Beyond bones
For the last 80 years we have focused on the link between vitamin D and calcium economy – vitamin D is necessary for calcium absorption and its deficiency leads to rickets. But this is just the visible part of the iceberg.
The bottom part of the iceberg is cell cycle and immune regulation, as vitamin D is involved in gene expression processes. This less known role actually accounts for about 85% of the metabolism of vitamin D everyday in our body (1).
This means that, depending on which type of cells it affects, vitamin D inadequacy can be involved in many other dysfunctions including cardiovascular diseases and cancer:
• CVD: Study including 1739 participants showed that vitamin D deficiency of < 15ng/mL and < 10ng/mL is associated with a 50% and 80% increase in the risk of developing cardiovascular disease respectively (2).
• Hypertension: Further evidence showed that vitamin D deficiency leads to a three fold increased risk of developing hypertension (3).
• Cancer: Additionally, vitamin D has been shown to reduce cancer risks for a number of cancers including colon, breast, prostate, lung cancers and lymphoma (1).
Availability, optimal levels and toxicity•
The two major forms of vitamin D are D2 (or ergocalciferol) and D3 (or cholecalciferol). Vitamin D3 is the one which is produced physiologically and is more effective at increasing the levels of circulating vitamin D hormone, but at appropriate levels both vitamin D2 and vitamin D3 are efficient. There is no difference in terms of availability between vitamin D in fortified food or in supplement (1).
• Evidence shows all functions are getting better up to serum levels of vitamin D of 80-100nmol/L (achieved with 4000-5000IU a day) – which is much higher than current recommendations (400-800IU a day for adults below 50).
• Vit D intake has been shown to be safe up to at least 10000 IU a day, and no case of intoxication has been reported below 30000 IU.
Groups at greater risk of deficiency (4)
• Elderly: A 70-year-old person has 25% of the capacity to produce cholecalciferol compared with a healthy young adult. In Ireland, evidence shows that inadequate vitamin D status during winter time is quite common in elderly women and adolescent girls5.
• Infants: While infant formula is generally fortified with vitamin D, breast milk does not contain significant levels of vitamin D. Infants who are exclusively breastfed are likely to require vitamin D supplementation beyond early infancy, especially at northern latitudes.
• Obesity: Obese individuals may have lower levels of the circulating form of vitamin D, probably because of reduced bioavailability, as the cholecalciferol is sequestered deep in the body fat.
• Sunscreen users: The use of sunscreen with a sun protection factor (SPF) of 8 inhibits more than 95% of vitamin D production in the skin. To avoid vitamin D deficiency it is recommended to consider supplementation along with sunscreen use.
• Dark skin: Dark-skinned individuals may require extra vitamin D to avoid deficiency at higher latitudes more particularly during the winter months.
• Renal/hepatic disorders: Conditions such as hepatic or renal disorders, or, rarely, a number of hereditary disorders can impair conversion of vitamin D into active metabolites.
References: 1- Professor Robert Heaney, personal communication
2- Wang TJ et al. Vitamin D deficiency and risk of cardiovascular disease. Circulation. 2008 Jan 29;117(4):503-11.
3- Forman JP et al. Plasma 25-hydroxyvitamin D levels and risk of incident hypertension. Hypertension. 2007 May;49(5):1063-9.
4- Holick MF. The vitamin D epidemic and its health consequences. J Nutr. 2005 Nov;135(11):2739S-48S.
5- McCarthy D et al. Vitamin D intake and status in Irish elderly women and adolescent girls. Ir J Med Sci. 2006 Apr-Jun;175(2):14-20.
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