What is the question?

• The question is: What is the relationship between dietary patterns before and during pregnancy and risk of gestational diabetes mellitus?

What is the answer to the question?

• Limited but consistent evidence suggests that certain dietary patterns before pregnancy are associated with a reduced risk of gestational diabetes mellitus. These protective dietary patterns are:
  • higher in vegetables, fruits, whole grains, nuts, legumes, and fish, and 
  • lower in red and processed meats. 
• Most of the research was conducted in healthy, Caucasian women with access to health care.
• Evidence is insufficient to estimate the association between dietary patterns during pregnancy and risk of gestational diabetes mellitus. 

Why was this question asked? 

• This important public health question was identified and prioritized as part of the U.S. Department of Agriculture and Department of Health and Human Services Pregnancy and Birth to 24 Months Project.

How was this question answered? 

• A team of staff from the Nutrition Evidence Systematic Review conducted a systematic review in collaboration with a group of experts called a Technical Expert Collaborative

What is the population of interest? 

• Women who are pregnant or able to become pregnant, ages 15-44 years.

What evidence was found? 

• This review includes 11 studies that were published since 1980. Some studies were small, including 12 participants, while others were really large and included 15,254 participants. Most of these women were Caucasians that had access to health care. The extent of overweight and obesity varied. In some studies 1 in 10 women were overweight or obese, while in others 1 in 3 were overweight or obese.
• Studies used different approaches to develop dietary patterns:
  • 5 studies used an existing index or score.
  • 4 studies used data-driven methods such as principal components analysis.
  • 1 study used both an index/score and a data-driven method.
  • The randomized controlled trial used an experimental diet.
• Studies showed that what women ate several years (2-10 y) before pregnancy was important for gestational diabetes risk.
• However, what women ate during pregnancy did not increase or decrease the gestational diabetes risk.

How up-to-date is this systematic review? 

This review includes literature from 01/1980 to 01/2017.

Background

• The goal of this systematic review was to examine the following question: What is the relationship between dietary patterns before and during pregnancy and risk of gestational diabetes mellitus? 
• Systematic reviews were conducted as part of the U.S. Department of Agriculture and Department of Health and Human Services Pregnancy and Birth to 24 Months Project. 
• Dietary patterns were defined as the quantities, proportions, variety or combinations of different foods and beverages in diets, and the frequency with which they are habitually consumed. 

Conclusion Statement and Grade

• Limited but consistent evidence suggests that certain dietary patterns before pregnancy are associated with a reduced risk of gestational diabetes mellitus. These protective dietary patterns are: 
  • higher in vegetables, fruits, whole grains, nuts, legumes, and fish, and 
  • lower in red and processed meats.
• Most of the research was conducted in healthy, Caucasian women with access to health care. 
  
  Grade: Limited
   
• Evidence is insufficient to estimate the association between dietary patterns during pregnancy and risk of gestational diabetes mellitus. 
  
  Grade: Grade Not Assignable

Methods 

• This systematic review was conducted by a team of staff from the Nutrition Evidence Systematic Review in collaboration with a Technical Expert Collaborative. Literature search was conducted using 4 databases (PubMed, Cochrane, Embase, and CINAHL) to identify articles that evaluated the intervention or exposure of dietary patterns before and during pregnancy and risk of hypertensive disorders of pregnancy. A manual search was conducted to identify articles that may not have been included in the electronic databases searched. Articles were screened by two authors independently for inclusion based on pre-determined criteria. 
• Data extraction and risk of bias assessment were conducted for each included study, and both were checked for accuracy. The body of evidence was qualitatively synthesized to inform development of a conclusion statement(s), and the strength of evidence was graded using pre-established criteria evaluating the body of evidence on internal validity/risk of bias, adequacy, consistency, impact, and generalizability.

Summary of Evidence

• This systematic review includes 10 prospective cohort studies and 1 pilot RCT, published between 1998 and 2016.
• The studies used multiple approaches to assess dietary patterns. Five studies used indices/scores to assess dietary patterns, four studies used factor or principal component analysis (PCA) and one study used both an index/score and PCA. In addition, one RCT assigned subjects to one of two experimental diets.
• Overall, 8 of the 11 included studies found statistically significant associations between dietary patterns and GDM risk among healthy Caucasian women with access to health care. Greater adherence to a protective dietary pattern before and during pregnancy was associated with a decrease in GDM risk of 24% to 56%. Higher adherence to a detrimental pattern was associated with an increase in risk of 23% to 63%.
• There is heterogeneity in terms of when dietary data were assessed. Five studies measured diet before pregnancy while the rest (n=6) assessed diet during pregnancy. 
  • Greater adherence to a healthy diet assessed 2-10 years before pregnancy showed a consistent inverse association with the risk of GDM in all the studies. These findings are also in agreement with the evidence linking dietary patterns and type 2 diabetes mellitus risk in non-pregnant populations.
  • There were mixed findings in studies that assessed diet during pregnancy: 1) three studies showed an association with GDM, 2) one showed an inverse association with blood glucose, only, and not with GDM, 3) one showed an effect on blood glucose and insulin response but did not study GDM, and 4) one other study showed no association with GDM.
• Generalizability of the studies is limited to healthy Caucasian women who have access to health care. Minority women and those of lower SES are underrepresented in this body of evidence.