Vitamin D is no ordinary nutrient.
Unlike any other vitamin, it acts as a hormone, with unusually broad effects in the body.
This explains why so many studies suggest that it's crucial to immunity, heart health, bone health, and much more.
Population studies suggest that it plays a role in diabetes prevention, and a small amount of clinical research supports that idea.
But some doubt remains, since it's not been clear whether (or how) vitamin D aids blood-sugar control … a major factor in diabetes prevention.
Given that mystery, and the shortage of clinical evidence, the outcomes of two new studies – one in rodents, and one in people – seem pretty significant.
Both studies were presented this past weekend, at the International Congress of Endocrinology and the Endocrine Society meeting in Chicago.
How much vitamin D is enough?
Most researchers involved in vitamin D studies recommend minimum blood levels ranging from 36 to 48 ng/mL.
The U.S. Institute of Medicine's Food and Nutrition Board says that vitamin D intakes of 5,000 IU/day raised vitamin D levels to 40–60 ng/mL but no higher.
But the average American's vitamin D level is a mere 24 ng/mL or less, and the U.S RDA for children and adults (600 IU) are proven unable to raise blood levels into the adequate range (36 to 48 ng/mL).
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 80 ng/mL (Vieth R 2004).
Vitamin D pills deterred pre-diabetes patients' downward spiral to the disease
Adding to the small body of clinical research, scientists from Calcutta, India report that vitamin D supplements aided people diagnosed with pre-diabetes.
The trial was led by Deep Dutta, MD, whose team recruited 136 people with poor blood sugar control for a two-year trial.
The volunteers were divided into two groups:
People with vitamin D levels at or below 30 ng/ml (inadequate)
People with vitamin D levels above 30 ng/ml (barely adequate or better)
Most vitamin D researchers recommend minimum blood levels ranging from 36 to 48 ng/ml.
The U.S. Institute of Medicine's Food and Nutrition Board notes that vitamin D levels below 30–48 ng/mL are associated with increases in death, cancer, adverse cardiovascular events, and falls or fractures among the elderly.
The people with low vitamin D levels (at or below 30 ng/ml) were then randomly assigned to one of two groups:
The participants with vitamin D levels above 30 ng/ml only took calcium supplements.
Encouragingly, the participants given high-dose vitamin D enjoyed reduced insulin resistance, as well as lower levels of pro-inflammatory immune system chemicals (TNF-alpha, IL-6, and CRP) associated with diabetes.
The high-dose vitamin D group also showed less progression from pre-diabetes to diabetes, and a greater return to normal blood sugar levels.
Dr. Dutta expressed his team's conclusion simply: “Our experience suggests that vitamin D supplementation was beneficial in improving glycemic [blood sugar control] outcomes.”
Rat study reveals vitamin D's effects no appetite and blood sugar
In a new lab study, when fat rats were given vitamin D, it acted in their brains to improve weight and blood glucose (sugar) control.
Lead author Stephanie Sisley, MD hails from Houston's Baylor College of Medicine.
As she said, “The brain is the master regulator of weight. Vitamin D deficiency occurs often in obese people and in patients with Type 2 diabetes, yet no one understands if it contributes to these diseases. Our results suggest that vitamin D may play a role in the onset of both obesity and Type 2 diabetes by its action in the brain.”
A region of the brain called the hypothalamus controls both weight and glucose, and has vitamin D receptors there.
In this study – funded by the U.S. National Institutes of Health – Dr. Sisley and colleagues from the University of Cincinnati delivered vitamin D directly to the animals' hypothalamus.
They administered the active, potent form – vitamin D3 – to obese male rats through a cannula (thin tube) surgically inserted (using anesthesia) into a narrow cavity within the hypothalamus.
The animals received nothing to eat for four hours (for a fasting blood sugar test), and were divided into two groups:
Test group - 12 rats received vitamin D dissolved in a solution.
Control group - 14 rats of the same body weight got only the solution, with no vitamin D.
One hour later, all rats had a glucose tolerance test, in which they received an injection of sugar (dextrose) in their abdomen, followed by measurement of their blood sugar levels.
Compared with the control rats, the animals that received vitamin D had improved glucose tolerance … a key measure of how well the body responds to dietary sugar.
The treated rats also had greatly improved insulin sensitivity … a key measure of the body's ability to use insulin to absorb blood sugar (glucose) into its cells.
Loss of this ability is called insulin resistance, which eventually leads to chronically high blood sugar – a defining feature of pre-diabetes – which can lead to diabetes.
Two of insulin's main effects are to clear glucose from the bloodstream and lower glucose production in the liver. In this study, adding vitamin D to animals' brains decreased the amount of glucose produced by the liver.
In a separate experiment, the researchers gave three rats vitamin D and four rats the D-free “placebo” solution for four weeks.
Compared with the rats that did not get vitamin D, the D-treated rats showed big drops in food intake and weight.
Over 28 days, the D-treated group ate nearly three times less food and lost 24 percent of their weight … despite not changing the way they burned calories.
In contrast, the D-free control group did not lose any weight.
“Vitamin D is never going to be the silver bullet for weight loss, but it may work in combination with strategies we know work, like diet and exercise,” Sisley commented.
Wisely, she called for more research, to determine whether obesity alters vitamin D transport into the brain or its effects in the brain.
Dutta D et al. Vitamin-D Supplementation in Prediabetes Reduced Progression to type2 Diabetes through Decreased Insulin Resistance and Systemic Inflammation: An Open Label Randomized Prospective Study from Eastern India. Poster Preview and Oral Session OR13-5; Saturday, June 21. International Congress of Endocrinology and the Endocrine Society ICE/ENDO 2014, Chicago, IL
Sisley S et al. CNS Vitamin D Improves Glucose Tolerance, Hepatic Insulin Sensitivity, and Reverses Diet-Induced Obesity. Poster Preview and Oral Session OR13-1; Saturday, June 21. International Congress of Endocrinology and the Endocrine Society ICE/ENDO 2014, Chicago, IL