пробил тут для интереса в гугле по vitamins overdose,
первая же ссылка, на аглицком правда
http://www.vitamins-nutrition.org/vitamins/vitamin-overdose.html
в целом подверждает то о чем я писал, вот например цитата:
"Water soluble vitamins - such as the B group vitamins and vitamin C - cannot be stored by the body to any great extent, so if you take in more than your body needs, they are simply excreted. However, the fat soluble vitamins A and D can accumulate in the body, but pose no danger to your health if consumed within the safe upper limits indicated for them. "