What is the issue?
Can dietary advice for pregnant women prevent the development of diabetes in pregnancy, known as gestational diabetes mellitus (GDM), which can cause health complications for women and their babies?
Why is this important?
Women with GDM have an increased risk of developing high blood pressure and protein in their urine during pregnancy (pre-eclampsia), and of having a caesarean section birth. Their babies may grow large and, as a result, be injured at birth, or cause injury to their mothers during birth. Additionally, there can be long-term health problems for women and their babies, including an increased risk of cardiovascular disease or type 2 diabetes. The number of women being diagnosed with GDM is increasing around the world, so finding simple and cost-effective ways to prevent women developing GDM is important.
Carbohydrates are the main nutrient affecting blood glucose after meals. The glycaemic index (GI) can be used to characterise the capability of carbohydrate-based foods to raise these levels. Some diets, for example, those with low-fibre and high-GI foods, can increase the risk of developing GDM. It has been suggested that dietary advice interventions in pregnancy may help to prevent women developing GDM.
What evidence did we find?
We searched for studies on 3 January 2016, and included 11 randomised controlled trials involving 2786 pregnant women and their babies. The quality of the evidence was assessed as low or very low and the overall risk of bias of the trials was unclear to moderate. Six trials compared dietary advice with standard care, four compared advice focused on a low-GI diet with advice for a moderate- to high-GI diet, and one compared dietary advice focused on a high-fibre diet with standard advice.
There was a possible reduction in the development of GDM for women who received dietary advice versus standard care across five trials (1279 women, very low-quality evidence), though no clear difference for GDM was seen between women who received low- versus moderate- to high-GI diet advice across four trials (912 women, low-quality evidence). Two trials (282 women) reported no clear difference between women who received dietary advice versus standard care for pre-eclampsia (low-quality evidence), though fewer women who received dietary advice developed pregnancy-induced high blood pressure (low-quality evidence). There was no clear difference between the groups of women who received low-GI and moderate- to high-GI diet advice, in the number of babies born large-for-gestational age across three trials (777 babies, very low-quality evidence). Only one trial comparing dietary advice with standard care reported on the number of babies who died (either before birth or shortly afterwards), with no deaths in this trial.
There were no clear differences for most of the other outcomes assessed in the trials comparing dietary advice with standard care. including caesarean section, perineal trauma, and child skin-fold thickness at six months. However, women who received dietary advice gained less weight during their pregnancy across five trials (1336 women) (low-quality evidence).
Similarly, there were no clear differences for other outcomes assessed in the trials comparing low- and moderate- to high-GI diet advice, including for caesarean birth and weight gain in pregnancy. The trial comparing dietary advice focused on a high-fibre diet with standard advice found no clear differences for any outcomes.
The included trials did not report on a large number of outcomes listed in this review, including outcomes relating to longer-term health for the women and their babies (as children and adults), and the use and cost of health services.
What does this mean?
Dietary advice interventions for pregnant women may be able to prevent GDM. Based on current trials, however, conclusive evidence is not yet available to guide practice. Further large, well-designed, randomised controlled trials are required to assess the effects of dietary interventions in pregnancy for preventing GDM and improving other health outcomes for mothers and their babies in the short and long term. Five trials are ongoing, and four await classification (pending availability of more information) and will be considered in the next update of this review.
Very low-quality evidence from five trials suggests a possible reduction in GDM risk for women receiving dietary advice versus standard care, and low-quality evidence from four trials suggests no clear difference for women receiving low- versus moderate- to high-GI dietary advice. A possible reduction in pregnancy-induced hypertension for women receiving dietary advice was observed and no clear differences were seen for other reported primary outcomes. There were few outcome data for secondary outcomes.
For outcomes assessed using GRADE, evidence was considered to be low to very low quality, with downgrading based on study limitations (risk of bias), imprecision, and inconsistency.
More high-quality evidence is needed to determine the effects of dietary advice interventions in pregnancy. Future trials should be designed to monitor adherence, women's views and preferences, and powered to evaluate effects on short- and long-term outcomes; there is a need for such trials to collect and report on core outcomes for GDM research. We have identified five ongoing studies and four are awaiting classification. We will consider these in the next review update.
Gestational diabetes mellitus (GDM) is a form of diabetes occurring during pregnancy which can result in short- and long-term adverse outcomes for women and babies. With an increasing prevalence worldwide, there is a need to assess strategies, including dietary advice interventions, that might prevent GDM.
To assess the effects of dietary advice interventions for preventing GDM and associated adverse health outcomes for women and their babies.
We searched Cochrane Pregnancy and Childbirth's Trials Register (3 January 2016) and reference lists of retrieved studies.
