not sure about the whole science behind it but when i did atkins you are allowed a lot of fat and protein but very limited carbs and this puts you into ketosis for example, i had a fry up every morning cooked in butter, i could only last a week at a time as the smell of fat turned me so i'm guessing that certain fatty foods don't necessarily have a high carb content and as cheese is high protein and fat, one of those foods allowed on atkins, the full fat milk is probably similar in grams of fat, etc to cheese as its from the dairy family. not sure if i'm right with this one though!