Vitamin D – The Facts

Sunset with branch obstructing

Vitamin D is classified as a fat-soluble vitamin but is actually a hormone that can be synthesised by the body with exposure to sunlight. Derived from cholesterol, Vitamin D needs to be activated from its pre-vitamin D3 form into Calcitriol by way of the liver and kidneys. Working in conjunction with Calcium, Vitamin D is required for bone growth and maintenance.

Biological function

For synthesis of vitamin D to take place exposure to UV light is required. Enzymes from the liver in the form of 25 – hydroxy and kidney in the form of 1,25 – dihydroxy activate the molecule producing Calcitriol (Tortora & Derrickson, 2011, p. 168).
When blood calcium falls, vitamin D works in conjunction with the parathyroid hormone to raise levels and when levels get too high Calcitonin inhibits the activation of vitamin D (Whitney et al., 2017, p 436). Vitamin D is stored in the liver and fatty tissues to be released into body cells when required (Wu-Wong, 2012, p. 7).

Alongside vitamins A, C and K, calcium, phosphorus, magnesium, collagen and fluoride, vitamin D assists in the development, repair and maintenance of bone (Berdanier, Berdanier & Zempleni, 2009, p. 337). The skin, muscles, cartilage, nervous system, pancreas, immune system, reproductive system and brain also rely on vitamin D to function effectively (Whitney et al., 2017, p 394).

Dietary Sources

The most accessible source of vitamin D is from the sun but when unavailable due to weather or lifestyle can also be obtained from food sources such as fatty fish, eggs and fortified foods (Whitney et al., 2017, p 397).

RDI and Therapeutic Doses

The Australian Health and Medical Research Council of Australia (2014) has set the following adequate intakes to meet daily requirements:

Birth to 18 years 5.0 μg
19 to 50 years 5.0 μg
51 to 70 years 10.0 μg
71 + years 15.0 μg

States of Excess

If vitamin D is consumed through supplementation toxicity can occur resulting in a rise in the level of blood calcium called hypercalcaiemia. Kidney stones may be a consequence or in extreme cases hardening of the major arteries of the heart and lungs can lead to death (Whitney et al., 2017, p 396).

States of insufficiency and deficiency

As with calcium deficiencies of Vitamin D in adults can lead to soft, brittle bones -osteoporosis and osteomalacia (Whitney et al., 2017, p 397). It has been reported in journal article Nutrition Review (2007) that Osteoporosis affected 10 million people aged over 50 with a further 30 million at risk in the USA. More vitamin D is required as we age due to our bodies inability to create and activate vitamin D along with a decrease of intake of milk and sun exposure (Whitney et al., 2017, p 395).
Bowing of the legs, deformities of the ribs and enlarged heads are all signs of rickets in children. This can be related back to a maternal deficiency (Paterson & Ayoub, 2015). Studies have shown that as much as 36% of healthy adults between the ages of 18 and 29 had low levels of vitamin D due to the use of sun protection (Holick, 2008).

Conclusion

To achieve the recommended daily intake of vitamin D and avoid or delay the onset of the breakdown of bone it is recommended to gain adequate exposure to natural sunlight. When sunlight is not available a diet rich in dairy, oily fish, eggs and foods fortified with vitamin D can help to achieve healthy quantities. Supplements can be consumed and are recommended along with calcium as we age.

References

Australian Government National Health and Medical Research Council. (2014). Nutrient reference values for Australia and New Zealand. Retrieved from https://www.nrv.gov.au/nutrients/calcium

Berdanier, C., Berdanier, L., Zempleni, J. (2009). Advanced Nutrition macronutrients, micronutrients and metabolism. Boca Raton, FL: Taylor & Francis Group

Holick, M. (2008). Vitamin D: a D-lightful health perspective. Nutritional Reviews. 66(2), 182-194.

Mundy, G. (2007). Osteoporosis and Inflammation. Nutritional Reviews. 65(12), 147-151.

Paterson, C., Ayoub, D. (2015). Congenital rickets due to vitamin D deficiency in the mothers. Clinical Nutrition. 34(5), 793-798.

Ross, A., Caballero, B., Cousins, R., Tucker, K., Ziegler, T. (2014) Modern Nutrition Health and Disease (11th ed.). Philadelphia, PA: Lippincott Williams & Wilkins

Tortora, G., Derrickson, B. (2011). Principles of Anatomy & Physiology
(13rd ed., Vol. 2). Hoboken, NJ: John Wiley & Sons.

Whitney, E., Rolfes, S., Crowe, T., Cameron-Smith, D., & Walsh, A. (2017). Understanding Nutrition (3rd Australia and New Zealand ed.). South Melbourne, Australia: Cengage Learning Australia Pty Ltd.

Wu-Wong. (2012). Why does Vitamin D matter. Illinois, CHI: Bentham books

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Tracey Talbot
Tracey Talbot

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