Vitamin D and Diabetes: No Bones About It


There has been a lot of recent interest in the role of Vitamin D (VitD) in the human body. Until recently, medical understanding of VitD has been limited to knowledge about bone health and calcium absorption. However studies over the past decade have now shown VitD has actions in the body unrelated to bones, including assisting in normal immune function, normal blood vessel health, normal organ function (including the pancreas), “normal” mood, and may protect against certain cancers [1]. VitD is required by the pancreas to make and release insulin.

Although we make VitD in our bodies, data shows VitD deficiency is common, especially in people with diabetes. Given recent concern over sunlight exposure and the risk for skin cancers, many people who live in areas where sunlight exposure is adequate use sunscreens that block VitD production, contributing to deficiency. For others (like me!), who live in higher latitudes where sun intensity is too low to make adequate VitD year-round, deficiency is even more common. How do you find out if you should supplement VitD? Read on.


Vitamin D Action: The Basics

Vitamin D (VitD) is poorly classified in human nutrition and physiology as a vitamin. It is true, it does assist with many bodily functions and enzymatic actions like other vitamins, but really it acts more like a hormone helping to regulate complex bodily functions including bone density, electrolyte balance (calcium and magnesium especially) for the cardiovascular system, and the release of insulin by the pancreas.

There are actually several forms of VitD in the human body, each with slightly different activities. The storage form of VitD is a 25-hydroxycholecalciferol (25-VitD). Although 25-VitD has physiologic actions, they are weak compared to the more active form of VitD, 1,25- dihydroxyvitamin D (1,25-VitD). 25-VitD is converted to 1,25-VitD when blood levels of calcium fall too low and more calcium is needed. 1,25- VitD helps increase the absorption of calcium from the food we eat and helps the kidneys reabsorb calcium from the blood stream. 


Where Does It Come From?

Vitamin D is made in the skin in response to exposure to ultraviolet light, i.e. sunlight, and from the diet. It is a fat-soluble vitamin and therefore is usually present in high fat foods. Dairy products are the most common source of VitD because many dairy products are fortified, however certain seafood including salmon and shrimp actually have higher concentrations. There are few vegetable sources of VitD (shitake mushrooms are one exception!), although because we make VitD this should not be a problem (unless you work inside all day and use sunscreen like most other people!).


Vitamin D Actions: More Modern Thinking

Medical science now knows of many more actions of VitD in addition to bone health. VitD helps regulate calcium (and to a lesser degree magnesium) absorption into all tissues and organs in the human body [2]. This may not sound that exciting, but given every heart beat and every muscle contraction require calcium, this is actually very exciting! Further more, every muscle relaxation, including relaxation of the smooth muscles inside of your blood vessels, requires magnesium- also regulated by VitD.

Have you ever noticed that you feel better after a day at the beach or after a walk on a sunny spring afternoon? There are many reasons for this sense of well-being - including the effects of exercise and the effects of relaxation on mood - but there is also an effect of VitD. VitD helps regulate the production of the neurotransmitter serotonin in the brain [3, 4]. The actions of serotonin have been popularized in recent decades by the popularity of SSRIs, or Selective Serotonin Reuptake Inhibitors, for the treatment of depression. In fact, there is a daily cycle of serotonin production and release which is balanced by a nighttime production of melatonin.

Both serotonin and melatonin share a common ancestor, an amino acid called tryptophan. Guess what? VitD helps throw the switch, directing tryptophan production more toward serotonin and less toward melatonin. Since melatonin makes us sleepy and increases our cravings for fatty foods (ever feel like hibernating?), it stands to reason that VitD may have an impact on food cravings, energy levels and overall mood by helping to make serotonin[3, 4].

There is mounting evidence that suggests VitD may also protect against certain cancers, including breast, colon and prostate cancers [5, 6]. This sounds surprising at first glance, but looking deeper we find that VitD actually binds to genes in a manner very similar to the hormones estrogen, helping to regulate the actions of many processes in the body.


Vitamin D in Diabetes

The importance of VitD in diabetes, both type 1 and type 2, has been recently reviewed by Mathieu in the medical literature [7]. VitD appears to function in several mechanisms to maintain normal insulin secretion. A vitamin D receptor (VDR) participates in the regulation of insulin gene transcription and appears vital to normal production, in part by maintaining normal calcium levels within the insulin-producing beta-cells [8]. Decreased levels of VitD in people with diabetes have been associated with more rapid progression of atherosclerosis, i.e. “hardening of the arteries”[9].

