Praise be to Allaah.
If some of the food comes back up from the stomach to the
throat, this is something that a person has no control over, rather it may
be due to some medical problem or because the stomach is filled with food.
This is called regurgitation and the one to whom that happens
should expel it from his mouth if he can. But if he cannot expel it and it
goes back to his stomach, there is no blame on him and it does not affect
his fast.
Ibn Hazm (may Allaah have mercy on him) said:
The fast is not invalidated if a person regurgitates and it
comes out of his throat, so long as he does not deliberately swallow it
after it reaches his mouth and he is able to spit it out. End quote.
Al-Muhalla (4/335).
For more details on this issue, please see question no.
40696 and
12659.