Low bone density (sometimes called osteopenia) refers to bone density that is lower than the normal peak density but not low enough to be classified as osteoporosis. Bone density is a measurement that shows how dense and strong the bones are.

Bones naturally become thinner as people grow older, because beginning at about 30 years of age existing bone cells are reabsorbed by the body faster than new bone is made. As this occurs, the bones lose minerals, heaviness (mass), and structure, making them weaker and increasing their risk of breaking. Both women and men are more likely to develop low bone density if they fail to reach their peak bone density by age 30. Women are more likely to develop low bone density than men. This is because women have a lower peak density and because the loss of bone mass speeds up as hormonal changes take place at the time of menopause.

Preventing and treating low bone density involves regular exercise, a nutritious diet with plenty of calcium and vitamin D, and lifestyle changes such as quitting smoking and avoiding excessive alcohol.