A recent meta-analysis of randomized controlled trials discovered that vitamin D supplementation helped control glycemic response and improved insulin sensitivity in individuals with type II diabetes.
Type II diabetes has become an increasing health concern over recent years, as overweight and obesity rates have skyrocketed. Furthermore, approximately 57% of healthcare expenditures are due to diabetes and related events in North America each year.
Diabetes is considered to be a major risk factor for many adverse health conditions and diseases. According to the available research, type II diabetes contributes to:
- 30% of stroke
- 40% of heart attacks
- 50% of kidney failure requiring dialysis
- 70% of non-traumatic lower limb amputations
While poor diet, lack of exercise and genetics are the main risk factors for developing type II diabetes, evidence also suggests that vitamin D levels plays a role in the metabolic status of type II diabetes patients. Research indicates that vitamin D deficiency is associated with abnormal glucose metabolism, decreased insulin sensitivity and overall risk of developing type II diabetes. However, some of the available evidence on the effect of vitamin D supplementation on glycemic control in those with type II diabetes is conflicting. Therefore, researchers from this meta-analysis decided to explore this relationship.
A total of 23 RCTs and 1,477 individuals were included in this analysis. Studies were included if they were randomized controlled trials, analyzed a type II diabetic population, supplemented participants with either daily or weekly doses of vitamin D for at least two months and measured 25(OH)D status.
Only 11 of the studies included subjects who were vitamin D deficient at baseline of the study, six of the studies administered supplementation weekly rather than daily and five studies included calcium supplementation along with vitamin D. Some common markers of metabolic status that were measured included:
- HbA1c: an indicator of blood glucose control – all studies measured HbA1c.
Fasting plasma glucose (FPG): measures the amount of glucose present in the bloodstream during a fasted period – 21 of the 23 studies measured FPG.
HOMA-IR: measures insulin sensitivity or resistance (i.e. how well insulin functions in the presence of glucose) – 12 of the 23 studies measured HOMA-IR.
This is what the researchers found:
- Vitamin D supplementation was associated with decreased HbA1c (95% CI: -0.45 to -0.15, p < 0.001). This was even more significant in those who also supplemented with calcium (p = 0.01).
- Vitamin D supplementation significantly reduced FPG by 4.9 mg/dL on average (p = 0.003).
- Patients who supplemented with both vitamin D and calcium had an even greater reduction in FPG [(9.5 mg/dL; p = 0.04).
- Those who supplemented with vitamin D experienced a significant decreased insulin resistance compared with controls (p = 0.001).
- There was no association between HOMA-IR and dual vitamin D and calcium supplementation.
- The average vitamin D dose across all included studies was 4,074 ±2,450 IU, therefore, a minimum dose of 4,000 IU was recommended for type II diabetic patients.
The researchers concluded:
“This systematic review showed that vitamin D supplementation can improve glycemic control, through lowering HbA1c, fasting plasma glucose and HOMA-IR.”
“A minimum dose of 100 μg/d (4,000 IU/d), which brings serum 25(OH)D values to above 40 ng/mL (100 nmol/L) is recommended to improve glycemic measures in type 2 diabetic patients.”
While this study’s design was very strong, there are some limitations that must be addressed. First, there was a lack of consistency in metabolic marker measurements in each of the studies. While markers such as HbA1c were measured in all of the studies included in this analysis, HOMA-IR was only included in 12. Additionally, not all of the participants were considered to be vitamin D deficient at baseline, which makes it difficult to observe how correcting vitamin D deficiency affects type II diabetic patients. Last, there were some confounding variables that were not accounted for, such as dietary intake and sun exposure,.,m nbv which makes it difficult to interpret the results based on vitamin D supplementation alone.
Further studies that explore this relationship in a vitamin D deficient population are needed to determine if supplementation can benefit those with type II diabetes.
Peterson, R. & Cannell, JJ. Meta-analysis of RCTs presents evidence that vitamin D supplementation improves markers of type II diabetes. The Vitamin D Council Blog & Newsletter, 10/17.