For children, these reactions should not be in balance. In a child, the anabolic reactions have to be greater than the catabolic. Boys start their growth spurt after girls. That’s why when they are in 7th or 8th grade, the girls are still taller than the boys. The boys change a couple years after the girls. Guys may gain 2-3 inches in that growth spurt between 14-17 years of age. During that growing period they will eat up all the food in your refrigerator. They are growing like crazy during that time. But what happens to both boys and girls at 18-19 is that if they keep eating the same way, they won’t grow taller anymore but only wider. This is the phenomena everybody notices.
The gains made by athletes in uncontrolled observations have been much more impressive. Weight gains of thirty or forty pounds, coupled with thirty percent increases in strength, are not unusual. Such case studies lack credibility because of the absence of scientific controls. However, it would be foolish to completely disregard such observations because the "subjects" have been highly trained and motivated see the articles on pharmacology of sport and sports medicine in the countries of the former Soviet Union for more information on anabolic steroids.
Parathyroid hormone (PTH) is essential for the maintenance of calcium homeostasis through, in part, its actions to regulate bone remodeling. While PTH stimulates both bone formation and bone resorption, the duration and periodicity of exposure to PTH governs the net effect on bone mass, that is whether it is catabolic or anabolic. PTH receptor signaling in osteoblasts and osteocytes can increase the RANKL/OPG ratio, increasing both osteoclast recruitment and osteoclast activity, and thereby stimulating bone resorption. In contrast, PTH-induced bone formation is explained, at least in part, by its ability to downregulate SOST/sclerostin expression in osteocytes, permitting the anabolic Wnt signaling pathway to proceed. The two modes of administration of PTH, that is, continuous vs. intermittent, can regulate, in bone cells, different sets of genes; alternatively, the same sets of genes exposed to PTH in sustained vs. transient way, will favor bone resorption or bone formation, respectively. This article reviews the effects of PTH on bone cells that lead to these dual catabolic and anabolic actions on the skeleton.