Cow milk (dairy) is a very well documented source of food intolerances. You’ll know you have a dairy intolerance by performing the following experiment:
Abstain from all dairy for a week, then drink a large glass of full cream milk. If you bloat, feel tired or shit your pants, you have an intolerance to milk.
This is usually because most humans don’t possess enough of the lactose digesting enzyme lactase. Lactose is the sugar in milk. If you can’t digest it, you get a sore, bubbly swollen guts, followed by thunderous diarrhoea.
Goats milk is a much better substitute because it has a lot less suspended fat and we use a different metabolic pathway to digest it (not relying on lactase somehow – scientists don’t even know why, because the lactose levels are less, but not entirely absent). Interestingly, patients who are diagnosed as lactose intolerant can drink goats milk without issue.