A bone mineral density (BMD) test measures the density of minerals (such as calcium) in your bones using a special X-ray. This information is used to estimate the strength of your bones. We all lose some bone mass as we age. Bones naturally become thinner (called osteopenia) as you grow older, because existing bone is broken down faster than new bone is made. As this occurs, our bones lose calcium and other minerals and become lighter, less dense, and more porous. This makes the bones weaker and increases the chance that they might break (fracture).