Fibre and pre-eclampsia

Monday July 21 2008

“How two slices of brown bread a day protects pregnant women against life threatening pre-eclampsia” is the headline in the Daily Mail . The newspaper discusses the results from a study of more than 1500 women, which suggests that eating a high-fibre diet protects against pre-eclampsia in pregnancy. The lead researcher, Dr Qiu, is quoted as saying that adding two slices of brown bread per day is the equivalent of adding 5g of fibre to the diet.

The researchers are cautious about the conclusions they draw from this study. However, they say that when considered alongside the results of other published studies, theirs suggests a link between intake of fibre in early pregnancy and reduced risk of pre-eclampsia. The design of this study means that it cannot provide concrete evidence that increasing fibre consumption reduces risk of pre-eclampsia. More research is needed to prove this conclusively. Pregnant women, and the population as a whole, should aim to eat a healthy balanced diet that is high in fibre.

Where did the story come from?

Dr Chunfang Qiu and colleagues from the Swedish Medical Center in Washington, and the University of Washington School of Public Health and Community Medicine carried out this study. The research was funded by the National Institutes of Health. It was published in the peer-reviewed medical journal, the American Journal of Hypertension .

What kind of scientific study was this?

This is a cohort study where researchers were interested in the link between maternal dietary fibre and pre-eclampsia. The participants were women from the Omega Study, where 1,538 women agreed to participate in an investigation of diet and pre-eclampsia between 1996 and 2002. Those with chronic high blood pressure and/or diabetes mellitus were excluded, as were those with missing information on fibre intake. Of these 1,538 women, 64 were diagnosed with pre-eclampsia (sustained high blood pressure during pregnancy with evidence of protein in urine). In a separate analysis, the researchers included another 46 women who met slightly different criteria for pre-eclampsia (according to newer screening criteria).

The women were given food frequency questionnaires at around 13.1 weeks of pregnancy to assess their diet before conception and during the first trimester. From this, researchers were able to determine total dietary fibre intake, and intake of other nutrients including vitamin C, fats and carbohydrates. The level of fats (lipids) in the women’s blood was determined through blood samples collected at 13.1 weeks’ gestation.

The researchers also collected demographic information about the women, including education, ethnicity, number of children, smoking and body mass index (BMI). Intake of fibre was categorised into quartiles (the participants were divided into four groups according to their intake, from low to high, with each group containing 25% of the participants). Risk of pre-eclampsia across these quartiles was then compared.

What were the results of the study?

The researchers found that total fibre intake was associated with risk of pre-eclampsia, with women in the highest quartile of intake (reporting more than 21g of fibre per day) experiencing a 66%-72% reduced risk compared with those in the lowest quartile of intake. This difference remained even after accounting for factors such as energy intake, maternal age, ethnicity, BMI, vitamin C and parity. They also found that women with the highest fibre intake had the lowest levels of blood triglycerides (excess fat) and higher levels of good cholesterol.

What interpretations did the researchers draw from these results?

The researchers conclude that their results, when taken together with previously published studies, suggest important health benefits associated with increased fibre consumption before and during early pregnancy. They say that if their findings are confirmed by other studies, this “may motivate increased efforts aimed at exploring lifestyle approaches, particularly dietary approaches, to lower the risk of pre-eclampsia”.

What does the NHS Knowledge Service make of this study?

The researchers raise important limitations associated with their study:

  • Firstly, they only assessed dietary fibre at one point early in pregnancy (during the first trimester). Women’s consumption of fibre is unlikely to have remained constant throughout their pregnancy. Also, women self-reported their consumption using a food frequency questionnaire, which may have resulted in some errors.
  • The researchers also state some problem in their measurement of blood lipids, in that the samples were “non-fasting” as fasting is contraindicated in pregnancy. However, they say that further analysis suggests that this has little effect on the final outcome. 
  • As with all non-randomised studies, there may factors that have not been accounted for, which are associated with both diet and outcome – i.e. confounders. The researchers acknowledge this, suggesting it is possible that unmeasured factors may be responsible, or partly responsible, for the relationship seen here.
  • Pre-eclampsia is a complex disorder and the causes are not known. There are likely to be a number of factors that interplay with one another to increase the risk in some women. Of importance is the fact that women with high blood pressure and diabetes were excluded from this study, both of whom are known to be at increased risk of pre-eclampsia. Other significant risk factors which do not appear to have been considered or adjusted for in the analysis are a history of pre-eclampsia in previous pregnancies, and family history of pre-eclampsia (though they did adjust for family history of hypertension).
  • The link between the two slices of brown bread and the quantity of fibre needed to be “protective” in the light of these results is unclear. Women in the highest quartile were eating about 10g of fibre more than those in the lowest quartile.

The researchers call for more studies of pregnant women “to demonstrate these potential causal relationships more conclusively”.

Analysis by Bazian
Edited by NHS Choices