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by dietitian Clare Wood

What is Vitamin D?

Vitamin D is a pro-hormone that is mainly produced in the skin from direct UVB sunlight rays, with a very small amount coming from naturally occurring and fortified food sources.

Vitamin D plays a vital role in the following:

• Maintaining calcium and phosphate balance in the body.
• Optimizing bone mineralization.
• Skeletal muscle function.

A deficiency in Vitamin D is known to be linked with a disease called Rickets in children and Osteomalacia in adults. It is also important for immune function and inflammation, and low levels have recently been linked to several diseases:

• The increased risk of multiple sclerosis
• Rheumatoid and osteoarthritis arthritis,
• Hypertension,
• Cardiovascular disease and
• Cancer

However, we still don’t fully understand if low levels of vitamin D actually cause these diseases, scientists need more research to determine this.

Vitamin D deficiency

The best indicator of vitamin D status is the concentration of 25-hydroxyvitamin D 25(OH)D in the blood, although there is still some confusion about what the ideal levels of Vitamin D are for optimal health.

• The definition of vitamin D deficiency is a serum level of 25(OH)D < 20 ng/mL (<50nmol/L). 
• Vitamin D insufficiency is defined as deficiency without clinical signs or symptoms. Insufficiency is defined between 21-32 ng/mL (~<75nmol/L),
• Sufficiency is a broad range not yet established (somewhere between 30-100 ng/mL).

Prevalence

Vitamin D deficiency is well recognized as a pandemic and a global public health problem with many high risk populations around the globe with deficiency even occurring in places like Australia where there is plenty of sun.

Vitamin D deficiency is becoming more prevalent due to cautious sun exposure, think “Slip Slop Slap”, the increasing obesity problem, and a reduction in foods that are naturally high, or fortified with vitamin D. Generally there are seasonal differences with lower Vitamin D levels seen in winter.

Effect on health

There are mixed reports in the literature, with some showing a link between low vitamin D status and increased risk of both upper and lower respiratory tract infections, plus longer durations of sickness. However, there is less evidence to show that supplementation will reduce the incidence or severity of sickness.

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Should we Supplement?

There are many factors effecting vitamin D status, including:

• Age
• Skin pigmentation
• Distance from the equator
• Sun exposure which is influenced by skin coverage
• Sunscreen
• Time spent in the sun
• Seasonal fluctuations throughout the year

With the high prevalence of vitamin D deficiency and insufficiency in the general population, plus the potential health benefits of supplementation, improving vitamin D status seems warranted. Current policies for vitamin D supplementation and fortification are possibly outdated requiring new guidelines.

The improvement of Vitamin D levels can be made using the following strategies:

• Dietary sources: There are only a few dietary sources of vitamin D from limited natural and fortified foods including fatty fish, egg yolks, sun-dried mushrooms, and fortified milk, margarine, and cereals. Dietary advice from a sports dietitian to optimize intake is important.
• Sun Exposure: to satisfy the body's requirement of vitamin D from dietary sources without supplementation would be difficult, if not impossible without sun exposure. Current sun exposure guidelines are cautious to avoid skin cancer, and provide challenges due to the variability of individual skin types, distance from equator, and time of the year. Although there is minimal risk of vitamin D toxicity through sun exposure, it is not the safest way to improve vitamin D status.
• Supplementation: studies have shown that individual supplementation can achieve vitamin D sufficiency and is recommended in the winter months to maintain levels for those at risk.
• Food Fortification: vitamin D food fortification is in different stages around the world. In Germany fortification is banned. Several studies have implemented programs successfully increasing 25(OH)D levels, while in the Americas food fortification is already happening, yet vitamin D insufficiency still occurs.  The solution may be to broaden the range of foods fortified, appealing to individual taste preferences, financial status, as well as using appropriate cultural choices.  

Risk of toxicity

Vitamin D toxicity can occur at concentrations of 25(OH)D reaching > 200 ng/mL, achieved with chronic intake of extremely high doses for several months. The result is high levels of calcium and phosphate in the blood, which if untreated can lead to kidney failure, soft tissue calcification and ultimately death. However, a prolonged intake of 10,000 International Units per day of vitamin D(3) is unlikely to pose a risk of adverse effects in almost all individuals in the general population.

Strategies

There is enough evidence to suggest that deficiency and insufficiency are widespread in the general population. Priority should be given to determining levels for optimal health and then standardizing the classification of vitamin D deficiency, insufficiency and sufficiency. First line strategy would be implementation of food fortification programs in high risk areas on a population level, to effectively achieve population improvements. Specific targeted strategies, such as a screening tool, should be implemented to identify at risk individuals who are deficient/insufficient. This will allow appropriate assessment, followed up with blood testing in the winter/autumn months of the year (when levels are lowest) where required. Then, appropriate supplementation on an individual level can occur to increase basal levels mainly to improve bone health, and possibly also to reduce illness, inflammation and to optimize muscle function. There is not enough evidence yet to prescribe vitamin D for prevention of chronic disease.

