I knew a woman[0] who ate nothing but ramen noodles for six months during her freshman year at college. She collapsed in class and had to be taken to the ER. I forget what all was wrong with her, but she was at least severely malnourished and, if I recall correctly, had also developed a proper antique disease like scurvy.
0 - loosely, in a friend-of-a-friend-of-a-friend way