My brother is 24 year old mentally challenged child with severe cp. He usually ate less almost 1/4th in comparison to other kids of his age.But gradually it reduced very much. Right now he is atmost taking 2 to cups of cooked rice in entire day(maybe around 75 to 80g). He refuses to take even water. Atmost he drinks around 1 litre of water. Even that takes a lot of effort for us to make him do that.But he is not loosing any weight either. And all his blood sugar and other levels, hp are normal.