[carlosfvp]

How did the doctor know her vitamin levels were low? I once asked a doctor if the vitamins in our body can be measured, and he said it can't be done, is not something you can measure.

[scarlac]

It can be measured with a blood test:

> The most accurate way to measure how much vitamin D is in your body is the 25-hydroxy vitamin D blood test.

https://www.webmd.com/diet/guide/vitamin-d-deficiency

[gnulinux]

I regularly blood-test my vitamin D and B levels since I live indoors a lot (not an outside person) and have a slight vit. B absorption problem (I don't absorb it as efficiently as normal people) so I need to take supplements even if I eat healthily.

[ryder9]

Blood test, had it done myself and found low levels of B12 & D, for which I took initially large doses and now take over the counter supplements

[James_Henry]

It depends on what vitamin or mineral you are testing.