Intake is tracked, imperfectly, and I am aware of the direction of the error.
I suspect the honest answer is that it depends, in which case I would like to know on what.
Assume I can obtain either option without difficulty, so availability is not the deciding factor.
Is there a defensible reason to prefer one, or is this a coin flip?
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.
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.
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.
Effective dose for a whole-body composition scan is published at a few microsieverts, well below a chest radiograph.
Ask the facility for its precision error. If it does not have one, that is information.
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Browse resultsAnswering this needs the precision error of the specific scanner, which good facilities publish and most do not.
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.
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.
Appendicular skeletal muscle mass index from DXA is the confirmatory measure in the consensus sarcopenia definitions.
Read the regional segments, not just the total.
The relevant detail is that regional analysis — trunk versus limbs — is more informative than the total for anyone tracking training effects.
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.
Specifically, 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.
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.
Take the bone density seriously; it is the part of the scan nobody reads.
Bone mineral density comes free with the scan and is worth reading, particularly during sustained weight loss.
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.
Same machine, same conditions, twelve weeks apart. Anything else is noise collection.
The relevant limitation is that lean mass includes water, so anything that shifts fluid shifts the reported lean figure without any tissue changing.
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.
Bioimpedance is not a substitute and is far more sensitive to hydration than DXA is.
Precision is good, accuracy is conditional. Track change, do not compare absolutes.
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