Randomised controlled trials (RCTs) and quasi-RCTs assessing the effects of dietary advice interventions compared with no intervention (standard care), or to different dietary advice interventions. Cluster-RCTs were eligible for inclusion but none were identified.
Two review authors independently assessed study eligibility, extracted data and assessed the risk of bias of the included studies. Data were checked for accuracy. The quality of the evidence was assessed using the GRADE approach.
We included 11 trials involving 2786 women and their babies, with an overall unclear to moderate risk of bias. Six trials compared dietary advice interventions with standard care; four compared low glycaemic index (GI) with moderate- to high-GI dietary advice; one compared specific (high-fibre focused) with standard dietary advice.
Dietary advice interventions versus standard care (six trials)
Considering primary outcomes, a trend towards a reduction in GDM was observed for women receiving dietary advice compared with standard care (average risk ratio (RR) 0.60, 95% confidence interval (CI) 0.35 to 1.04; five trials, 1279 women; Tau² = 0.20; I² = 56%; P = 0.07; GRADE: very low-quality evidence); subgroup analysis suggested a greater treatment effect for overweight and obese women receiving dietary advice. While no clear difference was observed for pre-eclampsia (RR 0.61, 95% CI 0.25 to 1.46; two trials, 282 women; GRADE: low-quality evidence) a reduction in pregnancy-induced hypertension was observed for women receiving dietary advice (RR 0.30, 95% CI 0.10 to 0.88; two trials, 282 women; GRADE: low-quality evidence). One trial reported on perinatal mortality, and no deaths were observed (GRADE: very low-quality evidence). None of the trials reported on large-for-gestational age or neonatal mortality and morbidity.
For secondary outcomes, no clear differences were seen for caesarean section (average RR 0.98, 95% CI 0.78 to 1.24; four trials, 1194 women; Tau² = 0.02; I² = 36%; GRADE: low-quality evidence) or perineal trauma (RR 0.83, 95% CI 0.23 to 3.08; one trial, 759 women; GRADE: very low-quality evidence). Women who received dietary advice gained less weight during pregnancy (mean difference (MD) -4.70 kg, 95% CI -8.07 to -1.34; five trials, 1336 women; Tau² = 13.64; I² = 96%; GRADE: low-quality evidence); the result should be interpreted with some caution due to considerable heterogeneity. No clear differences were seen for the majority of secondary outcomes reported, including childhood/adulthood adiposity (skin-fold thickness at six months) (MD -0.10 mm, 95% CI -0.71 to 0.51; one trial, 132 children; GRADE: low-quality evidence). Women receiving dietary advice had a lower well-being score between 14 and 28 weeks, more weight loss at three months, and were less likely to have glucose intolerance (one trial).
The trials did not report on other secondary outcomes, particularly those related to long-term health and health service use and costs. We were not able to assess the following outcomes using GRADE: postnatal depression; maternal type 2 diabetes; neonatal hypoglycaemia; childhood/adulthood type 2 diabetes; and neurosensory disability.
Low-GI dietary advice versus moderate- to high-GI dietary advice (four trials)
Considering primary outcomes, no clear differences were shown in the risks of GDM (RR 0.91, 95% CI 0.63 to 1.31; four trials, 912 women; GRADE: low-quality evidence) or large-for-gestational age (average RR 0.60, 95% CI 0.19 to 1.86; three trials, 777 babies; Tau² = 0.61; P = 0.07; I² = 62%; GRADE: very low-quality evidence) between the low-GI and moderate- to high-GI dietary advice groups. The trials did not report on: hypertensive disorders of pregnancy; perinatal mortality; neonatal mortality and morbidity.
No clear differences were shown for caesarean birth (RR 1.27, 95% CI 0.79 to 2.04; two trials, 201 women; GRADE: very low-quality evidence) and gestational weight gain (MD -1.23 kg, 95% CI -4.08 to 1.61; four trials, 787 women; Tau² = 7.31; I² = 90%; GRADE: very low-quality evidence), or for other reported secondary outcomes.
The trials did not report the majority of secondary outcomes including those related to long-term health and health service use and costs. We were not able to assess the following outcomes using GRADE: perineal trauma; postnatal depression; maternal type 2 diabetes; neonatal hypoglycaemia; childhood/adulthood adiposity; type 2 diabetes; and neurosensory disability.
High-fibre dietary advice versus standard dietary advice (one trial)
The one trial in this comparison reported on two secondary outcomes. No clear difference between the high-fibre and standard dietary advice groups observed for mean blood glucose (following an oral glucose tolerance test at 35 weeks), and birthweight.