Vitamin D is a fat soluble vitamin. It is one of the four vitamins in this group. The others are vitamin A, E and K. Fat soluble vitamins can be stored in the body for a prolonged time. While food sourced vitamin D is unlikely to cause toxicity due to accumulation in the body, vitamin D nutritional supplements if taken above the recommended dosages have the potential to cause the problem. This is the only vitamin that can be synthesized on the skin with the help of the sun’s ultraviolet light (UVL). For this reason it is also referred to as the sunshine vitamin.

Functions of vitamin D in the body

  • Regulation of gene expression. It is thought that the number of gene regulated by this vitamin can be as high as 1250
  • Supports calcium and phosphorus regulation for their incorporation in the bones
  • Has immune support properties
  • Supports the development of bones
  • Supports the mechanism of bone maintenance. This helps in the balancing of bone creation and natural loss processes.
  • Supports cell differentiation but inhibits cell proliferation. While differentiation helps fast multiplying cells to specialize into cells that form specific tissue or cells to replace those that have undergone normal biologically determined cell death, proliferation can be the genesis of cell mutations that can lead to cancer.
  • Supports the absorption of calcium from the gut

The control of vitamin D functions in the body is supported by a network of systems of the body that harmoniously work together in a complex network that include:

  • Parathyroid glands and the production of parathyroid hormone. These glands are responsible for sensing the level of serum calcium and depending on the status, releases or inhibits the release of the parathyroid hormone (PTH). This can result in:
  1. Increased absorption of calcium from the small gut
  2. Increased reabsorption of calcium that the kidneys have already filtered
  3. Sourcing calcium from the bones when there is inadequate dietary intake of the mineral. This is called bone demineralization and it can lead to weak bones that are prone to fractures even with minor injuries. Demineralization can also occur if there is inadequate vitamin D intake to support the absorption of calcium from the small intestines.
  • The liver
  • The kidneys
  • The small gut
  • The bones

Vitamin D has many more physiological functions that are beyond the scope of this article

Types or forms of vitamin D

There are two main types of vitamin D. These are:

  1. Vitamin D2 which is also known as ergocalciferol. This type is found in plants
  2. Vitamin D3 which is found in animal skins. The technical name for this type is cholecalciferol.

Knowing the type of this vitamin can help you to identify the source of the vitamin D in your supplement and avoid animal sourced products if you are vegan.

Symptoms of vitamin D deficiency

Vitamin D deficiency is more common than what many people might imagine. According to the Nutritional Research of January 2011, almost 47% of American adults have some degree of the vitamin’s deficiency. The percentage is much higher among Hispanics (about 69%) and Blacks (about 82%). The study in the journal mentioned concluded that vitamin D needs special consideration in view of the fact that, its deficiency is linked to some of the leading risk factors for death in the country. Signs and symptoms depend on the severity of the deficiency.

Severe deficiency of vitamin D


This is a condition that affects bone growth whereby there is an imbalanced and poor bone mineralization which leads to bow-legs among other bone disorders. If low serum calcium levels accompany the disorder, convulsions can be a presenting symptom. Rickets according to the Pediatrics of November 2008 is still much more common than generally expected. This is despite efforts at fortifying foods with vitamin D. this problem affects children because their bones are fast growing.


This is relative bone softening. If it is not corrected it leads to bone porosity also called osteoporosis. The condition affects adults whose bone growth has already stopped.

Painful and weak muscles

A study carried out in Denmark and published in the Mayo Clinic Proceedings of December 2003, suggested that these symptoms can be an indication of vitamin D deficiency.

Other signs and symptoms include:

In babies and children

  • Delayed closure of the fontanelle
  • Stunted growth
  • Soft and weak bones
  • Irritability, muscle spasms and maybe convulsions
  • Heart and respiratory problems in severe cases

In adults

  • Generalized and persistent tiredness
  • Bone pain
  • Hair-loss
  • Back pain
  • Poor wound healing
  • Mental issues such as depression
  • Recurrent infections due to impaired immunity

Who is at risk of vitamin D deficiency?

