Some of you may have recently read that you need to include impact exercise to prevent bone loss. I read that story too. But Dr. Mirkin came out with the following article refuting the earlier story. See below:

73 Art W2NRA

Dr. Gabe Mirkin's Fitness and Health E-Zine
Cycling Does Not Cause Bone Loss
September 12, 2010

During a six-day bicycle race, the bones of world class bicycle racers become stronger (Physiologie Appliquée, Nutrition et Métabolisme, June 2010). Bones are constantly changing. Certain cells called osteoblasts take calcium into bones to make them stronger, while other cells called osteoclasts take calcium out of bones to weaken them. During the race, hormones produced by osteoblasts to strengthen bones increased (osteocalcin increased by 300 percent, and C-terminal telopeptide of type I collagen increased by 43 percent).

The theory that cycling weakens bones flies in the face of our current understanding of bone metabolism. Any force on bones increases, and lack of force decreases, the rate of bone formation (Medicine & Science in Sports & Exercise, November 2009). 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.

I cannot find any studies showing that cycling weakens bones to increase fracture risk. Some studies show that competitive cyclists have lower bone mineral density in their spines than moderately-active, aged-matched men (Medicine & Science in Sports & 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. Bone density tests do not measure bones strength. They measure how much bones block X-rays that try to pass through them. The only way to measure bone strength is to see how much force it takes to break a bone.

The most likely explanations for broken bones in cyclists are high-impact crashes and/or lack of vitamin D. I recommend that all cyclists get a blood test called Vitamin D3 in December or January. If it is below 75 nmol/L, they are deficient in vitamin D and at increased risk for breaking bones. To prevent fractures, they should do winter training in the southern sunbelt or take at least 800 IU of Vitamin D3 per day.

A review of 12 controlled scientific studies showed that oral vitamin D reduced non-vertebral and hip fractures in patients over 65 years of age (Evidence-Based Medicine, October 2009). Blood levels of vitamin D below 75 nmol/L cause parathyroid hormone levels to rise too high, which causes osteoporosis. A main function of vitamin D is to increase calcium absorption from the intestines into the bloodstream. When blood levels of vitamin D fall below 75 nmol/L, levels of ionizable calcium drop. This causes the parathyroid gland to produce large amounts of its hormone. Higher than normal blood parathyroid hormone levels take calcium out of bones to cause osteoporosis.

A woman's bones are strongest when she is twenty years old. After that, she continues to lose bone for the rest of her life, and for the first few years of menopause, the rate that she loses bones more than triples. A study from the University of Erlangen in Germany shows that vigorous exercise during the menopause helps prevent osteoporosis (Archives of Internal Medicine, May 2004). In this study, fifty women lifted weights in group training sessions twice a week, and exercised by themselves twice a week. They also took calcium and vitamin D. As their muscles became stronger, so did their bones.

Sprint cyclists, and to a lesser extent distance cyclists, have greater tibia and radius bone strength than 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).