Last summer, an Indian team reported positive results from a small but well-designed trial testing the short-term effect of vitamin D3 supplements on insulin sensitivity in 71 healthy, middle-aged men with beer bellies… the so-called “central obesity” linked to higher diabetes risk.
Insulin resistance—also known as low insulin sensitivity—is a key diabetes risk factor.
The term insulin resistance means a reduced ability of your cells to respond to “absorb more blood sugar” signals communicated by insulin.
In other words, cells become “insensitive” to insulin released by the pancreas in response to a rise in blood sugar due to digestion of foods… especially carbohydrates.
As the authors wrote, “The trial indicates that vitamin D(3) supplementation improves postprandial [after-a-meal] insulin sensitivity in apparently healthy men likely to have insulin resistance... (Nagpal J et al. 2009).
And this month, a team from New Zealand's Massey University published similar, positive results from another clinical trial.
New Zealand trial yields encouraging signs
The study involved 81 South Asian women with insulin resistance and average (i.e., low) vitamin D levels, aged between 23 and 68.
Like the Indian trial, this clinical study was high quality (randomized, placebo-controlled, double-blind).
They were randomly assigned to receive either 4,000 IU of vitamin D3 or placebo daily for six months.
At the end of the test period, the vitamin D group experienced “significant improvements” in both insulin sensitivity and resistance, said the researchers.
These benefits were accompanied by two other good things: a decrease in fasting insulin levels compared to placebo, and no increase in insulin secretion.
The optimal effects were observed when blood vitamin D levels fell between 80 to 119 nanomoles per liter (nmol/L), which is much higher than the American average.
At the outset of the study, the women's D levels were about 50 nmol/l, or similar to the average American level (60 nmol/l).
As the researchers wrote, these results provide “…further evidence for an increase in the recommended adequate levels.”
How much is enough?
Most researchers involved in vitamin D studies recommend minimum blood levels ranging from 90 to 120 nmol/L (36 to 48 ng/mL)… which is the range the new diabetes study found helpful.
But as we said, the average American's vitamin D level is a mere 60 nmol/L or less, and the U.S RDAs (200 to 600 IU) are proven unable to raise blood levels into this range.
Though the current, official safe upper intake limit is only 2,000 IU per day, vitamin D intake is proven safe at daily levels some 25 times the current RDA for adults 51 to 70: that is, 10,000 IU per day vs. only 400 IU.
Most experts recommend taking at least 1,000 IU via food and supplements, and prefer an intake of 2,000 IU to 4,000 IU per day... unless most of your skin is exposed to 20 to 30 minutes of strong sunlight per day, which reduces the need for oral vitamin D.
Note: Darker skinned people, whose greater amount of skin pigment blocks the UV rays that make vitamin D, need either more sun exposure or more dietary vitamin D.
Leading vitamin D researcher Ronald Vieth, M.D., notes that normal human blood levels of vitamin D extend above 200 nmol/L or 80 ng/mL (Vieth R 2004).
Pittas AG, Lau J, Hu FB, Dawson-Hughes B. The role of vitamin D and calcium in type 2 diabetes. A systematic review and meta-analysis. J Clin Endocrinol Metab. 2007 Jun;92(6):2017-29. Epub 2007 Mar 27. Review.
von Hurst PR, Stonehouse W, Coad J. 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. Br J Nutr. 2009 Sep 28:1-7. [Epub ahead of print]
Nagpal J, Pande JN, Bhartia A. 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. Diabet Med. 2009 Jan;26(1):19-27.