Bad to the Bone

Elite cyclists participating in Pamela Hinton's bone density study thought they were perfect physical specimens. Her results made them think again.

by Natalie Fieleke

Each day the human skeleton, that nimble framework of mineralized protein that holds us together and propels us into motion, is broken down and put back together again. The process, a complex, two-stage cycle by which new bone is created as old bone is removed, is called remodeling. When it works as it should, this dance of creation and destruction achieves a rough equilibrium, and our bones maintain their strength and integrity. When destruction outpaces creation, however, bones become brittle and fractures more common.

Because this latter scenario has long been associated with aging, younger, active individuals -- or their physicians -- have seldom given bone problems a passing thought. This is changing, thanks to Pamela Hinton, an associate professor of nutritional sciences at MU, who co-authored something of a wake-up call in the February 2008 issue of the journal Clinical and Experimental Metabolism.

Hinton's study compared the bone densities of elite male runners to those of similarly accomplished cyclists. The bike riders were all relatively youthful, energetic men whose dedication to high-energy, low-impact exercise had, by all appearances, made them enviable physical specimens. Yet Hinton's findings told a different story, at least so far as subjects' bones were concerned. The cyclists, fit though they might be, consistently showed signs of osteopenia, a low bone mineral density condition that is a precursor to osteoporosis. It could double their risk of fracture.

"Unfortunately, some individuals who believe they are doing everything right in terms of their health might be surprised and upset by our finding," says Hinton. "On an individual level, I think the men in the study were surprised to learn they had osteopenia. They were somewhat disappointed and didn't know what to do about it. If anything, this study sends the message that osteoporosis doesn't only affect women. It's important for men to be aware that it's a disease they may have to someday deal with, in particular, if they don't get enough weight-bearing activity or are at risk for another reason."

Bone remodeling in its most basic form involves the work of two cell types, osteoclasts and osteoblasts. Osteoclasts are large, multi-nucleated cells that set in motion a process called "resorption," a secreting of enzymes and acids that dissolve bone mineral on a molecular level. As bone is dissolved, a second, smaller cell, called an osteoblast, simultaneously fills in what was lost with new bone tissue.

Around age 35, this process of remodeling can become imbalanced. By 50, the imbalance can lead to varying degrees of osteoporosis, a skeletal disease characterized by a microscopic breakdown of bone structure. Osteoporosis affects about 44 million U.S. women and men and leads to about 1.5 million fractures per year, according to the Washington, D.C.-based National Osteoporosis Foundation.

But what happens along the way? What occurs between the time people are at their healthy peak of bone mass and the time they are at greater risk for a fracture?

In May 2006, Hinton began her search for answers by designing a cross-sectional study comparing athletes who run but don't bike with those who bike but don't run. Hinton planned to bring the two groups into the lab and take a scientific snapshot of their bone turnover, specifically looking at markers of bone formation and breakdown, along with levels of the hormones affecting that process. The result would provide an accurate measure of how running or cycling differentially affected bones.

Along with her former doctoral student, Randy Scott Rector, and two nutritional science undergraduates, Robert Rogers and Meghan Ruebel, Hinton next set about finding participants. The researchers posted flyers on the MU campus and at various Columbia bicycle and sporting goods stores. They also advertised the study on the Web sites of local cycling and running clubs.

Hinton's connection to the cycling community helped, too. As an undergraduate at the University of Wisconsin-Madison, Hinton was a scholarship member of the cross-country and track teams, an All-American her senior year. When a car accident ended her distance running career, she took up cycling. She now competes in 60- to 80-mile road racing events.

Hours of pedaling gives a person time to think, a circumstance that eventually led Hinton to a key insight. "Being involved in cycling increased my awareness of this group of people who spend a lot of time essentially weightless," she says. This "weightlessness" of cycling, she notes, is the crucial distinguishing characteristic between the cyclists and runners, one with potentially profound consequences for the skeleton.

Hinton's study included only men, young and middle-aged athletic males, a group not generally believed to be at risk for bone loss. In a recent e-mail, Hinton's colleague Stephen Sayers, an assistant professor of physical therapy at MU and expert on physical performance and aging, put it this way: "Osteoporosis is often thought of as an 'elderly woman's disease.' ... Male athletes probably never considered that they would be at risk for this condition because they probably view their training as a panacea to all debilitating conditions."

As she sits balanced on an exercise ball in her McKee Gym office, Hinton explains the logic behind her curious study cohort: Of the 44 million Americans with osteoporosis, only about two million are men. Scientists believe this disparity occurs, at least in part, because women, whose bones are less dense to begin with, experience an abrupt loss of estrogen during menopause that speeds up bone turnover. Men's levels of testosterone, on the other hand, drop more gradually. But bone loss happens, and men need to be aware of it.

"For a while, it was thought that women didn't suffer from heart disease, and it took a while to change that belief," Hinton says. "And it seems like we're still in the process of changing the misconception about bone loss in men."

Connie Weaver, distinguished professor of foods and nutrition at Purdue University, couldn't agree more. "There haven't been many clinical studies on males, especially with any twist like this, when they're younger and active," says Weaver. "Her study won't allow you to determine the mechanism, but it was big enough to determine the phenomenon, so that's a lot of human interest."

The study is also important because it confirms that weight bearing exercise is beneficial to bone, and extends these findings to a new population, Weaver says. "Here you have healthy, mid-age males, and they've been less studied, so this adds to our understanding."