  1. People living in highly polluted regions where ultra-violet B rays are impaired. Areas further away from the equator are also thought to be a risk factor
  2. Too much and prolonged use of sunscreen
  3. Cultural concealing clothes as in Moslem women
  4. Exclusively breastfed babies
  5. Dark-skinned people
  6. Old people
  7. Renal disease
  8. Poor fat absorption
  9. Low or no fat diet
  10. Genetically determined vitamin D-binding proteins (DBP) status
  11. Malabsorption disorders
  12. Magnesium deficiency. One study indicated that magnesium is important in the metabolism of vitamin D
  13. Obesity

Vegan sources of vitamin D

  • Fortified foods and drinks such as orange juice
  • Whole meal cereals (fortified)
  • Some types of edible mushrooms exposed to UV light
  • Sun exposure
  • Nutritional supplements

Science-backed benefits of vitamin D

The benefits of vitamin D are many and varied. Every system in the body needs the sunshine vitamin. A few of these benefits include the following.

  1. Supports strong and healthy bones. This includes the teeth.
  2. Supports good sugar metabolism thus reducing the risk of diabetes in both children and adults. The benefits appears to be for both type 1 and 2 diabetes for both groups
  3. Supports cardiovascular health. One study in the journal Circulation of January 2008 found that people with low vitamin D were about 153% more at risk of developing heart problems.
  4. Reduces symptoms of depression
  5. Lowers the risk of cancer
  6. Lowers symptoms of multiple sclerosis and can be an important additionto the treatment of the condition.
  7. Supports the immune system and lowers the incidence and severity of allergies as in asthmatic attacks
  8. Healthy pregnancy and healthy babies. The BMJ of March 26 2013 reported a study that suggested that low vitamin D levels increased risk of gestational diabetes, preeclampsia and premature and low birth weight babies. Pregnant vitamin D deficient women were also found to be more prone to vaginosis.
  9. Reduces risk of age-related dementia
  10. Supports better wound healing including surgical wounds.

Vitamin D toxicity

Excessive consumption of vitamin D from nutritional supplements or from pharmacological preparations of the vitamin, can lead to accumulation in the body. This can present with serious health effects in a condition called hypervitaminosis D. Symptoms of toxicity include:

  • Digestive problems such as constipation and poor appetite
  • Vomiting
  • Dehydration
  • Generalized pain
  • Muscle weakness
  • Emotional disturbance and irritability
  • Calcium deposits may be demonstrated in various parts of the body and organs

Vitamin D can interact with corticosteroids. These medications impair calcium absorption. Cholesterol lowering medications also reduce the vitamin’s absorption from the gut. Phenobarbitone and some other types of anti-convulsant drugs also fall into this category.

Vitamin D toxicity, in the absence of other metabolic disorders can simply be avoided by not exceeding the dosages recommended by your doctor or those suggested on the label of your nutritional supplement package. That way, you will reap and enjoy the above and many more benefits of the sunshine vitamin.


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Schwalfenberg, G. K. (2011), A review of the critical role of vitamin D in the functioning of the immune system and the clinical implications of vitamin D deficiency. Mol. Nutr. Food Res., 55: 96–108. doi:10.1002/mnfr.201000174

Deng, X., Song, Y., Manson, J. E., Signorello, L. B., Zhang, S. M., Shrubsole, M. J., … Dai, Q. (2013). Magnesium, vitamin D status and mortality: results from US National Health and Nutrition Examination Survey (NHANES) 2001 to 2006 and NHANES III. BMC Medicine, 11, 187.

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Munger KL, Levin LI, Hollis BW, Howard NS, Ascherio A. Serum 25-Hydroxyvitamin D Levels and Risk of Multiple Sclerosis. JAMA. 2006;296(23):2832-2838. doi:10.1001/jama.296.23.2832

Llewellyn, D. J., Lang, I. A., Langa, K. M., Muniz-Terrera, G., Phillips, C. L., Cherubini, A., … Melzer, D. (2010). Vitamin D and Risk of Cognitive Decline in Elderly Persons. Archives of Internal Medicine, 170(13), 1135–1141.

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