This blog tracks updates to the Blood Sugar 101 Web site.

Monday, October 4, 2010

More Evidence that Vitamin D Supplementation Does NOT Lower Blood Sugar, IR or Improve Endothelial Function.

Page Changed: Helpful Supplements for People with Diabetes

This result [that supplementing with large infusions of Vitamin D3] was duplicated in a second placebo-controlled study of 61 participants given placebo, 100,000 IU or 200,000 IU of Vitamin D3.
Though the Vitamin D made a very small difference in their blood pressure (which was still too high after supplementation) the study found
here was no significant difference in the primary outcome of endothelial function at 8 weeks (placebo 5.2%, n=22; 100,000 IU 4.3%, n=19; 200,000 IU 4.9%, n=17) or at 16 weeks. Insulin resistance and glycosylated haemoglobin did not improve with either dose of vitamin D3.
The effect of different doses of vitamin D3 on markers of vascular health in patients with type 2 diabetes: a randomised controlled trial.M. D. Witham et al. Diabetologia Volume 53, Number 10, 2112-2119, DOI: 10.1007/s00125-010-1838-1


Julia said...

100,000 IUs over 8 weeks is not much! Your body can produce 20,000 to 50,000 IUs every day when exposed to UV B light.

Jenny said...

Julia, The missing piece is that they measured the people's vitamin D blood levels at the last measurement of the other parameters and there was no difference in the other measurements between those who had normal blood levels of Vitamin D and those that didn't.

The big shots did normalize blood levels in a significant number of people in this study because they aren't metabolized the same way that sunshine-made Vitamin D is.

Dan said...

This study doesn't mean anything. First of all, this study only addresses Vitamin D supplementation with single dose replacement therapy which is a stupid idea to begin with. Secondly, the study time of only 16 weeks may very well be too short to capture the blood sugar regulating effects of Vitamin D. Just because the 25-hydroxy Vitamin D level increased doesn't mean that the effects of long-term Vitamin D deficiency or years of suboptimal levels are rapidly reversed with supplementation.
Thirdly, the idea of studying doses of Vitamin D instead of supplementing to achieve certain 25-hydroxy Vitamin D levels is stupid. A better study would have supplemented patients with a steady, daily dose for 3 months, measured their 25-hydroxy Vitamin D level, and then kept the dose steady for another 3 months, measuring A1C, 25-hydroxy Vitamin D, and more at both 0, 3, and 6 months.
I've read much more solid data supporting Vitamin D's role in glucose metabolism and showing a benefit of Vitamin D supplementation in improving insulin resistance. Your advice on diabetes is strong, but you seem to have it out for Vitamin D, or at least that's my interpretation of the data. There's negative and positive data for everything, and on some level we're all victims of the confirmation bias, but this study is especially weak, and it is foolish to base any conclusions upon it.

Jenny said...


There is NO solid evidence that supplementing with Vitamin D improves insulin resistance or blood sugar control. I have put links to all the studies I could find on the page that discusses Vitamin D, and all that the studies show consistently is that low Vitamin D levels are associated with Diabetes and other conditions. Attempts to relieve the symptoms with supplementation have not worked.

Anectodatally, people who measure their blood sugar carefully report that there may be a transient effect on insulin resistance for a week or so when it is first started, but it fades out very quickly.

Dan said...

Hi Jenny,
Thanks for the thoughtful reply. The big problem with Vitamin D deficiency is that no one really knows what is the optimal blood range. Most studies out there still consider >30 ng/ml to be sufficient. It is clearly established that levels greater than 32 ng/ml are needed just to prevent secondary hyperparathyroidism which causes bone to be broken down to liberate calcium for its role as a cation in the blood, but the level at which serum Vitamin D may positively impact insulin resistance is far less certain. Unfortunately, good studies supplementing to levels of at least 45ng/ml are lacking. I agree the data showing an association between low Vitamin D and Type II diabetes is strong whereas the data showing a benefit to supplementation is weak, although there is some data out there:, It's my belief that the supportive data just doesn't exist yet because some are still nonsensically using Vitamin D2, some rely on megadoses of D3, most supplement to a still suboptimal range, most don't use a GTT to assess insulin resistance, and most studies don't follow their patients long enough to capture positive effects. My read of the data is that given what is currently known, Type II diabetics should supplement with Vitamin D to achieve blood levels of at least 40 ng/ml. I think the benefits outweigh the risks. There's pretty solid data showing a 7% reduction in all-cause mortality from supplementation of 700-2000 IU of Vitamin D3 per day (, so why not?

Jenny said...

The WHY NOT is because high supplementation with Vitamin D causes some serious problems, for example Milk Alkali Syndrome because it can mess up calcium metabolism.

This is a real concern. I ended up with elevated calcium levels after two years of supplementing with Vitamin D at the 2000 unit daily intake level.

Because we don't understand the underlying processes it is irresponsible to load our bodies with supplemental chemicals that may be having adverse effects it will take years to understand.

Nigel Kinbrum said...

A couple of comments.

1) Using large, intermittent doses of Vit D3 is probably a bad idea as most of the time, serum 25(OH)D level is reducing. See Vitamin D and UV fluctuations.

2) There are RCTs showing that Vit D3 reduces IR & postprandial glucose and improves endothelial function. See Vitamin D supplementation reduces insulin resistance in South Asian women living in New Zealand who are insulin resistant and vitamin D deficient – a randomised, placebo-controlled trial , A double-blind, randomized, placebo-controlled trial of the short-term effect of vitamin D3 supplementation on insulin sensitivity in apparently healthy, middle-aged, centrally obese men and Effect of vitamin D deficiency and replacement on endothelial function in asymptomatic subjects.

I do agree with you that serum 25(OH)D & Calcium should be monitored as some people do suffer adverse effects on 4-5,000iu/day.

Jenny said...

Nigel, There are problems with that study. The measure that they saw a positive change in is not one of the common measurements used in these studies, and as they report "No changes in secondary outcome measures (insulin secretion, basal indices of insulin sensitivity, blood pressure or lipid profile) were found with supplementation."

The evidence from other larger studies with megadoses like this one is that the long term result is more broken bones (because the calcium metabolism is disturbed).

And it was a very short study. Many of us have noticed transient changes in insulin sensitivity with Vitamin D that pass in a short time and are not sustained even when blood levels of Vitamin D are quite high.

Hence what I think they saw was an short term artifact of high dose Vitamin D, not a significant change in beta cell function.

Anonymous said...

Hi Jenny, first of all, wonderful blog - kudos to you.

You write, "This is a real concern. I ended up with elevated calcium levels after two years of supplementing with Vitamin D at the 2000 unit daily intake level."

What was your D blood level?


Jenny said...

The Vitamin D level was 55.8 ng/ml.

Nigel Kinbrum said...

Jenny said...
"Nigel, There are problems with that study."
I posted links to three studies in part 2) (sorry I didn't spread them out a bit more).

Sysy Morales said...

I get a blood test to see what my vitamin D level is and that is the only time I supplement because as you said, overdosing is a very real problem and some of us supplementing increase our levels faster than others so it's wise to know what the level is beforehand.