Selected References

• Hanley DA, Davison KS. Vitamin D insufficiency in North America. J Nutr. 2005 Feb;135(2):332-7.
• Autier P, Boniol M, Cecil Pizot B, Mulie P. Vitamin D status and ill health: a systematic review. The Lancet Diabetes Endocrinology. 2013;1-14. http://dx.doi.org/10.1016/S2213-8587(13)70165-7.
• Holick MF. 2012. Evidence-based D-bate on health benefits of vitamin D revisited. Dermatoendocrinol. 2012 Apr 1;4(2):183-90. doi: 10.4161/derm.20015.
• Souberbielle JC, Cavalier E. Supplementation, optimal status, and analytical determination of vitamin D: where are we standing in 2012? Anticancer Agents Med Chem. 2013 Jan;13(1):36-44.
• Holick MF, Chen TC. Vitamin D deficiency: a worldwide problem with health consequences. Am J Clin Nutr. 2008 Apr;87(4):1080S-6S.
• Palacios C, Gonzalez L. Is vitamin D deficiency a major global public health problem? Am J Clin Nutr. 2010 May;91(5):1255-60. doi: 10.3945/ajcn.2009.29094.
• Wacker M, Holick MF. Sunlight and Vitamin D: A global perspective for health. Dermatoendocrinol. 2013 Jan 1;5(1):51-108. doi: 10.4161/derm.24494.
• Calvo MS, Whiting SJ. Survey of current vitamin D food fortification practices in the United States and Canada. J Steroid Biochem Mol Biol. 2013 Jul;136:211-3. doi: 10.1016/j.jsbmb.2012.09.034.
• Daly RM, Gagnon C, Lu ZX, Magliano DJ, Dunstan DW, Sikaris KA, Zimmet PZ, Ebeling PR. Prevalence of vitamin D deficiency and its determinants in Australian adults aged 25 years and older: a national, population-based study. Clin Endocrinol (Oxf). 2012 Jul;77(1):26-35. doi: 10.1111/j.1365-2265.2011.04320.x.
• Jolliffe DA, Griffiths CJ, Martineau AR. Vitamin D in the prevention of acute respiratory infection: systematic review of clinical studies. J Steroid Biochem Mol Biol. 2013 Jul;136:321-9. doi: 10.1016/j.jsbmb.2012.11.017.
• He CS, Handzlik M, Fraser WD, Muhamad A, Preston H, Richardson A, Gleeson M. Influence of vitamin D
• Murdoch DR, Slow S, Chambers ST, Jennings LC, Stewart AW, Priest PC, Florkowski CM, Livesey JH, Camargo CA, Scragg R. Effect of Vitamin D3 Supplementation on Upper Respiratory Tract Infections in Healthy Adults. JAMA. 2012;308(13):1333-1339. doi:10.1001/jama.2012.12505.
• Laaksi I. Vitamin D and respiratory infection in adults. Proc Nutr Soc. 2012 Feb;71(1):90-7. doi: 10.1017/S0029665111003351.
• Urashima M, Segawa T, Okazaki M, Kurihara M, Wada Y, Ida H. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Am J Clin Nutr. 2010 May;91(5):1255-60. doi: 10.3945/ajcn.2009.29094.
• Holick MF, Binkley NC, Bischoff-Ferrari HA, Gordon CM, Hanley DA, Heaney RP, Murad MH, Weaver CM; Endocrine Society. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011 Dec;96(12):3908.
• Ben-Shoshan M. Vitamin D Deficiency/Insufficiency and Challenges in Developing Global Vitamin D Fortification and Supplementation Policy in Adults. J Am Geriatr Soc. 2013 May;61(5):707-14. doi: 10.1111/jgs.12213.
• Brown J, Sandmann A, Ignatius A, Amling M, Barvencik F. New perspectives on vitamin D food fortification based on a modeling of 25(OH)D concentrations. Nutr J. 2013 Nov 21;12(1):151. doi: 10.1186/1475-2891-12-151.
• Shakur YA, Lou W, L'Abbe MR. Examining the effects of increased vitamin D fortification on dietary inadequacy in Canada. Can J Public Health. 2014 Apr 17;105(2):e127-32.
• Yang Z, Laillou A, Smith G, Schofield D, Moench-Pfanner R. A review of vitamin D fortification: implications for nutrition programming in Southeast Asia. Food Nutr Bull. 2013 Jun;34(2 Suppl):S81-9.
• Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Am J Clin Nutr. 1999 May;69(5):842-56.
• Vieth R. Vitamin D toxicity, policy, and science. J Bone Miner Res. 2007 Dec;22 Suppl 2:V64-8. doi: 10.1359/jbmr.07s221.
• Black LJ, Seamans KM, Cashman KD, Kiely M. An updated systematic review and meta-analysis of the efficacy of vitamin D food fortification. J Nutr. 2012 Jun;142(6):1102-8. doi: 10.3945/jn.112.158014.
• Zarca K, Durand-Zaleski I, Roux C, Souberbielle JC, Schott AM, Thomas T, Fardellone P, Benhamou CL. Cost-effectiveness analysis of hip fracture prevention with vitamin D supplementation: a Markov micro-simulation model applied to the French population over 65 years old without previous hip fracture. Osteoporos Int. 2014 Jun;25(6):1797-806. doi: 10.1007/s00198-014-2698-1.

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