BACKGROUND AND PURPOSE: 

• Previous studies on the association between vitamin D levels and gestational diabetes mellitus (GDM) have been inconsistent  
• Previous meta-analyses may have included studies that were not free of the Hawthorne effect 
  • Hawthorne effect: Participants aware of their conditions and are therefore motivated to adopt healthier behaviors such as altering their vitamin D status 
  • Also known as ‘observer effect’, such that study participants modify their behavior because they are aware of being observed  
• Zhang et al. (BJOG, 2017) sought to design a meta-analysis that would include as many relevant GDM-related indices as possible to better determine the association between Vitamin D intake and risk for GDM 

METHODS: 

• Systematic review and meta-analysis 
• Literature review included 
  • Observational studies on the relationship between vitamin D and GDM, free from Hawthorne effect 
  • Randomized controlled trials (RCT) of vitamin D supplements during pregnancy for preventing or treating GDM  
• There was lack of agreement as to the definition of vitamin D deficiency 
  • Some researchers used 50 nmol/l (20 ng/ml) and 75 nmol/l (30 ng/ml), while others used 25 nmol/l (10 ng/ml) and 50 nmol/l (20 ng/ml) to differentiate vitamin D deficiency, vitamin D insufficiency and vitamin D sufficiency 

RESULTS: 

• 87 observational studies (55,859 participants) and 25 RCTs (2,445 participants) were included  
• Low blood vitamin D level during pregnancy was associated with a higher risk of GDM (odds ratio [OR] 1.850; 95% CI 1.471-2.328) 
• Blood vitamin D level was inversely correlated with HbA1c, FPG (fasting plasma glucose), HOMA-IR (homeostasis model of assessment for insulin resistance index) and FINS (fasting insulin) 
• Vitamin D supplementation during pregnancy impacted the blood levels of 25-hydroxyvitamin D, FINS FPG, GSH (glutathione), CRP (C-reactive protein), TC (total cholesterol), LDL, HDL and HOMA-IR 
  • Results seen regarding the effect of vitamin D on, CRP, TC, LDL and HDL may have been related to publication biases 

CONCLUSION: 

• There was an association between low blood vitamin D levels and risk for GDM 
  • Pregnant women with low blood vitamin D levels had 85% higher risk of GDM than those with regular vitamin D levels 
• Taking vitamin D supplements during pregnancy could help ameliorate GDM, although association strength is modest 
  • While further research on effectiveness of vitamin D supplementation during pregnancy is still needed, it is likely that other lifestyle interventions to prevent GDM will still be necessary 

Learn More – Primary Sources: 

Vitamin D and gestational diabetes mellitus: a systematic review based on data free of Hawthorne effect. 

Related ObG Topics:

Updated ACOG Guidance on Gestational Diabetes

One Step or Two Step GDM Screening – Comparing Outcomes

Does a Mediterranean Diet with Extra Olive Oil Decrease Risk of Gestational Diabetes Mellitus?