It is the silent thief that robs up to 25% of your skeleton by the time you reach fifty... The conventional explanation is that once a woman stops menstruating she produces little of oestrogen which helps keep calcium in her bones. But this is far from the truth. Oestrogen which simulates osteoclast cells doesn't help build new bones. Progesterone, on the other hand, simulates osteoblasts, which do build new bone. Taking natural progesterone increases bone density four times better than oestrogen. Some medical scientists now believe that a lifelong consumption of a high-protein, acid-forming diet may be a primary cause of osteoporosis.