(CBS) Most people are familiar with the bone-weakening effects of osteoporosis, but very few know that women can begin to lose bone density in their 20s.

This pre-cursor to osteoporosis is called osteopenia, and a report issued Thursday by the U.S. Surgeon General's office warned women about it.

Osteopenia is sometimes called the "young women's silent epidemic," because many young women fail to realize they are susceptible to bone loss.

Rheumatologist Dr. Harris McIlwain told The Early Show co-anchor Hannah Storm that 20 percent of college-aged women have osteopenia, and probably half the women over 45 have it.

What are the biggest risk factors?

"First," says McIlwain, who's co-authored a book called "Reversing Osteopenia," "if your mother or grandmother has osteoporosis or fractures or stooped-over posture. If your weight is less than 127 pounds. If you haven't been very active. If you smoke cigarettes. If you're nearing the time of menopause (or are experiencing early menopause, or have low estrogen levels, or have had a hysterectomy with removal of the ovaries). These and other ... factors increase your risk tremendously."

What does weight have to do with it?

"Weight has to do with the amount of fat in the body and as the weight gets lower for your height, it increases the risk of bone loss."

Dieting can also pose a problem. Many young women "diet hard," according to McIlwain, and "when you diet, you tend to lose those parts of the diet that are dairy, that have calcium, and you have less protein and may lose a lot of bone during that time," Mcllwain says.

There is a bone density test available. Who should take it?

"It's so simple and painless," McIlwain notes. "Anyone who has the risk factors that we spoke of. If you had a fracture as an adult, it's a big deal. If you're nearing the time of menopause, or have any of the risk factors we talked about, go ahead and have the test and you get the answer."

He recommends 1300 to 1500 milligrams a day in calcium supplements as well.

Also helpful, he says, are two types of exercise: "One is weight-bearing, which makes the bones get stronger, and back exercises, because if the back muscles get stronger, the bones get harder.

"Medications may be added. Your doctor can tell you. In fact, it was shown to decrease the risk of fractures by 75 percent."