To be eligible for Hinton's investigation, athletes had to have been consistently active for at least six hours per week in either running or cycling, but not both, for the past two years. Potential participants with current or previous medical conditions were excluded, as were those who had implanted metal devices or who used medication impacting bone density. Applicants who smoked cigarettes were also left out, because research shows smoking reduces bone mineral density and increases fracture risk. Those who made the cut were, well, a cut above.

"Some of the cyclists may train upwards of 15 to 20 hours per week; it's not just someone riding down the trail," Hinton says.

Hinton and her team began work on the investigation by collecting data in four parts. They first weighed and measured the men to determine their body mass index. Then they collected blood samples to determine serum markers of bone turnover in the blood, indications of how much bone resorption and bone formation were taking place. They also measured levels of hormones important to bone, such as testosterone and estrogen. Next, Hinton and her team used "dual X-ray absorptiometry," or a DXA scan, to measure bone mineral content in various key areas: the lumbar section of the spine, hips, arms, legs and full bodies of participants.

The researchers asked the runners and cyclists to record their daily activities and nutritional intake. Finally, Hinton asked each participant to fill out a medical history so that researchers could control for the effects of past activity.

Hinton hypothesized that cyclists would have lower bone density than runners. Cycling has cardiovascular benefits and puts only a small amount of stress on the bones, but the fact that cyclists aren't physically hitting the pavement has long been thought detrimental to bone strength.

The reason why involves what scientists call "fluid shear stress." Bone cells are organized in a fluid-filled, lace-like network. Rapid movement of the fluid across the surface of the bone cells during movement -- fluid shear stress -- signals the production of new bone-forming cells. High-impact activities, such as running and jumping, produce a large amount of fluid shear stress, thus stimulating bone growth. Low-impact exercises have the opposite effect.

Each time the runner comes in contact with the ground, Hinton says, an "impact force" or "ground reaction force" is exerted on the bones of the hips, legs and feet. This stress peaks and fades with each stride, creating a rhythmic series of compression and relaxation. It is this rhythm that causes fluid flow across bone cells, thus stimulating them to adapt to the stress by adding new bone.

Barring disaster, a cyclist never comes in direct contact with the ground: The only forces exerted on the skeleton during cycling are those produced by contraction of the leg muscles during pedaling. Because the magnitude of these forces is much less than those of high-impact ground reactions, this no-impact form of exercise is much easier on joints. But over time the lack of strain and the resulting deficiency of fluid shear stress could contribute to bones' deterioration.

Still, not many of the athletes showed concern about their bones prior to the study, Hinton recalls. After all, they were all young and fit. "A few that had had fractures were a little more accepting of my hypothesis," Hinton recalls. "The others were like, 'Yeah, whatever, my bones are fine.'"

For these dismissive athletes in particular, Hinton's findings came as a shock. More than one-half of the cyclists, 63 percent, had osteopenia of the spine or hip as compared to 19 percent of the runners. Put another way, the cyclists were seven times more likely to have osteopenia of the spine than runners, despite similar height, weight and body composition between the two groups.

"To be honest, I didn't expect to find that many (cyclists) with osteopenia," Hinton said. "I would expect this in professional cyclists who are very thin and spend 30 hours a week on the bike, but I was surprised to see it in this level of athlete."

After completing the DXA scans, Hinton shared the results in a letter to participants. "The message was, this is not something that you should ignore," Hinton says. "You're at increased risk for low bone density now, and time is just going to magnify that effect, so it's probably a good idea for you to do something about it."

The letter got their attention, and many participants asked how they could counteract their newly discovered osteopenia. One of them was Nathan Means, 35, an assistant professor of biology at Columbia College and an avid cyclist. He said besides the fun, stress relief and cardiovascular benefits exercise provides, doing something good for the whole body is one of the main reasons he's active. Still, Means says, "You don't want to choose something that will have deleterious effects on your body. I was alarmed. I called Pam up and said 'What do I need to do for mitigation, what can I do to stop it?'"

Hinton told Means and the other at-risk athletes to modify their diet and exercise. Make sure your diet includes sufficient calcium and vitamin D, she advised. These nutritional elements can help stem future bone loss though they won't correct an already existing problem. For that, she said, you need weight-bearing activity on a regular basis.

More generally, Hinton says cyclists should follow the recommendations of the American College of Sports Medicine and the National Strength and Conditioning Coaches' Association. These professional organizations recommend that cyclists incorporate conventional strength training into their existing regimens. Cyclists might also think about incorporating plyometrics -- "explosive" jumping, bounding and hopping exercises -- into their daily workout schedules.

Research has shown, for example, that plyometrics is an effective way to increase bone mass in adolescent women, Hinton says. But she concedes there remains much to learn about how various forms of weight-bearing activity affect mens' bone mass.

"There have been studies in women, especially postmenopausal women, and kids," Hinton says. "But relatively little has been done on the effects of resistance training or plyometrics on bone density in men."

That will change with Hinton's next project, a year-long study to determine which activity, traditional strength training or plyometric exercise, is more beneficial for building bone among men with low bone density. This should allow researchers to make better fitness recommendations, she says.

Means isn't waiting. He has already added plyometric activity to his weekly exercise mix of cycling and yoga. And he now jumps rope about four times a week, for short bursts of time.

"Jumping rope is tough," Means says. "I thought I was in pretty good shape for a fellow my age. And then I started doing this."