There has been a tremendous amount of research on the natural compound icariin in recent years.  The substance is usually isolated from Horny Goat Weed (what a name) also known as yin yang huo.  I'll highlight the research by category here:

Osteoporosis

Several studies indicate that icariin both stimulates the growth of new bone, and slows the destruction of old bone.  As a result a recent 2-year study on menopausal women showed that using the supplement increased bone mass in the hip and low back by 1.6 and 1.3 percent, where as the placebo group lost bone by 1.8 and 2.4 percent.  These percentages make a big difference in fracture risk.  Several other studies in rats have further demonstrated the benefits of icariin.

Alzheimer's

New research in rats shows that icariin improves learning and memory in rat-models of Alzheimer's.  Author's in one of these studies concluded "In the space probing test, icariin increased remarkably the searching time and searching distance in the quadrant where the platform was originally located. All tests indicated icariin improved the ability of spatial learning and memory in Alzheimer's disease model rats."

Stroke

Rats supplemented with icariin and deprived of oxygen have less damage than non-supplemented rats.  Icariin improved blood flow, decreases oxidative "collateral damage" after a stroke, and promotes acetylcholine-- a neurotransmitter tightly linked to memory.  Read the study.

The strong, consistent research on this herb in building bone and protecting the brain, as well as an excellent safety profile, make it worth giving a try for individuals with osteopenia or osteoporosis.