If a person was deficient in a vitamin to the point of causing adverse problems with their body, what vitamin deficiency would likely cause notable symptoms first?
Well, start with some basic information from Wikipedia, and use that as a starting point:
Okay, so from there, we can rule out diseases caused by deficiencies in A, D, and B[sub]12[/sub], which all take months to deplete. I used to think a deficiency in B[sub]12[/sub] would lead to notable fatigue in short order, but apparently that takes a lot of time to kick in.
Scurvy might be a contender, because Vitamin C is water-soluble, and is not much stored in the body. You’re constantly sweating/peeing the C out of your system.
You can get vitamin D through exposure to sunlight.
So that probably narrows the list to thiamine, niacin or Vitamin C.
Scurvy (vitamin C deficiency), beriberi (thiamine deficiency), and pellagra (niacin deficiency), are the most likely candidates. Niacin, also known as vitamin B3, isn’t stored in the body in any appreciable amounts, and symptoms of pellagra may begin to manifest within a few weeks (as mentioned in the Wikipedia article bordelond quoted). Scurvy typically appears within a couple of months of complete vitamin C deprivation.