Praise be to Allah.
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 Allah 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 from Al-Muhalla, 4/335)
For more details on this issue, please see these answers: (40696, 12659, 228431, 192316, 38579, 38205)
And Allah knows best.
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