Clinical trials of VitD replacement in diabetes are few. Borissova et al. administered just over 1000 IU per day to diabetic patients over one month; 70% of diabetic patients were VitD deficient at the beginning of the study [10]. Significantly improved insulin response at meals was observed. Case reports have described beneficial results including improved glucose tolerance and increased beta cell function [11].


How Common Is Vitamin D Deficiency?

VitD deficiency is very common, especially during the winter months and in northern states. Current estimates suggest the prevalence of VitD deficiency in the general population is between 36-57% with estimates in people with diabetes range from 34-71% [1, 9, 10].


How Do I Find Out If I Have Vitamin D Deficiency?

Blood tests for VitD are possible and “should” be covered by medical insurance when ordered by your physician. Laboratory assessment of VitD status should be performed by measuring 25-Vit D. 1,25-Vit D is very active in the body and it is removed from the body quickly. Current literature suggests a value of 32 mg/L (80 nmol/L), however many clinical laboratories have not modernized their normal ranges[12, 13]. If you have your VitD levels checked, ask your doctor for your value and compare it the recommendation above; feel free to show your doctor this article and ask him/her to read the references below. VitD replacement beyond a suggested optimal level of 32mg/L (80 nmol/L) cannot be recommended.


Sounds Like I Need More Vitamin D. Why Not Just Supplement?

Vitamin D is a very active compound and taking too much can cause damage. Therefore I always recommend patients have their blood levels checked before supplementing and monitor regularly once replacement therapy has started. Taking too much VitD can dangerously elevate calcium, causing heart palpitations and lead to kidney damage. Reviews of the safety of vitamin D3 supplementation in the medical literature suggest doses less than 4,000 IU per day are not known to produce adverse effects, but safe long term doses have not been well studied.


Take an Active Role

Vitamin D deficiency is common in the general population and perhaps more common in people with diabetes. Given the now recognized role of vitamin D in cancer prevention, insulin production in diabetes, high blood pressure and many other health conditions, ask your doctor to check your vitamin D status and consider guided supplementation if you are deficient. Always work with your conventional physician, licensed naturopathic physician or licensed complementary health provider to ensure regular testing and response to replacement therapy.

Ryan Bradley, ND, MPH     March, 2007



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3.   Partonen, T., Vitamin D and serotonin in winter. Med Hypotheses, 1998. 51(3): p. 267-8.

4.   Partonen, T., Extrapineal melatonin and exogenous serotonin in seasonal affective disorder. Med Hypotheses, 1998. 51(5): p. 441-2.

5.   Garland, C.F., et al., The role of vitamin D in cancer prevention. Am J Public Health, 2006. 96(2): p. 252-61.

6.   Holick, M.F., Vitamin D: its role in cancer prevention and treatment. Prog Biophys Mol Biol, 2006. 92(1): p. 49-59.

7.   Mathieu, C., et al., Vitamin D and diabetes. Diabetologia, 2005. 48(7): p. 1247-57.

8.   Chiu, K.C., et al., Hypovitaminosis D is associated with insulin resistance and beta cell dysfunction. Am J Clin Nutr, 2004. 79(5): p. 820-5.

9.   Targher, G., et al., Serum 25-hydroxyvitamin D3 concentrations and carotid artery intima-media thickness among type 2 diabetic patients. Clin Endocrinol (Oxf), 2006. 65(5): p. 593-7.

10.  Borissova, A.M., et al., The effect of vitamin D3 on insulin secretion and peripheral insulin sensitivity in type 2 diabetic patients. Int J Clin Pract, 2003. 57(4): p. 258-61.

11.  Kumar, S., et al., Improvement in glucose tolerance and beta-cell function in a patient with vitamin D deficiency during treatment with vitamin D. Postgrad Med J, 1994. 70(824): p. 440-3.

12.  Heaney, R.P., Functional indices of vitamin D status and ramifications of vitamin D deficiency. Am J Clin Nutr, 2004. 80(6 Suppl): p. 1706S-9S.

13.  Vieth, R., Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Am J Clin Nutr, 1999. 69(5): p. 842-56.