My sister, she got gastritis when she drinks coffee she feels poignant in the area of her stomach. So, I want to know that why do people who have gastrointestinal problems when having coffee got to have stomach ache?
In the article titled "Coffee and gastrointestinal function: facts and fiction. A review.", it is stated that it has pharmacological effects and coffee promotes gastro-oesophageal reflux.
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Caffeine is a stimulant which is the family of drugs that is used for suppressing appetite for diet pills. It's possible that her reaction is a physiological to the stimulant, and not the chemicals in coffee, itself.
When I have too much coffee, my stomach feels off, myself.