydration than DXA is.
Read the regional segments, not just the total.
edited 3 Jul 2025 by Dr_Yusuf_Adeyemi — added a caveat about sampling
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