Researchers suggest vitamin D protects against cardiovascular disease and insulin resistance, but can they prove it?
High doses of vitamin D may have no benefits for endothelial function among obese teens, according to the results of a new small, open-label, prospective trial.
“Our study demonstrated that once monthly treatment with 100,000 IU of vitamin D3 over a period of 3 months did not have an effect on endothelial function in obese adolescents, despite a post-treatment increase in 25(OH)D [25-hydroxy vitamin D] levels,” stated researchers from the Department of Pediatric and Adolescent Medicine, Mayo Clinic, Rochester, MN, led by senior author Seema Kumar, MD, a pediatric endocrinologist.
Endothelial dysfunction is a marker for cardiovascular disease in teens and a risk factor for the development of diabetes.
Previous observational studies have noted links between vitamin D deficiency and a host of weight-related medical complications, including cardiovascular diseases and insulin resistance.
Researchers have postulated that vitamin D might exert protective effects on the vasculature through direct and indirect effects on renal and vascular cells, as well as on mediators of inflammation and oxidative stress and calcium metabolism.
There has been a suggestion of a link between vitamin D insufficiency and endothelial activation by elevated levels of soluble vascular adhesion molecule-1 in obese white children with 25(OH)D <50 nmol L-1.
Kumar and colleagues treated 19 obese adolescents, 13–18 years of age, with 25-hydroxy vitamin D 25(OH)D levels <75 nmol L−1 with 100,000 IU vitamin D3 orally once a month for 3 months. Endothelial function was assessed by flow-mediated dilatation (FMD) of the brachial artery at study entry and 1 month after the third dose of vitamin D3.
They also obtained biochemical parameters, including calcium, fasting lipids, glucose, insulin, and high-sensitivity C-reactive protein levels.
While mean 25(OH)D levels increased from 55.9 ± 12.2 to 86.9 ± 16.7 nmol L−1 there was no correlation between 25(OH)D levels and brachial artery FMD. “The brachial artery FMD percentage did not change significantly following vitamin D3 treatment,” they state.
After 3 months of having vitamin D boosted into the normal range with supplements, the teenagers showed no changes in body weight, body mass index, waistline, blood pressure, or blood flow.
The researchers said their findings of an increase in total cholesterol following treatment of adolescents with vitamin D was not particularly surprising, and are consistent with those seen reported in adults.
“Vitamin D receptors are found ubiquitously, including in adipose tissue, and 25(OH)D plays an important role in lipid metabolism via several mechanisms including induction of an increase in lipoprotein lipase activity, increased lipogenesis and lipolysis and enhanced intestinal calcium absorption, which could reduce the formation of calcium fatty soaps in the gut and increase the absorption of fat,” they state.
Obese adolescents are often a difficult group to treat, needing multiple and higher doses of vitamin D. The good news is the study found no evidence for hypercalcemia or hypercalciuria after once monthly vitamin D3 treatment.
The researchers acknowledge the study was limited by the small number of participants and its relatively short time frame, and they call for larger, placebo-controlled studies to examine the impact of vitamin D treatment in adolescents with vitamin D deficiency and established endothelial dysfunction.