"Osteo" means bone and "pena" indicates a state of being low in quantity. The term "osteopenia" refers to a bone density which is somewhat less, but not excessively less, than a "standard" young person of the same sex. If your bone density measurement indicates that your bone density is between 1.0 and 2.49 standard deviations below the "standard" young person, then you are said to have a bone density in the "osteopenic range." You are said to have osteopenia.

Osteopenia, however, is not a disease or even a true diagnosis. It merely indicates a state of relative low-side bone mass. You could have "osteopenia" because you never developed a high peak bone mass in your youth. It does not have to mean that you are currently losing bone. You simply could have a low-side bone density.

On the other hand, some of us with "osteopenia" are currently undergoing bone loss and on our way to having a higher degree of bone loss, known as "osteoporosis."

Recent surveys suggest that a large percentage of individuals in the US have a low-side bone density and are classified as having "osteopenia." According to the National Osteoporosis Foundation some 21.8 million American women and another 11.8 million men have osteopenia.

While low bone density is one of the risk factors for osteoporotic fracture, having osteopenia does not predict future fracture. In fact various studies document that nearly half of those who fracture do not have excessively low bone mass (osteoporosis), much less moderately low bone mass (osteopenia).

So what does it mean that your bone density is 1.0 standard deviation (SD) below that of a young person? One way to look at this is to realize that for most bone density tests -1 SD equals a 10–12 percent decrease in bone density. Another perspective is that you probably have a bone density that is lower than 84% of the young people of your same sex. A final way to look at it is to say that in a young, healthy population, the statistics being used determine that 16% of all young women will have a T-score that is less than -1, and thus have "osteopenia."

Remember,  if you are not young you will most likely not have the bone density of a young person. Also know that there is a great deal of controversy regarding how the "ideal young person reference bone mineral density" is established. Currently each manufacturer of each bone density measurement machine decides on its own "ideal young person bone density reference range."

Often studies using locally developed reference ranges come up with very different results than those using the manufacturers' reference range. For example, in the U.S. NHANES 111 the bone mineral density of a diverse sample of young women was used as the reference range, and this cut the prevalence of osteoporosis, as defined by bone mineral density, by more than half. If the manufacturers' reference range had been used in this study, the prevalence of osteoporosis of the hip would have been 49%, rather than the 28% they reported. For more information on the controversy surrounding ideal bone mineral reference ranges see Gillian Sanson's book, The Myth of Osteoporosis.