Caffeine is a diuretic, meaning it makes you need to pee more frequently. Although it's only temporary this does work against your hydration goals. The lactose in milk also tends to cause tummy upsets: according to the Dietitians Assocation of Australia lactose intolerance is more common among people from Asia, Africa, Middle East, some Mediterranean countries and Australian Aborigines. But even in Caucasians, approximately 1 in 20 people have some degree of lactose intolerance. Symptoms include bloating, gas and cramps.