The lesson is not which marker is accurate but how to ask: the question comes first and the test second, never the other way round. Point the same question at supplements and it becomes 'does taking it actually do anything?'
An abnormal blood calcium is not telling you about intake; it says the regulatory system itself is off — parathyroid, kidney, certain tumours — and that is a signal for a doctor. How calcium moves in and out of bone, and how to read bone density, is where skeletal status is actually judged.