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.
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Youthful risks
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.
Even perspiration can be a factor, Bemben says: "Cyclists may lose a lot of calcium in their sweat," she says. "Even if they're taking in amounts [of liquid] that are seemingly high for the average man, that might not be enough to balance what they're excreting."
