A surprising new study reports that in men, the skeletal system is involved in testosterone production.
Osteocalcin
It turns out that osteoblasts, bone cells that generate new bone tissue, also produce a hormone called osteocalcin. And osteocalcin, the researchers found, seems to promote testosterone production. The surface of testosterone-producing Leydig cells of the testicle have osteocalcin receptors, and testosterone production in these cells seems to be upregulated or enhanced by the presence of osteocalcin (see diagram).
When scientists injected osteocalcin into mice, testosterone levels increased accordingly. Mice with low levels of osteocalcin were found to have low levels of testosterone, low sperm counts, and poor rates of reproduction.
While it's been known for some time that sex hormones like estrogen and testosterone can affect bone growth, there wasn't any research on the influence of bone on sex hormone levels. Bones were thought to be the product of metabolism (growth, calcification, etc) rather than producers.
Bone hormone osteocalcin was discovered in 1975 (Hauschka Laboratory of Boston's Children's Hospital). Just a few years ago, in 2007, osteocalcin was found to increase insulin sensitivity and secretion, help to control glucose levels and bodyweight.
Protein, Bone Health and Testosterone
So how does protein fit into the osteocalcin-testosterone equation?
A high protein diet has been shown, in animals, to increase bone mineral content. Human studies have shown that a high protein diet improved recovery from hip fractures. Researchers explain that protein stimulates human growth factor, which in turn promotes bone growth. A 2004 study in the Journal of Endocrinology and Metabolism confirmed that a diet high in protein and low in carbohydrates increased bone growth factor IGF-1in both men and women.*
So what is good for the bones - like protein seems to be- may be good for men's testosterone levels too.
*Interestingly, although the lower estrogen levels women experience after menopause is associated with bone loss, researchers have not yet found evidence of the reverse. There is no documented effect at this time of osteocalcin on women's estrogen levels or fertility, although future research may prove otherwise. It would make sense, given that low estrogen-osteoporosis connection, but remains to be proven.
