I yesterday watched the documentary Vitamania by Dr Derek Muller, which is about the production, benefits and risks of vitamin supplements. In the film, he briefly talked about people being blood tested in order to get some insights if they are vitamin and/or mineral deficient.
So I was wondering: When we as humans have to pretty much constantly get the vitamins our body needs to function right, can a blood test really give meaningful insights of a vitamin deficiency?
For example, if the day (or even week) before the blood test I eat less water-dissolvable vitamins like vitamin C than usual and than needed. Would that show a deficiency in the blood test thus leading to false results? Or would a deficiency only appear after a month, two, three? Or even half a year?
I already tried to search for something relatable on google. Only found some forum posts or non-scientific articles of random people claiming things like "Vitamin B gets flushed out like every day or so", which is not really specific or backed up by any literature/study. I don't really have access to any physical literature or something similar.