Time to Think about Vitamin D - Again!


Labor Day is well behind us, and as we all begin planning our strategy to survive the busy Fall and Winter, it’s also time to reconsider our health. As we step away from the beach in favor of the fireplace, it’s important to remember that having less time outside impacts our health too: specifically it results in lower vitamin D! Over two years ago, I discussed some recent research on Vitamin D's Role in Diabetes just after the Institute of Medicine summit on the topic:

It’s now time for an update, as the vitamin D literature continues to be provocative (and unfortunately inconsistent). First, let’s recap:

“Vitamin D” is really a hormone that is produced in the skin during sunlight exposure. Although many people associate vitamin D with bone health (because it regulates calcium absorption), vitamin D regulates the function of over 200 genes and has many additional actions1 including:

  • Regulation of cell division and proliferation

  • Regulation of immune function

  • Regulation of blood pressure control in the kidney (via renin)

  • Regulation of insulin production in the pancreas

  • How do I know if I need to have my vitamin D checked?

Because of the many actions of vitamin D, the symptoms of vitamin D insufficiency and deficiency are widely varied. There are several very serious symptoms that are associated with severe vitamin D deficiency, including loss of bone mineralization, severe muscle and bone pain, and mouth, jaw and tooth pain. However, numerous more mild and non-specific symptoms of low vitamin D status also occur, and may include:

  • Fatigue

  • Depression

  • Muscle pain and fatigue

  • Bone pain

  • Loss of appetite

  • Diarrhea

  • Insomnia

  • Vision problems

  • Burning in mouth and throat

Other than staying inside, what else influences vitamin D status? Numerous factors impact vitamin D status1,2, including:

  • Using sunscreen prevents vitamin D formation

  • Body weight: Increased body weight leads to lower vitamin D due to sequestration in fatty tissue

  • Diet: Increased fortified dairy and cereals, egg yolks, fish liver, and beef liver all contain high amounts of vitamin D.

  • Physical activity: The more active, the higher vitamin D

  • Interestingly, age alone does not lead to lower vitamin D status - rather the factors above account for differences by age!


How do I know if I have enough vitamin D?

The only way to know for sure is to have a blood test to measure your blood concentrations of “25-hydroxycholecalciferol” (a storage form of vitamin D). This test can be ordered by your primary care doctor (although you may have to be convincing in your request!) or it is also available through several clinical laboratories who market tests directly to the public (these labs can be found online, although I recommend talking with your doctor first to ensure the test is performed in a lab of good quality).

If you get tested, there are several definitions to know in order to understand whether or not you need to supplement extra vitamin D. Vitamin D “insufficiency” is defined as a serum 25-hydroxycholecalciferol concentration less than 30 ng/ml, while “deficiency” is a 25-hydroxycholecalciferol concentration less than 20 ng/ml. Don’t allow your doctor to simply tell you that your results “were fine” or “within normal limits”, rather learn your actual value so that you know for certain. Also, because serum concentration tends to decline during the Fall and Winter, if you are borderline in your status entering into the Fall, chances are you will need to supplement at some point during the season!


What’s new regarding vitamin D in diabetes?

type 1 diabetes

In reviewing my 2011 article, I realized I did not include an update on the importance vitamin D in type 1 diabetes, so let’s start there…

Several studies investigating the link between vitamin D status and type 1 diabetes support a relationship between vitamin D status and risk of developing type 1 diabetes, however clinical trials of people with type 1 diabetes actually taking vitamin D are less convincing! A few highlights are below:

Increasing D3 intake during pregnancy reduced risk of Type 1 diabetes in offspring.3

2,000 IU D3/day x 1 year after birth reduced incident Type 1 diabetes by 80% over 30 years later.4

Children with vitamin D insufficiency had a 200% increased risk of developing Type 1 diabetes.4

However, one clinical trial of a vitamin D3 intervention in people with type 1 diabetes demonstrated no improvement in insulin secretion (measured by c-peptide) after 2 years of intake.5 These findings were supported by a second, very similar trial in people with type 1 diabetes taking vitamin D for 18 months showed no benefit in insulin secretion, glucose control, or dosing requirements for exogenous insulin. 6

How can we interpret these findings? Although it is too early to conclude that supplemental vitamin D offers no benefit in type 1 diabetes, the short-term trials highlighted above suggest that if there are benefits, they are not due to improved blood sugar control. However, the highlights above do suggest the importance of preventing vitamin D insufficiency during family planning and early pregnancy. Once again, prevention is best!

Now onto type 2 diabetes…

Since my past articles on vitamin D included discussion of the literature up through 2011 (See Complementary Corner archives from 20072009 and 2012 on vitamin D), here I’ll highlight research released since then, which has focused on several outcomes relevant to people with type 2 diabetes, including blood sugar control, inflammation, and vascular function.

