Bone growth and shrinkage
As children grow, so do their bones, as new bone tissue is placed on top of existing bones, particularly in the legs and backbones. This process of lengthening bones is called modeling. During the teenage years, sex hormone (estrogen and testosterone) levels rise, helping to strengthen bones, which become thickest in early adulthood.
In adults, an activity called bone remodeling is what chiefly affects bone strength and density. Bone remodeling repairs microdamage within the skeleton. It also helps to release calcium stored in bone so that the amount of calcium in the blood remains within an ideal range.
Remodeling can be triggered by microdamage on bones that are heavily used or that have accumulated stress from wear and tear. Insufficient calcium in the diet forces the body to absorb calcium from bone. Prolonged poor dietary intake of calcium causes the body to leech calcium from bones, and this leaves them thinner and weak.
When the body does not obtain enough calcium from food the parathyroid glands (located in the neck) release higher-than-normal levels of parathyroid hormone (PTH). Prolonged elevated levels of PTH cause the body to absorb calcium from the skeleton and the kidneys to resorb calcium from the urine. PTH also helps the body convert vitamin D2 to its active form, vitamin D3. All of these steps increase the amount of calcium absorbed from the intestine and available for use by tissues. However, no amount of PTH or vitamin D is going to make up for missing calcium over the long term.
Hit by hormones
Remodeling of bone is also affected by many hormones or hormone-like compounds, including the following:
- vitamin D
- parathyroid hormone
- tumour necrosis factor (TNF)
In HIV infection, the immune system undergoes excessive activation and inflammation and produces many chemical signals or cytokines (interleukins, TNF and so on). Studies in men at high risk for HIV or who have recently become HIV positive have found unexpectedly high rates of osteopenia and osteoporosis compared to HIV-negative men of similar age.
HIV infection appears to be associated with premature menopause in some women.
All of this information suggests that HIV-positive people are at increased risk for thinning bones.
Although many studies have reported small changes in bone density in HIV-positive people, surprisingly, such small changes can have a big impact on the ability of bones to carry the body’s weight. For instance, among HIV-negative people small changes in bone density affects the architecture of bones, making them more porous and weaker.
— Sean R. Hosein
- Lindsay R, Cosman F. Chapter 354. Osteoporosis. In: Longo DL, Fauci AS, Kasper DL, Hauser SL, Jameson JL, Loscalzo J, eds. Harrison’s Principles of Internal Medicine. 18th ed. New York: McGraw-Hill; 2012.
- Ofotokun I, McIntosh E, Weitzmann MN. HIV: inflammation and bone. Current HIV/AIDS Reports. 2012 Mar;9(1):16-25.
- Cotter AG, Powderly WG. Endocrine complications of human immunodeficiency virus infection: hypogonadism, bone disease and tenofovir-related toxicity. Best Practice & Research. Clinical Endocrinology & Metabolism. 2011 Jun;25(3):501-15.