In 2006, Aaron Smathers, then 29, was a graduate student in the Department of Health and Exercise Science at the University of Oklahoma, gathering data for a study of brittle bones in cyclists. One of his subjects was himself, since he’s been a bike racer for years. A recent scan had revealed that his bones were less dense than usual for a man his age. Not long after those results came in, he crashed during a race, snapping his collarbone. Six weeks later, in his first post-injury race, he was engulfed by a multi-rider pile-up, crashed again, and re-broke his collarbone. Worse, he fractured his hip so badly that the ball of the ball-and-socket joint broke off. “Later I thought, well, this reinforces my study,” he says.

Is cycling bad for the bones? A number of intriguing studies published in the past 18 months, including Smathers’, have raised that possibility — an issue that has special resonance now, with this weekend’s start of the 2009 Tour de France. Certainly, the toll of broken bones among top-level racers is high. Famously, Lance Armstrong broke his collarbone this year, while Christian Vande Velde, another of America’s premier Tour hopes, fractured six bones, including three in his spine, during a crash at the Giro d’Italia in May.

Of course, slamming into the pavement at 40 miles per hour can be expected to break anyone’s bones. But Smathers’ research suggests that other factors may be at work as well. “If you have low bone mineral mass, you can wind up with a much more serious break from a crash” than if your bones are thicker, he points out.

In his study, the bone density of 32 male, competitive bike riders, most in their late 20s and early 30s, was compared to that of age-matched controls, men who were active but not competitive athletes. Bone scans showed that almost all of the cyclists had significantly less bone density in the spine than the control group. Some of the racers, young men in their 20s, had osteopenia in their spines, a medical condition only one step below full-blown osteoporosis. “To find guys in their twenties with osteopenia was surprising and pretty disturbing,” Smathers says.

Another recent study, this one published last year, had similar results. It followed competitive cyclists over the course of a race season in Colorado. The riders, aged 27 to 44, began with slightly below-average bone density. By the conclusion of the race season, they had lost a significant portion of their total, already-low bone mass in their hips, though not in their spines. At a three-month follow-up exam, however, they showed a small amount of bone recovery in the hips.

Other broader studies suggest that cycling is unique in its impact on skeletal health. Other endurance sports don’t seem to hurt bones in the same way and are typically beneficial. A study published in March, for instance, that compared the bone densities of weight lifters, runners, and cyclists found that the cyclists had lower bone mineral density than the runners or the weight lifters. In another study, triathletes added moderately to their bone mass over the course of a season.

Cycling, unlike running or weight-lifting, causes little impact to skeletons. Bones react to external stresses by strengthening bone. Smathers and other researchers aren’t sure why cyclists have lower bone density. From their studies they have found that body mass is a central factor. In Smathers’ study, the lightest riders had the lowest bone density. In addition, many serious riders burn more calories in a day than they consume, an energy imbalance that is being studied to determine its impact on bone loss. And sweat could play a role. A rider can lose hundreds of milligrams of calcium an hour through sweat. Although the riders in both Smathers’ and the Colorado study were ingesting more than the recommended daily allowance of calcium for their age, they may still have had a deficit of the mineral, which is essential to bone-building. Some researchers theorize that calcium must be taken during exercise to be most effective. A 2004 laboratory study of cyclists who were given either tap water or calcium-enriched water during a 50-minute, stationary-bicycle ride found that the riders drinking the tap water had much higher levels of blood chemicals related to bone loss than did the riders swigging the calcium. Researchers suspect that drinking calcium-enriched waters or sports drinks during long, hot bike rides may help to stave off some bone loss.

Even more encouraging, most recreational cyclists probably don’t need to worry too much about their bones. “The studies to date have looked primarily at racers,” Smathers says. “That’s a very specialized demographic. These guys train for hours at a very high intensity. They sweat a lot. They never go for runs. They don’t usually do much weight-lifting,” to avoid adding bulk. “They’re strange.” He knows. “For competitive riders, I’d recommend spending some time weight-training.” If you do race or train hard and often on a bike, consider a bone scan, he says. “It’s good to know your status.” For himself, his racing career ended with hip surgery and four metal pins in the joint after his second severe crash. “I do miss racing,” he says. On the plus side, his latest bone scan, completed just weeks ago, shows that his bone density, while still low, is increasing.