Recent clinical trials of vitamin D in people with type 2 diabetes have found:

Clinical research by Harris et al. in 2012 demonstrated increased insulin secretion when participants with pre-diabetes and early type 2 diabetes were given 4,000 IU D3/day for 12 weeks; notably despite the apparent increases in insulin release, glucose tolerance after a standard oral glucose tolerance test was not improved. 7

Although supplementing 2,000 IU/day of vitamin D3 in people with type 2 and HbA1c values > 7% measured a non-significant average change of -0.4% in HbA1c, unless HbA1c was greater than 9%, in which case participants had a 1.4% reduction. 8

A clinical trial of vitamin D-fortified yogurt in people with type 2 diabetes demonstrated that those participants in the vitamin D group had significantly lower inflammation after 12 weeks (measured as reductions in c-reactive protein and pro-inflammatory immune messengers, called cytokines). 9

A randomized clinical trial in type 2 diabetes published in 2012 by Witham et al. sought to determine if there was a dose response to vitamin D3 on biomarkers of blood sugar control, vascular function and circulation to the heart itself by comparing placebo, to 100,000 IU and 200,000 IU of vitamin D3 administered as a single dose. Their findings suggested no differences in blood sugar control or insulin resistance, but demonstrated improved heart perfusion and blood pressure. 10


What is the take home message?

Vitamin D expert Dr. Robert Heaney captures the take home message best in the title of his recent review article: “Health is better at serum levels [25-hydroxycholecalciferol] above 30 ng/ml.”! 11 As a medical and scientific community, we don't understand all the reasons why health status declines with lower vitamin D status, and the proverbial “jury” is still out regarding whether low vitamin D status represents poorer health status because people of poorer health status tend to be outside less, be less active, be more overweight, and have diets that are lower in vitamin D… or if there is a true biological effect of vitamin D that improves long-term health status and reduced disease risk. However, experimental evidence is starting to demonstrate at least 2 (maybe 3!) likely mechanisms of health improvement: immune regulation (which may account for lower type 1 diabetes risk in children when moms take vitamin D… and may account for reduced inflammation) and improved vascular functioning, including increased dilation of vessels resulting in lower blood pressure and increased blood flow.

Without question, the vitamin D saga will continue. More research and replication research is necessary before we really “know” the true health effects of vitamin D, and to determine optimal blood concentrations for human health.

Regardless of whether or not you choose to have your vitamin D measured, or take supplemental vitamin D, remember the best way to ensure your vitamin D levels are adequate has nothing to do with supplements and lab tests, but your health behaviors, including staying physically active outdoors, maintaining a close to ideal body weight, using sunscreen sparingly during seasons of low sunlight intensity, and maintaining a healthy diet that includes sources of vitamin D!

In health, Ryan Bradley, ND, MPH

November, 2013



  1. Holick MF. Vitamin D Deficiency. N Eng J Med 2007;357:266-81.

  2. Jorde R, Sneve M, Figenschau Y, Svartberg J, Waterloo K. Effects of vitamin D supplementation on symptoms of depression in overweight and obese subjects: randomized double blind trial. J Intern Med 2008;264:599-609.

  3. Chiu KC, Chu A, Go VL, Saad MF. Hypovitaminosis D is associated with insulin resistance and beta cell dysfunction. Am J Clin Nutr 2004;79:820-5.

  4. Hypponen E, Laara E, Reunanen A, Jarvelin MR, Virtanen SM. Intake of vitamin D and risk of type 1 diabetes: a birth-cohort study. Lancet 2001;358:1500-3.

  5. Bizzarri C, Pitocco D, Napoli N, et al. No protective effect of calcitriol on beta-cell function in recent-onset type 1 diabetes: the IMDIAB XIII trial. Diabetes care 2010;33:1962-3.

  6. Walter M, Kaupper T, Adler K, Foersch J, Bonifacio E, Ziegler AG. No effect of the 1alpha,25-dihydroxyvitamin D3 on beta-cell residual function and insulin requirement in adults with new-onset type 1 diabetes. Diabetes care 2010;33:1443-

  7. Harris SS, Pittas AG, Palermo NJ. A randomized, placebo-controlled trial of vitamin D supplementation to improve glycaemia in overweight and obese African Americans. Diabetes, obesity & metabolism 2012;14:789-94.

  8. Soric MM, Renner ET, Smith SR. Effect of daily vitamin D supplementation on HbA1c in patients with uncontrolled type 2 diabetes mellitus: a pilot study. Journal of diabetes 2012;4:104-5.

  9. Shab-Bidar S, Neyestani TR, Djazayery A, et al. Improvement of vitamin D status resulted in amelioration of biomarkers of systemic inflammation in the subjects with type 2 diabetes. Diabetes Metab Res Rev 2012;28:424-30.

  10. Witham MD, Dove FJ, Dryburgh M, Sugden JA, Morris AD, Struthers AD. The effect of different doses of vitamin D(3) on markers of vascular health in patients with type 2 diabetes: a randomised controlled trial. Diabetologia 2010;53:2112-9.

  11. Heaney R. Health is better at serum 25(OH)D above 30 ng.ml. J Steroid Biochem Mol Biol 2012.