Vitamin D: Implications in Diabetes and Beyond

Vitamin D2

Vitamins are essential organic compounds that the body cannot make on its own and has to obtain through diet or supplementation. Vitamin D was discovered in early 1920s, in an experiment performed by Elmer Verner McCollum and his co-workers. This group of scientists noticed that heated, oxidized cod-liver oil could cure rickets in rats but was not effective in preventing xerophthalmia.1 The ‘’antirickets factor’’ was initially misclassified as the fourth vitamin due to its presence as a trace component in the diet. Years later, scientists realized that vitamin D is actually a steroid hormone synthesized in the skin upon exposure to UVB light from the sun. It exists in two main forms, cholecalciferol (Vit. D3) and ergocalciferol (vit D2); cholecalciferol is also known as the active form.

The predominant function of vitamin D in the body is the increase of serum calcium and phosphate levels and promotion of bone mineralization.2 Although vitamin D deficiency has been traditionally linked with bone conditions such as osteomalacia and rickets, recent research has shown that inadequate levels of vitamin D may have implications in a variety of health conditions and diseases, such as: immune deficiency, respiratory infections, inflammatory bowel disease, diabetes, cardiovascular disease, cancer and neurocognitive disorders.3 In addition, several meta-analyses have pointed to an increased risk of mortality associated with both low and high 25(OH)D levels.4-6

A recent meta-analysis published in the Diabetes Research and Clinical Practice journal found serum 25(OH)D levels to be significantly lower in patients with type I diabetes than in healthy controls.7 This meta-analysis included 23 studies, with participants divided among two groups – the children and adolescents group (1900 total), and the adults group (3494 total). The association between low serum 25(OH)D levels and type one diabetes was observed in both groups, but it was not clear whether a causative relationship exists. Study limitations included not taking into account several potentially important factors, such as differences in latitude or sun exposure and genetic factors.

A different article published in the February 2015 issue of the Endocrine Society’s Journal of Clinical Endocrinology & Metabolism, also attracted substantial attention of the scientific community.8 This cross-sectional study conducted by Spanish researchers included 148 participants from two hospitals in Spain. The study participants were classified according to their BMI and the presence of diabetes and prediabetes, or absence of glycemic disorders. Vitamin D levels, as well as vitamin D receptor gene expression, were assessed in study participants. The study found higher levels of vitamin D in obese subjects who did not have glucose metabolism disorders than diabetic subjects. Similarly, low levels of vitamin D were found in lean subjects suffering from diabetes or another glucose metabolism disorder. The study concluded that vitamin D levels are associated more closely with glucose metabolism than obesity.

Due to targeted research efforts of the international scientific community, new insights about the connection between vitamin D levels and specific chronic conditions, beyond diabetes, are becoming available by the day. At the same time, the deficiency/insufficiency of vitamin D has become a public health concern, especially in developing countries.A high prevalence of hypovitaminosis D was observed in China and Mongolia, especially in children. In addition, according to research published within the past ten years, one-third to one-half of individuals from Sub-Saharan Africa and the Middle East have serum 25(OH)D levels below 25 nmol/L, despite adequate sunshine throughout the year.

The available data on vitamin D’s role in a variety of physiological and preventative functions are in accordance with the recent recommendations to monitor and improve vitamin D levels in children and adults. The Endocrine Society clinical practice guidelines from 2011 define optimal serum 25(OH)D levels to be above 30 ng/mL, with preferred range being 40-60 ng/mL.10 The recommended daily allowance varies depending on the age.  Safe sunlight exposure, consumption of foods fortified with vitamin D and/or supplementation are effective ways of maintaining optimal serum 25(OH)D levels.


  1. McCollum EV, Simmonds N, Becker JE, Shipley PG. Studies on experimental rickets. XXI. An experimental demonstration of the existence of a vitamin which promotes calcium deposition. J. Biol. Chem.1922;53:293-312.
  2. Garg M, Lubel JS, Sparrow MP, Holt SG, Gibson PR. Review article: Vitamin D and inflammatory bowel disease – established concepts and future directions. Aliment. Pharmacol. Ther. 2012;36(4):324-44.
  3. Gröber U, Reichrath J, Holick MF. Live Longer With Vitamin D. Nutrients. 2015;7:1871-1880.
  4. Melamed ML, Michos ED, Post W, Astor B. 25-hydroxyvitamin D levels and the risk of mortality in the general population. Arch. Intern. Med. 2008;168:1629–1637.
  5.  Dobnig H, Pilz S, Scharnagl H, Renner W, Seelhosrt U, Wellnitz B, Kinkeldei J, Boehm BO, Weihrauch G, Maerz W. Independent association of low serum 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D levels with all-cause and cardiovascular mortality. Arch. Intern. Med. 2008; 168:1340–1349.
  6. Zittermann A, Iodice S, Pilz S, Grant WB, Bagnardi V, Gandini S. Vitamin D deficiency and mortality risk in the general population: A meta-analysis of prospective cohort studies. Am. J. Clin. Nutr. 2012;95:91–100.
  7. Feng R, Li Y, Li G, Li Z, Zhang Y, Li Q, Sun C. Lower serum 25(OH)D concentrations in type 1 diabetes: A meta-analysis. Diabetes Res. Clin. Pr. 2015, in press (
  8. Clemente-Postigo M, Muñoz-Garach A, Serrano M, Garrido-Sánchez L, Bernal-López MR, Fernández-García D, Moreno-Santos I, Garriga N, Castellano-Castillo D, Camargo A, Fernández-Real JM, Cardona F, Tinahones FJ, Macías-González M. Serum 25-Hydroxyvitamin D and adipose tissue vitamin D receptor gene expression: Relationship with obesity and type 2 diabetes. J Clin. Endocrinol. Metab. 2015;100(4):E591-E595.
  9. Arabi A, Rassi RE, Fuleihan G E-H. Hypovitaminosis D in developing countries – prevalence, risk factors and outcomes. Nat. Rev. Endocrinol. 2010;6(10):550-61.
  10. 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; 96:1911–1930.

Jasenka Piljac Zegarac is a scientist and freelance writer. She holds a PhD in biology and a BS degree in biochemistry, and contributes on a regular basis to several health and science blogs. Her scientific publications have gathered more than 1100 citations. She may be contacted for assistance with a variety of science and medical writing projects.

One thought on “Vitamin D: Implications in Diabetes and Beyond

  1. Greg Eaton

    Dear Jasenka,
    I like the article, but would like to know the correlation between nmols/litre and ng/mL cited in the article.
    Without this I cannot compare recommended minuimum levels in ng/mL with the cited lower level of 25nmol/litre

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