Study identifies absorbent inner-bone material that may explain osteoporosis

Human bone is a remarkably durable material in healthy men and women, but researchers have never been able to determine exactly what makes it so. Children grow up learning about the benefits of a diet high in calcium, but a rigid structure of hardened materials does not provide much flexibility in terms of pressure resistance.

Researchers from the University of Cambridge's Department of Chemistry used a combination of nuclear magnetic resonance spectroscopy, X-ray diffraction and high-level molecular modeling to discover the presence of large volumes of citrate in human bone. Though citrate is a natural by product of the metabolic processes of the body's cells, according to the study, it plays a crucial role in providing much-needed flexibility to the body's skeletal structure. While more research is necessary, citrate loss may indicate a root cause of osteoporosis and other conditions that lead to brittle bones in aging men and women everywhere.

Discovering the compound
The study, which was published in the journal Proceedings of the National Academy of Sciences of the United States of America, sought to codify the purpose of citrate in relation to bone structures. Human bone is primarily made up of calcium phosphate, which hardens into crystals. When a large number of these crystals fuse together, however, the bone becomes a stiffened, brittle mass that does not support weight or shock very well. The researchers believe that this could be the root cause of conditions such as osteoporosis.

Citrate, however, appears to function as both a shock-absorbing material and a way to keep calcium phosphate crystals from fusing together. When mixed with water, citrate enters the bone and bonds to the crystals, forming a mesh of tightly fastened yet flexible material. Researchers described this material as similar to two panes of glass with a layer of water between them – individually solid, but able to move and slide in response to various external pressures.

It is the presence of this citrate-mesh that the researchers believe makes human bone able to withstand such pressure. Without it, bones may be at a higher risk of fractures.

"This nano-scopic layering of citrate fluid and mineral crystals in bone means that the crystals stay in flat, plate-like shapes that have the facility to slide with respect to each other," Melinda Duer, Ph.D., lead author of the study, said in a statement. "Without citrate, all crystals in bone mineral would collapse together, become one big crystal and shatter. It's this layered structure that's been missing from our knowledge, and we can now see that without it you're stuffed."

Keeping citrate around
Citrate does not necessarily last forever, though. Healthy bone includes a fine protein-mesh punctuated with microscopic holes. This net keeps citrate confined within the internal structures of the bone so it can keep bonding to calcium phosphate. However, as the body ages or bone suffers repeated heavy use and trauma, this protein-mesh breaks down, the holes enlarge and citrate leaks out of the bone.

Without lowered citrate levels, calcium phosphate is free to bond to itself. Duer and her colleagues believe that this may be the first step in identifying a key component in the onset of osteoporosis in older people.

According to orthopedic documentation from the American Academy of Orthopaedic Surgeons, 10 million people have osteoporosis, with another 18 million at risk. Every year, 1.5 million bone fractures are thought to be caused by complications associated with osteoporosis, and the accompanying medical costs are thought to exceed $17 billion, according to figures gathered by the AAOS in 2005.

With Duer's findings on the chemical and biological significance of citrate, scientists may be one step closer to a cure for osteoporosis.