Vitamin A

There was a boy with with really big teeth. His name was Abraham Alfonzo, but everyone called him Vitamin-Abe because he ate so many carrots. His friends made fun of his giant teeth, but one day the evil witch Xerophthalmia came to town. She announced that she would put a curse on all the orange and yellow vegetables and fruits so that they would be too hard to eat.

"Why should we care if we can't eat those foods!" shouted the peasants.

"You will see in time," said the witch mysteriously.

As the days passed the people soon learned how terrible their curse really was. Without access to the Carotanoids such as Beta-Caratine contained in orangy plants, their bodies couldn't synthesise enough Vitamin A. In time, some of the peasants started developing Xerophthalmia, which they named after the wicked witch. Their eyes became dry from a lack of mucus, which caused them to become susceptible to infection. Over time lesions developed and spread, leading to irreversible blindness.

In the meantime, Abe was still able to enjoy the same foods he had always had, thanks to his unusually hard and strong teeth. This was very fortunate, not only because he didn't go blind. When the giant man-eating walking trees came in the middle of the night, he had excellent night vision, so he was able to evade the attack which killed half the town. When we was found, the trees could not crush him, for the vitamin A had helped him grow healthy bones. And finally, when the trees exploded and released their deadly spores, his mucus forming cells were doing their job and his immune system was able to fight off what was left.

All in all, two million people died, leaving only three (besides Abe) who managed to survive. But then the first one, a cute girl, took a massive dose of Vitamin A supplements and suffered the painful consequences. The second one did not take nearly as much, but still had a lot on a chronic basis. The third was pregnant, and she was wise enough to eat a normal amount, which prevented Teratogenic side-effects in her baby.

Abe also consumed a lot, but it was in the form of caratonoids, so his only side effect was orange skin.