New study assesses increased risk of type 2, most beneficial method of raising vitamin D levels.
Overall, 77% of US adults have been found to have vitamin D deficiency, defined as less than 32 ng/mL, with the prevalence doubling since 1980.
For the purposes of a recent study, participants from the Rancho Bernardo Study of Health Aging in California were gathered to examine the association between 25(OH)D levels and incidence of diabetes or prediabetes in a cohort with an unusually high median vitamin D concentration. Specifically, they included 903 primarily older, middle-income, community-dwelling white adults without a history of diabetes.
Researchers looked at more than 900 individuals and found that having plasma levels of 25-hydroxyvitamin D greater than 30 mg/mL was associated with a significant and substantial reduction in later diabetes risk.
It was found that participants with blood levels of 25-hydroxyvitamin D that were above 30 ng/mL had one third of the risk of diabetes, and those with levels above 50 ng/mL had one fifth of the risk of developing diabetes (compared with those whose levels were < 30 ng/mL).
The research also showed that every 10 ng/mL increase in 25(OH)D levels above 30 ng/mL was associated with a 36% reduction in diabetes risk. However, it was emphasized that the epidemiological nature of the study means the findings are not able to demonstrate causality.
Further research is needed on whether high 25-hydroxyvitamin D levels might prevent type 2 diabetes or the transition from prediabetes to diabetes. But this paper and past research indicate there is a strong association.
In their review of the article, they noted that higher plasma 25(OH)D levels have previously been associated with a lower risk of type 2 diabetes. However, the results to date have been mixed, and no adequate data based on a cohort are available for the high end of the normal range, above approximately 32 ng/mL (80 nmol/L).
Researchers explain that the cohort may have a lower than usual prevalence of vitamin D deficiency because of year-round sunshine and good weather in a sunny and clear area of southern California, where the study was done and because of a higher standard of education, and a greater socioeconomic status and proportion of whites.
The cohort has the highest known published median 25(OH)D concentration — 42 ng/mL (105 nmol/L) in men and 39 ng/mL (98 nmol/L) in women — of any population that has reported data on diabetes incidence by vitamin D levels.
At baseline (1997–1999), participants underwent anthropomorphic assessments and completed standardized questionnaires on current medications, cigarette smoking, alcohol consumption, physical exercise, and any history of myocardial infarction, stroke, angina pectoris, or peripheral claudication. In addition, plasma 25(OH)D levels were measured. Blood samples were then collected every two years for 8-h fasting plasma glucose (FPG) testing, with an oral glucose tolerance test performed if the 8-h FPG level was ≥ 100 mg/dL (5.5 mmol/L). Mean age was 74.1 years and an average BMI of 25.4 kg/m2.
Over a mean follow-up of 12.5 years, 47 individuals developed incident type 2 diabetes, defined as an 8-h FPG ≥ 126 mg/dL (7.0 mmol/L). A further 337 developed prediabetes, defined as an 8-h FPG 100–125 mg/dL (5.5–6.9 mmol/L).
The association of 25(OH)D with diabetes persisted after the exclusion of participants who reported at baseline that they usually took vitamin D or calcium supplements.
On multivariate analysis, the team found that increased plasma 25(OH)D levels were associated with a reduced risk of diabetes. Compared with a plasma 25(OH)D level < 30 ng/mL, individuals with levels of 30–39 ng/mL had a hazard ratio (HR) of developing incident type 2 diabetes of 0.31, which decreased to an HR of 0.29 among those with levels of 40–49 ng/mL, and an HR of 0.19 in those with levels ≥ 50 ng/mL (P = .005 for trend).
The association between plasma 25(OH)D levels and prediabetes was, in contrast, weak and nonsignificant.
The findings for this study are in contrast with those of a recent well-designed multicenter cohort study that found no association between plasma 25(OH)D levels and risk of diabetes (Diabet Med.2014;31:564-569). A possible explanation for these seemingly disparate findings, they say, is that the mean 25(OH)D concentration in that study was 23 ng/mL (58 nmol/L), as participants came from much more northern latitudes of the United States, compared to 42 ng/mL (105 nmol/L) in the present study.
- In the current study, researchers assumed the minimum healthy plasma level of 25(OH)D to be 30 ng/mL, which is 10 ng/mL above the level recommended in 2010 by the US Institute of Medicine.
- The study does not solve the basic question of whether individuals may need to seek vitamin D supplementation if needed to maintain a concentration of 30 ng/mL (75 nmol/L).
- The researchers indicated that sufficient 25(OH)D levels obtained naturally from sunlight and food, not supplementation, might be more relevant to reduce diabetes risk.