Julie Grimm was training for a marathon last April when she started feeling a twinge in her hip.

She assumed that some moments in her training would be painful, so she ignored it and kept running. By the end of that 16-mile run, she couldn’t even walk. She had to be carried home and stayed in bed all weekend.

Grimm had broken her femur at the hip joint. The break required emergency surgery and a metal plate, bolt and two pins to piece the bone back together.

Grimm was devastated. After all, she had competed in two triathlons and weight-trained for many years. She was thin and in good shape.

Her surgeon agreed: “This doesn’t happen to 34-year-old women who are fit,” he said.

Six weeks after the surgery, he sent her for a bone density scan. The scan is a painless type of X-ray that checks a part of the body, such as the hip, for loss of minerals or thinning bones. It turned out Grimm’s bones were weaker than normal. She was diagnosed with osteopenia, a precursor to osteoporosis, if left untreated.

“Osteoporosis never crossed my mind,” Grimm, now 35, said. “I have old-lady insides and young-lady outsides.”

Porous bones

Osteoporosis is a weakening of the bones due to loss of minerals that make up bone mass, or density. The more porous bones become, the more likely they are to fracture.

For years, the face of the disease was considered a hunched-over, tiny old woman. That is no longer the case, according to a national expert in the field. While the disease still primarily affects older, post-menopausal women, cases such as Grimm’s are becoming increasingly common, according to Dr. Kathryn Diemer, director of the Bone Health Program at Washington University’s School of Medicine in St. Louis.

“My clinic used to be little old ladies, but now I’m seeing a lot of younger women, men and young men,” she said.

In her typical clinic routine, Diemer sees 20 patients a day. Several years ago, a younger person with low bone mass might turn up once in a month. Now, she sees at least one or two young women in each clinic session. She has treated a 21-year-old with multiple fractures due to low bone mass.

Why now?

There may be several reasons for the increasing diagnosis of osteoporosis, Diemer said:

There is greater awareness of women’s health issues, and people can get their bones scanned at health fairs or at their doctor’s office.

Younger girls are not drinking as much milk as they should and are not as physically active as they should be.

Very athletic girls may stop menstruating, which can also affect bone mass.

Many women do not realize that a prenatal vitamin during pregnancy usually does not provide enough calcium to maintain strong bones.

Grimm, a stay-at-home mother who lives in Glendale, had nursed each of her three children for a year, which also depletes a woman’s calcium reserves. She had no idea that her prenatal vitamin wasn’t providing enough calcium.

“Because girls are not developing peak bone mass by 30, they hit the fracture threshold sooner,” Diemer said.

In older, post-menopausal women, the controversy surrounding hormone replacement therapy made many discontinue using it. But the lack of estrogen undermines bone tissue, and the difference needs to be made up with nutrition and exercise, Diemer said.

More than 40 percent of post-menopausal women are at risk for developing osteoporosis and up to 50 percent are at risk for a preventable fracture. The National Osteoporosis Foundation Web site (www.nof.org) says there are 10 million people in the United States who already have the disease and 18 million with low bone mass.

Osteoporosis causes more than 1.5 million fractures a year. And a recent study found that an average of 24 percent of hip-fracture patients, age 50 and over, die within a year of the fracture. The disease is a major cause of disability and frailty in older people.

Who’s at risk

Certain risk factors make a person more susceptible to low bone mass. Those factors include:

A family history of fractures or osteoporosis.

A thin frame.

Smoking.

Heavy drinking.

Certain medications, such as steroids.

Irregular periods, or loss of periods.

Anorexia.

In Grimm’s case, despite her thin frame and pregnancy and nursing, she had no symptoms before she fractured her hip. Now, she takes extra calcium supplements every day and stays committed to her weight-training program.

“Everyone sees me as so fit and healthy, but my bones aren’t,” she said.

By taking care of the problem now, she says, she will make sure that by the time she’s 50, she doesn’t have the body of an 80-year-old.