Women reporting daily vitamin D supplementation of at least 400 IU per day before becoming pregnant were nearly 30 percent less likely to develop gestational diabetes than women who did not report taking a vitamin D supplement, according to an analysis of the Nurses’ Health Study II cohort.
In the United States, 78 percent of nonpregnant women of childbearing age have vitamin D insufficiency, defined as a 25-hydroxyvitamin D level of 75 nmol/L or less, indicating that a substantial number of women may enter pregnancy with suboptimal vitamin D status, Dr. Wei Bao, assistant professor of epidemiology at the University of Iowa College of Public Health, and colleagues wrote in the study background.
“Evidence from previous studies in animals and humans suggests that vitamin D may play a pivotal role in regulating insulin secretion and insulin sensitivity,” the researchers wrote. “In addition, improved vitamin D status by vitamin D supplementation could normalize fasting glucose levels and attenuate insulin resistance in pregnant women. Some, although not all, previous studies found that low maternal concentration of 25 – (OH)D during early pregnancy was associated with increased risk of [gestational diabetes].”
Dr. Bao and colleagues analyzed diet information from 15,225 women with 21,356 singleton pregnancies between 1991 and 2001 participating in the Nurses’ Health Study II cohort. Participants reported information regarding disease outcomes and lifestyle behaviors and completed food frequency questionnaires in 1991 and every four years, from which researchers assessed vitamin D intake from food sources. Information on the current use and doses of vitamin D supplements was collected biennially. Researchers used log-binomial models with generalized estimating equations to estimate relative risk for gestational diabetes based on vitamin D intake, adjusting for age, family history of diabetes, overall diet quality, physical activity and BMI.
During 10 years of follow-up, researchers observed 865 incident cases of gestational diabetes in the cohort, as well as an inverse association between supplemental vitamin D intake and gestational diabetes risk.
Compared with women who did not report taking daily supplemental vitamin D before pregnancy, those who reported taking at least 400 IU per day were nearly 30 percent less likely to develop gestational diabetes once pregnant (RR = 0.71; 95 percent CI, 0.56 – 0.9). Women reporting daily vitamin D supplementation between one IU and 399 IU also saw a reduced risk for developing gestational diabetes (RR = 0.8; 95 percent CI, 0.67 – 0.96). Results persisted after adjustment for age, parity, race, family history of diabetes, diet and lifestyle factors, and BMI.
Researchers also observed an inverse relationship between dietary and total vitamin D intake and gestational diabetes risk, but this association did not reach statistical significance. Other risk factors for gestational diabetes, including age, family history of diabetes, overall diet quality, physical activity or being overweight, did not modify the association between total, dietary and supplemental vitamin D, according to researchers.
“Our study indicates potential benefits of increasing vitamin D intake from supplements in the prevention of [gestational diabetes] in women of reproductive age,” the researchers wrote.
[Photo: Dr. Wei Bao]