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What Happens If the Body Is Low on Vitamin D?

Vitamin D plays a crucial role in many of the body’s systems, including the skeletal, immune and cardiovascular systems. However, since the chief source of vitamin D is sunlight, you may be at risk for low vitamin D levels if you don't have enough sun exposure and don't take any supplements. Low levels of vitamin D are strongly linked to bone disorders, and they may play a role in immune system dysfunction and heart disease.

Forms of Vitamin D

Your body has two forms of vitamin D. Vitamin D-3, which may be the more absorbable form, is present in your skin in an inactive form. It becomes activated when ultraviolet rays from the sun hit your skin. Vitamin D-3 is also in foods such as fortified milk. Vitamin D-2 comes mainly from vegetables. Vitamin D is a fat-soluble vitamin, meaning that it is stored in your fat tissue and can be recruited when your body needs it. Therefore, if you have extremely low levels of body fat or have impaired fat absorption, your ability to absorb vitamin D may be compromised.

Bone Disorders

The most well-known role of vitamin D is its regulation of calcium in your bones; without vitamin D, your bones could not use calcium efficiently. When blood serum calcium levels become too low, the parathyroid glands produce parathyroid hormone, which stimulates an enzyme in the kidney to activate the stored vitamin D in the body. This vitamin D increases calcium absorption from the diet, reduces calcium loss through the kidneys, and mobilizes calcium from the bones to maintain blood calcium levels. Prolonged vitamin D deficiency can lead to softened bones, or osteomalacia, due to lower calcium uptake into your bones. The lowered bone calcium also reduces bone mineral density, leading to osteoporosis.

Immune System Dysfunction

Most of the cells in your immune system have proteins that bind specifically to vitamin D. When your immune cells encounter a foreign pathogen, your body activates a series of signals to execute a response. These signals increase the number of vitamin D-specific proteins in the immune cells, and the vitamin D binding to immune cells stimulates immune cell development, thereby strengthening the immune system. Low vitamin D may reduce this coordinated response and may even lead to autoimmune disorders in which the immune system inappropriately attacks its own tissues.

Heart Disease

A main contributor to heart diseases such as heart attacks, strokes, and heart failure is atherosclerosis, or plaque buildup in the arteries. This plaque includes calcium and calcified tissue. Low levels of vitamin D reduce the body’s ability to absorb calcium efficiently, leaving excess calcium in the bloodstream. This calcium may be more likely to form an atherosclerotic plaque. A 2013 review, published in the "American Journal of Physiology" notes that the vitamin D receptor -- the protein that allows your cells to respond to vitamin D -- might play a role in hindering the development of atherosclerosis. However, another review, published in the October 2013 issue of the "International Journal of Clinical Practice," notes that more research is needed to better understand vitamin D's role in heart health.

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This article reflects the views of the writer and does not necessarily reflect the views of Jillian Michaels or JillianMichaels.com.