Vitamin D, Insulin Secretion, Sensitivity, and Lipids

Abstract

OBJECTIVEVitamin D deficiency is associated with an unfavorable metabolic profile in observational studies. The intention was to compare insulin sensitivity (the primary end point) and secretion and lipids in subjects with low and high serum 25(OH)D (25-hydroxyvitamin D) levels and to assess the effect of vitamin D supplementation on the same outcomes among the participants with low serum 25(OH)D levels.RESEARCH DESIGN AND METHODSParticipants were recruited from a population-based study (the Tromsø Study) based on their serum 25(OH)D measurements. A 3-h hyperglycemic clamp was performed, and the participants with low serum 25(OH)D levels were thereafter randomized to receive capsules of 20,000 IU vitamin D3 or identical-looking placebo twice weekly for 6 months. A final hyperglycemic clamp was then performed.RESULTSThe 52 participants with high serum 25(OH)D levels (85.6 ± 13.5 nmol/L [mean ± SD]) had significantly higher insulin sensitivity index (ISI) and lower HbA1c and triglycerides (TGs) than the 108 participants with low serum 25(OH)D (40.3 ± 12.8 nmol/L), but the differences in ISI and TGs were not significant after adjustments. After supplementation, serum 25(OH)D was 142.7 ± 25.7 and 42.9 ± 17.3 nmol/L in 49 of 51 completing participants randomized to vitamin D and 45 of 53 randomized to placebo, respectively. At the end of the study, there were no statistically significant differences in the outcome variables between the two groups.CONCLUSIONSVitamin D supplementation to apparently healthy subjects with insufficient serum 25(OH)D levels does not improve insulin sensitivity or secretion or serum lipid profile.

The serum 25-hydroxyvitamin D response to vitamin D supplementation is related to genetic factors, BMI, and baseline levels

“…As expected, there was a clear difference between the genotype risk quartiles regarding serum 25(OH)D but not regarding age or BMI ( Table 1). As previously published, the intake of vitamin D in the three studies was safe with no serious side effects, and in all three studies, there was a significant decrease in serum PTH after vitamin D supplementation (18,19,20).…”

“…If this window is narrow, tailoring of supplementation will be of great importance and dose adjustments have to be made. However, it is more likely that the window is rather wide as it has been difficult to show a clinical effect of increasing the serum 25(OH)D level from 50-70 nmol/l to close to 150 nmol/l (18,19,20,34,35,36 This study has some weaknesses. We pooled data from three different studies, and due to different study profiles, some heterogeneity would be expected, and only for the bone density study had we more than one measurement after starting the vitamin D supplementation.…”

“…Serum PTH was measured as described previously (18). Serum 25(OH)D was measured with an LC-MSMS method (19). This assay has a within-day precision (coefficient of variation (CV)) !3.2% and between-day precision (CV) !8.8%.…”

Vitamin D and Its Effects on Glucose Homeostasis, Cardiovascular Function and Immune Function

“…On the other hand, other studies did not find any effect of vitamin D supplementation [44,45,46]. Grimnes et al [47] could not find any effect of 6 months of 20,000 IU vitamin D 3 administrated orally twice a week to a healthy Caucasian cohort who had vitamin D levels lower than 40.3 ± 12.8 nmol/l on both insulin sensitivity and secretion. This was consistent with another study which showed that the injection of two doses of 100,000 IU vitamin D 3 did not lower fasting glucose or insulin sensitivity in a Caucasian cohort [48].…”

The Relationship between Vitamin D and Type 2 Diabetes Is Intriguing: Glimpses from the Spect-China Study

“…A cross-section study has reported that 25(OH)D was significantly decreased in group T2DM compared to the control group [14]. Another nested case-control study has found that participants with high serum 25(OH)D levels showed a higher insulin sensitivity index and a lower HbA1c than those with low serum 25(OH)D, but the difference in insulin sensitivity index was no longer significant after adjustment for confounders [15]. Therefore, whether the relationship between vitamin D deficiency and T2DM is causal or caused by confounding is uncertain.…”