What is the question?

• The question is: What is the relationship between dietary patterns before and during pregnancy and risk of hypertensive disorders of pregnancy?

What is the answer to the question?

• Limited evidence in healthy Caucasian women with access to health care suggests that dietary patterns before and during pregnancy higher in vegetables, fruits, whole grains, nuts, legumes, fish, and vegetable oils and lower in meat and refined grains are associated with a reduced risk of hypertensive disorders of pregnancy, including preeclampsia and gestational hypertension. Not all components of the assessed dietary patterns were associated with all hypertensive disorders.
• Evidence is insufficient to estimate the association between dietary patterns before and during pregnancy and risk of hypertensive disorders of pregnancy in minority women and those of lower socioeconomic status. 

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? 

• These studies assessed the relationship between dietary patterns before and during pregnancy and hypertensive disorders of pregnancy (including preeclampsia and gestational hypertension). 
• 5 of the 8 included studies showed that the diet eaten before and during pregnancy was related to risk of hypertensive disorders of pregnancy (HDP). 
  • Beneficial diets had higher amounts of vegetables, fruits, whole grains, nuts, legumes, fish and vegetable oils
  • Harmful diets had higher amounts of meats, potatoes and processed foods
• One other study showed an association between diet during pregnancy and blood pressure but not preeclampsia or gestational hypertension.
• The body of evidence is limited in several ways. For example, there was little racial/ethnic and socioeconomic diversity in these studies, and it is difficult to compare studies due to inconsistency in how dietary patterns were measured.
• Additional research is needed to assess the relationship between dietary patterns before and during pregnancy and hypertensive disorders of pregnancy.

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 hypertensive disorders of pregnancy? 
• 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 evidence in healthy Caucasian women with access to health care suggests that dietary patterns before and during pregnancy higher in vegetables, fruits, whole grains, nuts, legumes, fish, and vegetable oils and lower in meat and refined grains are associated with a reduced risk of hypertensive disorders of pregnancy, including preeclampsia and gestational hypertension. Not all components of the assessed dietary patterns were associated with all hypertensive disorders. 
  
  Grade: Limited
   
• Evidence is insufficient to estimate the association between dietary patterns before and during pregnancy and risk of hypertensive disorders of pregnancy in minority women and those of lower socioeconomic status.
  
  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 8 studies (sample size ranging from 290 to 72,072) within 4 cohorts and 1 RCT, published between 2005 and 2016.
• The studies used multiple approaches to assess dietary patterns, which makes it difficult to compare findings across studies. Three studies used indices/scores to assess dietary patterns, four studies used factor or principal component analysis and one RCT assigned participants to either an experimental or control diet. 
• Despite this variability, 5 of the 8 included studies reported statistically significant associations between dietary patterns and hypertensive disorders of pregnancy (HDP) risk among healthy Caucasian women with access to health care. An additional study showed an association between dietary patterns and blood pressure but not preeclampsia or gestational hypertension.
  • Dietary patterns characterized by higher intakes of vegetables, fruits, whole grains, nuts, legumes, fish and vegetable oils were associated with a 30-42% decreased risk of HDP and a 14-29% decreased risk of preeclampsia. 
  • Two of the dietary patterns assessed were reported to be detrimental: traditional and processed food patterns, characterized by higher intakes of meats, potatoes and processed foods. One was associated with a 21% increased risk of preeclampsia and the other was associated with an increased risk of high blood pressure during pregnancy.
• Generalizability of the included studies is limited to healthy Caucasian women who have access to health care. Minority women and those of lower socioeconomic status are underrepresented in this body of evidence.
• The body of evidence has several limitations: 
  • The evidence base includes eight studies from only four unique cohorts and one RCT
  • All but one of the studies were conducted outside the United States in predominantly Caucasian women
  • Dietary patterns vary considerably across studies, making it difficult to compare findings
  • No adjustment was made for many key confounding factors, and 
  • The data are primarily observational, limiting the ability to draw any casual inferences. The RCT was done among 240 women and was not powered to examine HDP (addressed by the authors as "hypertensive complications"). Additionally, the timing of the intervention in the RCT may have been too late in pregnancy to have seen an effect.
• Additional research is needed that should: 
  • Include diverse populations from the U.S. and elsewhere with varying racial/ethnic and socioeconomic backgrounds
  • Develop and validate novel epidemiological tools that can accurately capture the complexity of dietary habits
  • Improve comparability across studies by increasing the uniformity of 1) diet assessment tools used across different regions and populations, 2) methods used to define dietary patterns, and 3) timing of dietary assessment
  • Adjust for key confounding factors in observational studies, including parity, educational attainment, smoking status, race/ethnicity, maternal age, family poverty income ratio, pre-pregnancy BMI, mean total energy intake, and gestational diabetes mellitus (and/or justify how and why confounders were chosen)
  • Include well-designed and sufficiently powered RCTs
  • Include and assess effect measure modification in multiethnic subgroups within the U.S., and 
  • Use robust statistical methods such as multiple imputation to handle missing data.