Cycling, Bone Strength, and Vitamin D

Winter training in the PNW isn’t anywhere close to winter training in the sunbelt, so pay careful attention to Dr. Gabe Mirkin’s recommendations for vitamin D supplementation for cyclists in the following article posted today:

Dr. Gabe Mirkin’s Fitness and Health E-Zine
November 1, 2009

No Evidence Cycling Weakens Bones

No data exists in the scientific literature showing that
any type of exercise weakens bones. Bone growth depends on the
forces exerted on them by gravity and contracting muscles. So any
activity or exercise that causes you to contract your muscles will
strengthen bones (Medicine & Science in Sports & Exercise,
November 2009).

Previous studies showed that world class cyclists had
reduced bone densities in their spines. However, 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 recent review of 12 blinded, 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.

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