Let me tell you why this is bullshit. The reason why your dog got stomachache when you suddenly switch brands is because we ourselves conditioned our dog to eat the same food every single day!
Just like humans. When you are used to eating rice every single day in your life, you will suffer with stomachahe when you are suddenly given potatoes to substitute your rice.
But, if you are used to eating different kind of food every day, you are used to eating potatoes and rice and bread in turn every other day, you are just fine!
And how about those times when dog food have not been invented yet? What do dogs eat? Didn't they eat just like we do? Imagine if every single dog in the world got stomachache because they eat different kind of food. Maybe dogs would cease to exist by now!
And how about those dogs that the owners refuse to feed them dog food/ live in the country somewhere with no access to dog food and feed them boiled chicken with rice, etc?
How come their dogs are healthy and got no upset stomach whatsoever?
To conclude, i can say i firmly believed that this is just dog food companies' marketing strategy to hook customers into brand loyalist. And what they did is wrong. They 'reeducate' customers with the wrong beliefs to gain profits.