“Sprint cyclists, and to a lesser extent distance cyclists, had greater tibia and radius bone strength surrogates than the controls, with tibial bone measures being well preserved with age in all groups.
This suggests that competition-based cycling and the associated training regimen is beneficial in preserving average or above-average bone strength surrogates into old age in men” (Medicine & Science in Sports & Exercise, March 2009).
Several readers responded, quoting other studies that showed competitive cyclists have lower bone mineral density in their spines than moderately-active, aged-matched men (Medicine and Science in Sports and Exercise, February 2009; Osteoporosis International Reports, August 2003). These studies have been interpreted to mean that cycling increases risk for bone fractures beyond what you would expect from just falling off the bike.
I cannot find any studies showing that cycling weakens bones to increase fracture risk. Bone density is associated with bone strength, but does not measure it. The only way to measure bone strength is to see how much force it takes at break a bone. For example, birds have strong bones that are not very dense.
The theory that the act of cycling weakens bones flies in the face of our current understanding of bone metabolism. If indeed cyclists suffer from weak bones (and I do not believe that they do), the cause would be something other than riding a bicycle.  Bones are constantly remodeling. Cells called osteoblasts bring in calcium to bones while cells called osteoclasts take calcium out. Any force on bones increases, and lack of force decreases, the rate of bone formation. Astronauts in space lose bone because lack of force blocks their ability to respond to Insulin Like Growth Factor-1 that stimulates bone growth (Journal of Bone and Mineral Research, March 2004).  All competitive cyclists know that hammering on the pedals while pulling up on their handle bars puts tremendous force on every muscle and bone in their bodies, and this should stimulate bone growth.
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Update (3/6/09)
I was reading the latest edition of RoadBikeRider.com Newsletter (Issue No. 383 – 03/05/09) where I found this dissenting view on Dr. Mirkin’s article.
ED’S NOTE: I was stunned late last week to read this Q&A in the widely circulated medical newsletter by Gabe Mirkin, M.D., a doctor in his 70s who does about 200 miles (320 km) a week on a tandem and in spirited group rides.
Q:Â Does bicycling reduce bone density, increasing risk for fractures and osteoporosis?
Dr. Mirkin: “This is a myth that is not supported by any good data. A study from Manchester Metropolitan University in the UK shows that sprint cyclists have denser bones than long distance cyclists, who have denser bones than sedentary control subjects (Medicine & Science in Sports & Exercise, March 2009).
“While cyclists have less dense bones than weight lifters and football players, they still have denser bones than people who do not exercise. The greater the force on bones during exercise, the denser the bone. So any type of exercise is good for your bones and a sedentary lifestyle is bad for bones.”
I can agree with some of that, but the first sentence is outrageous. One study that produces differing evidence does not make a “myth” out of all that’s come before.
In fact, a different issue of the journal cited by Dr. Mirkin reports a study that found competitive male road cyclists had significantly lower bone mineral density in their spines than a control group of men who were moderately physically active. Even though the cyclists had a greater calcium intake, they were still more likely to have osteopenia (lower-than-normal bone density) or osteoporosis (very low bone mineral density) than those in the control group.
A report about this by Newswise is found at http://tinyurl.com/cmj67x. It includes this interesting note, “A similar study in Bone illustrated that male road cyclists had lower bone mineral density than male mountain bikers after adjusting for body weight and age. The distinction could be found in that mountain biking, with its variable terrain, provides more impact for bone growth than road cycling or racing.”
The report adds, “When it comes to the risk of thinning bones, however, it’s the weight-bearing nature of exercise that signals bones to create more mass. Without such stress, bones do not get stronger, and become more prone to injury.”
Cycling isn’t very weight-bearing. I wish I could cite the source, but I recall reading that when spinning along at 90 rpm — typical cruising conditions — a cyclist exerts only about 14 pounds of force on the pedals, often less. Someone running, however, experiences 3 times body weight on each footfall.
Personally, my list of physical imperfections includes osteopenia, diagnosed in the early 1990s after road cycling had been my primary sport for most of the preceding dozen years. When RBR polled male subscribers a few months ago, 1 in 10 said they have been diagnosed with osteopenia or osteoporosis — the “old ladies’ disease.” Twenty-four percent said they were concerned enough about bone density to seek a DXA scan.
Diet, age and heredity also play roles. So it’s a complicated issue. But it’s not a myth that cyclists as a group have been found to suffer from low bone density. I sent that objection to Dr. Mirkin and asked him to respond.
Here’s what he wrote back: >>>SOS