Researchers report several factors which contribute to bone loss among serious cyclists. These include:
(1) Cycling is not weight-bearing and yields comparatively low skeletal strain (the skeletal strain of exercise encourages bone formation).
(2) Endurance cyclists do a great deal of this non-weight-bearing exercise, averaging over 13 hours per week, and perhaps do this activity instead of other exercise which might be weight-bearing and bone-building.
(3) Cyclists experienced an increase in parathyroid hormone, likely subsequent to excessive loss of calcium through the skin with sweating. Excess parathyroid hormone tends to increase bone breakdown.
(4) Cyclists likely did not consume enough calories for their heavy training. Also, I would add that they likely did not consume enough of the 20 key bone-building nutrients.
(5) The physiological stress of such training produces bone-damaging stress hormones and pro-inflammatory cytokines.
The Better Bones perspective on this research finding would include, but also go beyond, the above five proposed causal factors. As we see it, intense physical activity places various stresses on the body resulting in increased oxidative damage, increased bone-depleting low grade metabolic acidosis, and increased losses of many nutrients in the sweat. All these factors suggest the need for a higher level of not only calcium, but of all the 20 key bone nutrients. Further, ample antioxidants should be consumed by endurance athletes. These nutrients should be taken, perhaps in liquid form, just before or during the exercise itself. In addition, special attention should be given to reducing any exercise-induced metabolic acidosis with the Alkaline for Life® Diet and the use of alkalizing mineral compounds as necessary.
Reference:
Barry, DW, and Kohrt, WM. 2008. BMD decreases over the course of a year in competitive male cyclists. J Bone Miner Res, 23(4):484-491.