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Fried foods generally don't agree with my body.
Literal
Fried-things [topic-は] I [possessive-の] constitution [to-に] usually agree-not-[explanatory-んだ].
揚げ物 ('fried food,' literally 'fried thing') + 合う ('match / agree / fit') takes に for the thing being matched. 体質 ('physical constitution / body type') is medical/nutritional vocabulary. A common way to explain dietary restrictions.