Cyclists are no strangers to breaks and fractures, but Andrew Coggan could be forgiven for not expecting a hip fracture from a bike crash at age 30. He may have been less surprised than most of his peers though, having recently been diagnosed with low bone density.
For many cyclists, an injury like Coggan's is the first sign that bones are not as strong as they should be. Although cyclists are known for staying on top of their training heart rate zones and pedal cadence, increasing research suggests they should also pay attention to their risk of thinning bones.
"Sometimes athletes in their late 20s and early 30s will come in for a femur or a hip fracture, and they'll be surprised because the fall was really not that bad," says Dr. Max Testa, a sports medicine physician at the Orthopedic Specialty Hospital in Salt Lake City who routinely treats elite cyclists. "But we'll look at the X-rays and see that there is some osteopenia [lower-than-normal bone density] there."
Many factors contribute to osteopenia or osteoporosis (very low bone mineral density) in cyclists, but one of the culprits is the nature of the exercise itself. Cycling is a low-impact sport that puts little mechanical load on the bones. That's great if you have joint problems, but it's the weight-bearing nature of exercise that signals bones to create more mass. Without such stress, bones don't get stronger, making them more prone to injury.
Avid cyclists, both amateur and professional, seem to be especially at risk of bone injuries if they don't do any type of cross-training. (Swimmers may also be in danger, since that sport requires little mechanical loading as well.) The lower spine is a particularly susceptible area, since it gets almost no loading. The hips may get some from the action of pedaling.
Coggan, now a senior scientist and exercise physiologist at Washington University in St. Louis, had been cycling one to two hours a day for about 15 years when he crashed in 1989. "And I recall prior to that," he says, "when I'd be chatting with a group of cyclists, I'd be taking note of the fact that everybody had scars from things like broken arms and broken collarbones."
A recent study in the journal Medicine & Science in Sports & Exercise found that competitive male road cyclists had significantly lower bone mineral density in their spines than a control group of men who were moderately physically active while doing other recreational activities. They were also more likely to have osteopenia and osteoporosis than those in the control group, despite the fact that the cyclists had a greater calcium intake.
Another study, published in the journal Bone in 2002, found that male road cyclists had lower bone mineral density than male mountain bikers after adjusting for body weight and controlling for age. The difference there could be that mountain biking, with its bumps and jumps, perhaps provides more impact and stimulation for bone growth than does road cycling.
Young cyclists aren't immune.
"You don't achieve peak bone mass until your late 20s," says Debra Bemben, co-author of the more recent study and an associate professor in the health and exercise science department at the University of Oklahoma. "If cyclists are in their early 20s and they're not doing anything else for exercise that's going to load their spine and help them achieve peak bone mass, it may put them at risk if they fall, since they'll have a greater chance of fracture. This is a pretty important health issue."
Further, some hard-core cyclists may not be eating enough to offset what they burn when exercising, depriving their bodies of bone-strengthening nutrients such as calcium and vitamin D. Especially at risk are women who have disordered eating, menstrual disruptions and bone loss -- known as the "female athlete triad."
"If there's a deficit in the energy balance," Bemben says, "then the body is not able to build things up, like bone."
That caloric shortfall could also trigger other physiological problems, such as hormone imbalances. For women this could mean lower estrogen levels; for men, lower testosterone levels. Bemben says estrogen and testosterone have protective effects on bones, slowing the rate of bone breakdown.
But hormones aren't only affected by calories. "If people overexercise, that can suppress testosterone in men, as it can suppress estrogen in women," says Dr. Aurelia Nattiv, director of the Santa Monica-UCLA Osteoporosis Center, although studies have not always borne out the low testosterone-low bone density connection.
"Too much of a good thing can cause negative effects on bone. We do see that sometimes not only do women lose their periods and have low levels of estrogen, but elite male runners can have low testosterone. So adequate hormonal balance is important." She adds that a family history of osteoporosis can contribute as well.