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).
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