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.

Fasting Does Not Increase Endurance

Some people think (incorrectly) that fasting before a race or competition will increase their endurance. Fasting weakens and tires you. How long you can exercise a muscle depends on how much sugar, called glycogen, you can store in that muscle and how long you can keep it there. When a muscle runs out of its stored glycogen, it slows down because it requires more oxygen to burn more fat. It also accumulates more lactic acid to become acidic which causes a burning pain, and it becomes more difficult to coordinate.

Every time you move a muscle, some of the stored glycogen is used up. Every time you eat, some of the food can be stored as glycogen in that muscle. When you go for more than an hour without eating, you use up glycogen without replacing it. If you don't eat before you compete, you start with reduced stores of glycogen in your muscles and you will not be able to compete at your best.

It is nonsensical to claim that fasting increases endurance by causing muscles to burn more fat and less glycogen so muscles can retain their stored glycogen longer. When you start with less glycogen, you still use it up faster and run out of fuel earlier. You can increase endurance by cutting back on exercise four days before your competition and eating as much or more than usual. Eat one to three hours before competing. If your event lasts more than an hour, take fluid, sugar and protein (whole grain bars, sugared drinks, etc.) during your event.