Is it legal for players in a tournament to eat something while they play. If it isin't, then are there any special considerations for certain individuals who have health problems? (such as a diabetic needing to eat a candy bar for instance).

It's called en passant.

Oh, ... i have no idea.

It is legal (well, there's nothing in FIDE regulations about it). However, it is illegal to distract your opponent, so it may be necessary to be discreet about it.

FIDE rule 12.6 - It is forbidden to distract or annoy the opponent in any manner whatsoever.

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Look for the word "preserved".

Atatürk Istanbul: Hou Yifan wins Atatürk Masters

Nalchik Final: Kosteniuk leads 2:1, needs just a draw

Istanbul: Humpy wins, Zhao loses for an exciting finish

Whatever preserved dates are, they must not be noisy when chewed.

Yeah of course you can eat, but away from the board.