The trend is what I care about; individual weeks are obviously noise.
I want to know what the trade-off actually is rather than which option is fashionable.
I would rather have a defensible reason than a marginal improvement.
So which one, and on what grounds?
4 weeks is 28 days, and the only question is whether 4 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 — 4 weeks accumulates 0.5 kg. 0.5 kg is under the 0.9 kg threshold, so a repeat every 4 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.
The short version: same machine, same time of day, fasted, rested, and then the series means something.
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.
| Method | Measures | Sensitive to | Least significant change |
|---|---|---|---|
| DEXA | Three-compartment by attenuation | Hydration, positioning | ~2–3 % regional lean |
| BIA (consumer) | Impedance, modelled | Hydration, food, temperature | Not usable at this timescale |
| Air displacement | Two-compartment by density | Lung volume, hair, clothing | ~1–2 % fat mass |
| Tape and scale | Circumference, mass | Technique | Surprisingly usable as a trend |
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.
Effective dose for a whole-body composition scan is published at a few microsieverts, well below a chest radiograph.
Same machine, same conditions, twelve weeks apart. Anything else is noise collection.
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Visit GL BiochemTo be exact about it, this is an instrument question rather than a physiology question, and treating it as one resolves most confusion in this tag.
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.
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.
Ask the facility for its precision error. If it does not have one, that is information.
Put another way, regional analysis — trunk versus limbs — is more informative than the total for anyone tracking training effects.
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.
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.
A body-fat percentage from one machine and one from another are not the same measurement and should never be compared.
Read the regional segments, not just the total.
The relevant limitation is that lean mass includes water, so anything that shifts fluid shifts the reported lean figure without any tissue changing.
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.
Between-manufacturer differences in body-composition output on the same subject are documented in cross-calibration studies and are not small.
Precision is good, accuracy is conditional. Track change, do not compare absolutes.
edited 30 Apr 2025 by Dr_Yusuf_Adeyemi — added the method parameters
Specifically, bone mineral density comes free with the scan and is worth reading, particularly during sustained weight loss.
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.
Take the bone density seriously; it is the part of the scan nobody reads.
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