While it is yet to be fully understood how vitamin D helps the body regulate blood glucose, there seem to be several ways. Because vitamin D is present in the insulin-producing beta cells, and insulin secretion is calcium dependent (and therefore indirectly vitamin D dependent), it is theorized that vitamin D has a direct effect on how much insulin the body makes. Both in vitro studies (studies done on cells in a laboratory) and animal studies have shown that vitamin D deficiency impairs insulin secretion and that correcting vitamin D status restores the function.
Furthermore, when insulin is released, it must make contact with insulin receptors on cells to lower blood glucose. Other laboratory studies have shown that vitamin D may help the body produce and activate these insulin receptors.
Finally, diabetes and other chronic conditions are associated with increased levels of inflammation throughout the body, which may be the root of some of their long-term complications such as atherosclerosis (hardening of the arteries). Vitamin D and calcium are both shown to reduce the production of inflammatory chemicals in the body called cytokines.
With so much evidence of a role for vitamin D in regulating blood glucose levels, the obvious question is whether vitamin D can prevent, cure, or treat diabetes.
Several large observational studies have shown that vitamin D intake might prove to be a preventative factor in diabetes. For example, a study of over 10,000 Finnish children showed that taking supplements of 2,000 IU per day of vitamin D (5 times the current US recommendation) resulted in an 80% reduction of risk of Type 1 diabetes. Further strengthening the notion of this relationship, the children in the study who had suspected rickets had the highest rates of Type 1 diabetes. Although this magnitude of risk reduction sounds amazing, remember that Finland gets relatively little sunlight due its northern latitude (so people may need more vitamin D from food and/or supplements). Also, because this was only an observational study, one cannot assume this tremendous risk reduction from supplements for all children.
Apparent risk reduction has also been shown with Type 2 diabetes. In 2006, the journal Diabetes Care published a report from the Nurse’s Health Study, which followed over 80,000 women for 20 years. Women in the study who had both a calcium intake of more than 1,200 mg and a vitamin D intake of more than 800 IU had a 33% lower risk of developing Type 2 diabetes.
These are promising reports, but to prove that vitamin D helps prevent diabetes, clinical trials, or studies on humans, are still needed. Clinical trials will show whether the relationship between vitamin D and diabetes is direct or indirect. In other words, they will determine whether it’s vitamin D deficiency or some other factor, such as genetics, that causes the high rate of Type 1 diabetes among Finnish children. They will also determine whether high levels of vitamin D protect women from Type 2 diabetes, or whether women who lead the types of healthy lifestyles that help prevent Type 2 diabetes (getting lots of outdoor activity, for example) just happen to have higher levels of vitamin D.
So far, it doesn’t appear that diabetes can be cured with vitamin D supplementation. The few intervention studies in which vitamin D has been administered in hopes of reversing diabetes have not been successful.
But what about using vitamin D to help manage diabetes? Observational research shows that blood glucose levels rise in the winter, which is when vitamin D levels fall. This doesn’t prove cause and effect, since many factors could contribute to higher blood glucose during the winter, but it raises the possibility of a connection. The few human intervention studies that have been done to try to establish cause and effect have been inconclusive. But most have been done over short periods with only small numbers of subjects, and some critics believe the amounts of vitamin D taken in the studies were too low to make a difference.