Calcium is a critical component of bone, whereby it comes together with phosphate to mineralize bone and hence give it strength. However, calcium plays an even more important role in normal cellular function throughout the body, such that blood levels are maintained within a very narrow range.
In this respect, bone becomes a very useful reservoir of calcium when dietary intake is low. That is, when insufficient calcium is consumed in our diet the body will remove calcium from bone. Calcium absorption from the intestine is also a regulated process that is primarily dependent upon vitamin D.
Vitamin D is produced on our skin under the action of UV light, derived from the sun, which is then converted to the active form by the kidneys and to a lesser degree the intestines. Consequently when vitamin D is deficient, calcium is not optimally absorbed and the process described above ensues.
In our current social environment, which recommends avoiding excessive sun exposure to protect against skin cancer (which is most appropriate), the rate of vitamin D deficiency has significantly increased. Vitamin D also communicates closely with the most important organ that is responsible for controlling the level of Calcium in our blood, the parathyroid gland.
The parathyroid gland, as the name suggests, resides adjacent to the thyroid gland in the neck. In fact there are, most commonly, four separate glands present (two on each side). This endocrine organ monitors the level of calcium circulating in the blood and responds according to the level. For example, if the calcium level is low then the parathyroid releases a hormone, creatively called, the parathyroid hormone, which removes calcium from bone, decreases calcium loss in urine, and increase calcium absorption in the gut. Occasionally a parathyroid gland can begin to function of its own accord, independent of the feedback it receives about the calcium level, which is termed hyperparathyroidism. This can then result in excessive removal of calcium from bone causing bone fragility.
The kidney is another organ that has an important role in bone health, as alluded to in the discussion above about vitamin D and the function of the parathyroid gland. Consequently, kidney disease can also predispose to abnormal bone, which has been termed renal osteodystrophy. This condition, as in osteoporosis, causes a weakness in bone predisposing to fracture.
The thyroid gland is also not exempt from relating to bone health. Again, creatively, the thyroid gland produces thyroid hormones. These hormones have an important role in facilitating a number of bodily functions, including bone health. Consequently diseases of the thyroid, which are relatively common, represent another risk factor for the development of bone disease, especially when overactive (a state termed hyperthyroidism).