Vitamin D and Gestational Diabetes

Maternal Plasma 25-Hydroxyvitamin D Concentrations and the Risk for Gestational Diabetes Mellitus
Original source: PLoS ONE 3(11): e3753
Authors: Cuilin Zhang (corresponding author), Chunfang Qiu, Frank B. Hu, Robert M. David, Rob M. van Dam, Alexander Bralley, Michelle A. Williams
Publication date: November 18, 2008
DOI (digital document ID): 10.1371/journal.pone.0003753
Requires subscription? No
Publicly-available abstract: Yes

Synopsis:
There is increasing evidence that Vitamin D plays a role in maintaining stable blood glucose levels, but the studies directly examining circulating levels of 25-hydroxyvitamin D (25-[OH] D) are limited and inconsistent. This study aims to determine the association between levels of (25-[OH] D) earlier in pregnancy than previously examined, and the risk of the mother's developing gestational diabetes later on in the pregnancy.

A nested case-control study was conducted among 953 pregnant women. 57 had diabetes at the start of the study, and 114 women who were not diagnosed with GDM were selected as controls.

Blood samples were taken at 16 weeks gestation, with known time since last meal, and frozen at -80C until analysis. Per American Diabetes Association guidelines, participants were screened for gestational diabetes at 24-28 weeks gestation, using fasting blood glucose and an oral glucose tolerance test. The process used to measure 25-[OH] D levels produces a maximum 12% probability of error.

The results were categorized as sufficient, insufficient, or deficient.

The odds ratios were adjusted based on mother's age at conception, season of conception, diet, vitamin supplementation, mother's pre-conception body mass index, family history of Type 2 diabetes, physical activity, and whether or not the mother smoked.

The women who developed gestational diabetes tended to follow the known risk patterns: overweight, older, being non-white or Latino, and having a family history of diabetes. The heavier these women were, the lower their circulating levels of 25-[OH] D.

At 16 weeks' gestation, 25-[OH] D levels of the women who went on to develop gestational diabetes were, on average, 20% lower than those of the controls. This difference remained even when filtering out the known and suspected risk factors for diabetes. About one third of the women who developed diabetes, versus one seventh of the women who did not, had 25-[OH] D levels considered to be deficient.

Further analysis showed an inverse relationship between vitamin D levels and risk of developing gestational diabetes was observed and remained even after considering all the factors mentioned above.

Reliability:
The number of women studied was moderately large and restricted to women who sought prenatal care prior to the halfway mark in the pregnancy. The study was also limited to women who read and wrote English, and who planned to deliver at one of two study hospitals. This may or may not filter out less-affluent women in higher-risk populations.

A 12% error in the assay method may skew the results, despite statistical attempts to minimize them.

The authors submit that a single blood test for 25-[OH] D levels may not provide an accurate picture of the mother's levels of Vitamin D throughout the pregnancy; also, that some of the 25-[OH] D deficient women may have developed impaired glucose tolerance or gestational diabetes earlier in the pregnancy (but had not been screened for it). The authors would like to see a much larger study conducted to confirm (or refute) their findings.

Practical Applications:
If a firm relationship between 25-[OH] D and gestational diabetes (or diabetes in general) is confirmed, then lifestyle changes and dietary supplementation can be advised earlier in the pregnancy (or pre-pregnancy) to minimize the risks of developing gestational diabetes.

This study also contributes to the general body of research regarding Vitamin D metabolism and diabetes in general. There may be good reasons for people with diabetes, or at risk for developing diabetes, to screen for Vitamin D levels and to increase Vitamin D intake. [Please note that Vitamin D is a fat-soluble vitamin and that excess intake is not easily excreted from the body, so please consult your physician before adding additional vitamin D supplements to your diet.]


This synopsis was based on the entire article.