It depends how each individual's body reacts to changes in food, drink and mineral intake.
I've spent my life travelling as my father was in the airlines and I was brought up in different countries until I was 12 when we returned to the UK for my school. We always ate local food and drank local water except in India and Ceylon where the water truck came every day. I'm 47 now and I still eat local foods and drink local water all around Europe, North Africa etc. I've never had any problem except in the USSR when I drank some cold alcoholic drink brew from a samovar on a very hot day. That made me ill for the only time in my life but I think it was the cold on a parched stomach.
There are some basic rules to follow. Never eat food that has been cooked and then allowed to cool. If it's piping hot then it's generally fine. Never drink water where there's an obvious risk of waterbourne diseases or parasites. With regard to the change in mineral content in water some people's bodies are quite sensitive while others are more tolerant. That's why some people can tolerate lots of spices, garlic, peppers etc. while others become ill.
I would suggest a general rule that if you and your family are sensitive to spicy foods then you will also be sensitive to different water sources. In that case, play it safe and buy bottled water.