D-fined: what is vitamin D and where does it come from?
Vitamin D is a fat-soluble vitamin, which is made within our bodies when UVB rays from the sun penetrate our skin. Most of us rely on this to meet our requirement; typically, over 90% of our vitamin D comes from this mechanism (Holick 2004). However, factors such as skin pigmentation, sunscreen use, as well as the time of day and seasonality, can influence the amount of vitamin D that is produced in our skin; more on this later.
We can also obtain vitamin D from a limited number of foods. Like other fat-soluble vitamins, vitamin D is found in fats and oils, such as milk, oily fish (salmon, mackerel and sardines) and cod liver oil (Holick, 2004). Whilst eggs are also a potentially good source, the amount of vitamin D measured in them has been variable (Holick, 2004). Fortification of food items is rare, although some products such as orange juice, bread and cereal, do sometimes have vitamin D added to them.
D-lightful: what are the benefits of vitamin D?
Vitamin D is vital for bone health. This is because it increases calcium absorption and maintains blood calcium concentrations within an acceptable range. Vitamin D deficiency results in calcium being withdrawn from bones instead to maintain normal concentrations of calcium in the blood. This puts individuals at risk of developing osteoporosis or fractures later in life (Holick, 2004).
D-ficiencies and dangers: do I need to think about my intake?
Over the last 50 years, it appears that many women have shunned oily fish and dairy products (Whitton et al., 2011). This has meant that the amount of vitamin D obtained from the average female’s diet in the UK is below recommended levels (PHE 2018). Whilst worrying, is not necessarily a disaster because, as previously mentioned, 90% of our intake comes from the action of sunlight on our skin (Holick 2004).
However, vitamin D deficiency is widespread, especially in those living at higher latitudes, where seasonality puts constraints on vitamin D production, and in those of African American origin, whose skin is less efficient at making vitamin D. However, other groups are at risk, such as those who work indoors or who always wear sunscreen. For example, Sullivan et al. (2003) showed that 17% of adolescent females were vitamin D deficient at the end of the summer, possibly because this group diligently protected themselves from the sun. Indeed, sun protection is important in preventing skin cancer, so a balance is needed to achieve a healthy amount of sun exposure.
It is also vitally important to determine whether an individual has sufficient but safe levels of vitamin D, as it is potentially the most toxic of all vitamins. Intoxication can never occur as a result of sun exposure; excess vitamin D is degraded into inactive products (Holick 2007). However, if too much vitamin D is consumed in our diet, this can result in elevated blood calcuim and calcification of soft tissues, including the kidneys, lungs, heart, and arteries (Consequently, be sure to consult a healthcare professional before taking supplements or altering your diet.
British Pediatric Association (BPA) (1956) Hypercalcemia in infants and vitamin D. British Medical Journal, 2, 149–151
Holick, M.F. (2004) Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease. American Journal of Clinical Nutrition, 80(6), 1678S-1688S
Holick, M.F. (2007) Vitamin D deficiency. New England Journal of Medicine, 357(3), 266-281
Perez-Barrios, C., Hernández-Álvarez, E., Blanco–Navarro, I., Perez-Sacristan, B. and Granado-Lorencio, F., (2016) Prevalence of hypercalcemia related to hypervitaminosis D in clinical practice. Clinical Nutrition, 35(6), 1354-1358.
Public Health England (PHE) (2018) National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme.
Sullivan, S.S., Rosen, C.J., Chen, T.C. and Holick, M.F. (2003) Seasonal changes in serum 25(OH)D in adolescent girls in Maine. In: Proceedings of the American Society for Bone and Mineral Research Annual Meeting. Washington, DC: American Society for Bone and Mineral Research
Whitton, C., Nicholson, S.K., Roberts, C., Prynne, C.J., Pot, G.K., Olson, A., Fitt, E., Cole, D., Teucher, B., Bates, B. and Henderson, H. (2011) National Diet and Nutrition Survey: UK food consumption and nutrient intakes from the first year of the rolling programme and comparisons with previous surveys. British journal of nutrition, 106(12), 1899-1914