If there are no other illness, heartburn / reflux, burps, stomach rumbling, stomach pain etc. is all quite common if you regularly eat outside. Taste sells, so food in restaurant use copious amounts of oil / butter, sugar, salt, refined flour, MSG, etc. all of which are unhealthy when consumed in quantity and regularly. Eating home-cooked food really makes a difference to your gut health.
A lunch for me could either be a salad from the grocery store or raw broccoli, tomatoes, carrots, almonds, and maybe one of hard-boiled eggs/canned ham/cheese.
That sounds healthy except perhaps for the canned ham, if it is processed; diary products like cheese can cause bloating, pain, flatulence etc. if you are lactose intolerance (according to this article - https://www.everydayhealth.com/digestive-health/foods-that-c... - Broccoli and carrot can also gas, but not a real cause for concern with healthy foods like these). Apart from that you still face the common issue of hygiene and when the food was prepared. When eating out, we don't know what goes in the kitchen where the food is prepared and can only make a best guess that the food is handled hygienically in a clean environment. So that's out of our hand. But in grocery stores, we can certainly keep an eye out for the date / time the food was prepared and try to buy it "fresh". Foods that are kept out for long or not stored properly in the right temperature do undergo change (fermentation for e.g.) and that can also affect our tummy.
I said they are unhealthy when eaten regularly in the quantity that restaurants use. That, and the fact that restaurants (even the good ones) sometimes serve leftovers often cause these issues. (And a healthy person won't remain healthy for long if they eat regularly eat unhealthy food from restaurants).
