Harm to coffee for the stomach and metabolism

Harm coffee of various types - natural and soluble. How much coffee can you drink per day, so as not to harm yourself? The damage of coffee is not only in caffeine. How do digestive organs suffer from the effects of coffee? What substance, contained in coffee, harms the stomach? Why does coffee often cause heartburn? Why can not I drink coffee on an empty stomach? Can coffee lead to cancer? Why can not calcium preparations be taken with coffee? How can coffee lead to dehydration?