That said, here are some basic guidelines to help you not watch all your expensive food flush right down the toilet bowl.
Since I have a sensitive digestive system and have problems processing lactose and casein, my number one is milk and all its derivatives.
Milk products can come in many forms. Butter, ice cream and so on and so forth. It can also come in forms you might not expect like sour cream, certain types of cheeses, and as an added ingredient to foods like mashed potatoes, or buttermilk put in cole slaw, or Rice Krispy treats and certain types of chocolates.
To get around the lactose and casein, I drink pea protein while I workout. The taste is like cow manure but it's cheap (around $15 for a tub of it on Amazon) and one scoop of it carries approximately 27 grams of protein. One scoop mixed with water in a shaker bottle is all I can tolerate per workout. Anymore than one scoop will cause me to get the sh*ts and make a run to the bathroom.
Second food I avoid is soybean oil and soy. They also liquefy my guts into the toilet. Unfortunately the only way to avoid it is I have to scrutinize every ingredients list on every food label before I consider buying it. Soybean oil tends to creep in about 80% of the available food supply. They put it in peanut butter, mayonnaise, certain types of Asian Sauces for stir-fry cooking, some brands of protein shake mixes, cans of tuna and other canned fish and meats, and even in ramen noodle instant and certain artificial butters. The list goes forever and it's rare that a processed food from a can or plastic bag doesn't have soybean oil or soy but like I said I avoid it as best I can.
So it begs the question what the hell do I eat then?
I do mostly boiled chicken or rotisserie chicken I get from the grocery store. I sometimes buy cuts of beef at the grocery store and I salt and pepper to taste or I put BBQ sauce on my protein. If I want carbohydrates, I'll boil up some noodles like pasta or maybe cook rice. If I feel like I need some vitamins I'll eat fruits and vegetables canned or frozen or fresh. For fats, I like to eat natural peanut butter. It has the added benefit of increasing my level of satiety which gives me a nice full feeling after I eat lean pieces of chicken and get hungry again, so I reach for the natural peanut butter and put two cups of it on a plate and eat it raw. It's called "natural" because there's no soybean oil in it. Natural peanut butter uses palm oil instead.
Another way to avoid emptying your guts into a toilet is to have good food hygiene. What I mean is how you prepare your food and what cooking utensils you use to process your food. To wash dishes, buy antibacterial grease lifting dish soap and then mix with a 1/2 cup of bleach into a tub or sink that doesn't leak. Adding bleach can only be done with antibacterial dish soap because it doesn't have ammonia which when mixed with other soaps that do have ammonia can bond to the bleach to make something akin to the type of mustard gas used in chemical warfare during World War I. Definitely something you don't want to breathe in while you're washing dishes.
Probably the best investment you can make is to buy a high quality digital food thermometer, It should go without saying that proteins have a risk of food borne viruses and bacteria, and the only way to kill them is to make sure your foods reach safe eating temperatures. Chicken with bone in should be cooked to 180 degrees Fahrenheit. Boneless chicken no more than 145 degrees Fahrenheit. Beefs are tough to comment on because who wants to eat a leather shoe from beef that's been overcooked, but then again nobody wants beef that squirts blood into your eye when you poke it with a knife either.