The ash-diet hypothesis forms the basis of alkaline diet. It says that some foods containing acid-forming substances (such as animal protein from milk or meat and some plant foods) cause the blood pH to drop or become less alkaline. This effect is then buffered by minerals released from bones. That loss of minerals from the bones is supposedly responsible for mineral bone loss and the resulting osteoporosis.
Supporters of this hypothesis believe that a typical Western diet predominantly consists of these acid-forming foods and that following this diet puts us in a permanent state of mild acidosis which in the long term leads to gradual mineral bone loss. (1, 2)
One of the myths derived from this hypothesis (related to alkaline diet), is that milk/dairy products weaken the bones and may lead to osteoporosis.