Whey isn't necessary to grow muscle. Stop taking it if you want, but I don't see why you would. It's a cheap, conveinent, and often tasty source of bioavailable protein. Blend into milk, raw oats, and ANPB or some oils of choice and you've got a killer shake.
Muscles don't eat fat for energy. Stop saying this people.
You will lose muscle without adequate levels of protein. Eat enough protein to maintain muscle mass, at the least. You will only lose weight if you eat under maintenance calories. Whey has no effect on weight gain, other than the fact that it's more calories. It has the same effect oats, raisins, apples, and milk.