How much carbohydrate does a woman with Gestational Diabetes Mellitus need?

What is the evidence for the effectiveness of Medical Nutrition Therapy for GDM?

MNT for GDM in Australia has been predominating directly by the evidence-based guidelines produced by the Academy of Nutrition and Dietetics (A.N.D) Guidelines (USA)⁶ as there are no Australian guidelines. The A.N.D guidelines include eighteen recommendations which are all graded according to the strength of the evidence. The recommendation with the strongest grading is that of the effectiveness of individualised MNT for GDM⁶.This grading was based on evidence from five studies.^6-10 All studies included individualisation of MNT by a dietitian, as well as at least two of the following – diabetes education, education on self-monitoring of blood glucose levels (SMBG), physical activity, and follow-up by a physician.

Should women with GDM follow a carbohydrate restricted diet?

Despite the fact that MNT for GDM has historically been based on restricting carbohydrate intake,⁶ there is limited evidence to support this approach.^{1, 13} The A.N.D first evidence-based nutrition practice guidelines for GDM were published in 2008 and recommended that carbohydrate intake be restricted to less than 45% of total energy. However, this recommendation appeared to be based on just two studies,^{6, 7} only one of which was randomised.¹⁴ Since then, only two randomised controlled trials (RCT) of reasonable quality comparing lower- versus higher-carbohydrate intakes have been published.^{6, 7} Neither study find any differences in pregnancy outcomes between diets. A subsequent pilot randomised crossover trial, RCT study, in which all food was provided, compared the impact of lower versus higher-carbohydrate diets on glycaemic management using continuous glucose monitoring (CGM).¹⁸ This study again found both diets effectively managed maternal glycaemia to within current therapeutic targets, however, the higher-carbohydrate diet achieved significantly lower postprandial free fatty acids. The authors theorise that, given that protein intake remains remarkably constant, a lower-carbohydrate diet generally results in a higher fat intake. Maternal triglycerides and free fatty acids can be used by the placenta to promote fetal fat gain. Although a pilot study, the findings challenge the current recommendations of carbohydrate restriction and demonstrate potential for improved glycemic management and metabolic parameters with a more liberalised carbohydrate intake.⁶ The most recent (2016) A.N.D guidelines confirm the lack of evidence to stipulate a specific carbohydrate intake, stating that positive findings have been found with a range of carbohydrate intakes from moderate to high carbohydrate (36.7-65% of total energy).⁶

What about diet quality?

With the prime focus of MNT for GDM being on maternal hyperglycaemia, it is easy to see why MNT for GDM could primarily focus on restricting carbohydrate quantity. But what about the type of carbohydrate? Carbohydrate-rich food can be categorised as high, moderate and low glyceamic index (GI) according to their impact on post prandial blood glucose levels. A low GI diet has the potential to improve blood glucose levels – potentially without the same degree of carbohydrate restriction. Low GI diets (characterised by intake of high-quality, complex carbohydrates) have been found to lower insulin use and reduced risk of macrosomia in multiple studies.¹⁸

Evidence from several RCTs on the DASH (Dietary Approaches to Stop Hypertension) diet provides further support for a focus on diet quality. The DASH diet is a low energy density diet, rich in fruits, vegetables, and low-fat dairy products, and low in saturated fat and cholesterol. This diet was originally designed to lower blood pressure, and has been shown to be effective in doing so.^{6, 7} The DASH diet has also been found to be beneficial for women with GDM – resulting in a substantial reduction in the need for insulin therapy and caesarean delivery.^6-8 However, it is difficult to determine which dietary components are responsible for the positive outcomes, as there were multiple differences between the DASH and control diets in all three studies. It is worth pointing out that the DASH diet is in fact a high carbohydrate diet (up to 400 grams per day). However, this carbohydrate intake was in the context of a diet low in simple sugars (equivalent to less than one tablespoon of added sugar per day), refined grains and saturated fat, as well as high in fruit (4-5 serves per day), vegetables (6 serves per day), wholegrains (more than half of all grain based carbohydrate), low-fat dairy food (4 serves per day), and nuts (2/3 cup per day). It is also worth noting that a meta-analysis on dietary modification in GDM¹ excluded DASH diet studies from their sensitivity analysis due to methodological concerns. Although the carbohydrate content of the DASH diet could be considered extreme, it does provide proof of concept – it may be possible to liberalise carbohydrate intake with a greater focus on diet quality.  Further, an underlying driver of hyperglycaemia in pregnancy is insulin resistance.⁶ The DASH diet appears to have beneficial effects on fasting plasma glucose, serum insulin levels, and insulin resistance.²³ A whole diet approach such as this may directly address the underlying cause of GDM – insulin resistance, rather than on the symptom of it, namely maternal hyperglycemia. Further high-quality research is clearly needed to explore this hypothesis.

How much weight should women with GDM gain?

What is a healthy maternal weight gain for women with GDM? The Institute of Medicine (IOM) maternal weight gain guidelines⁶, although not specific for GDM pregnancies, are usually used in GDM clinics as no GDM-specific gestational weight gain (GWG) guidelines currently exist (see Table 1). Research has shown that women with GDM who exceed the IOM maternal weight gain guidelines were more likely to have a caesarean section, LGA and macrosomia compared to those who achieved these targets.^{6, 7} These results were despite treatment of maternal hyperglycemia.

Table 1: Institute of Medicine Maternal Weight gain guidelines (IOM, 2009)^{25, 26}

*Calculations assume a 0.5–2 kg weight gain in the first trimester

I have been doing a PhD in MNT for GDM part-time in addition to my clinical role at Bankstown-Lidcombe Diabetes Centre since 2015. My research has included investigating the contribution of excessive gestational weight gain (EGWG) to the outcomes of women with GDM. It is important to emphasis at this point that insulin resistance in pregnancy affects more than glucose metabolism.²⁴ Increased free fatty acids and triglycerides are hallmarks of insulin resistance. Dietary modification aimed at healthy weight gain in GDM may have a positive effect on altering maternal lipids including free fatty acids, triglycerides and cholesterol.⁶

I am fortunate to work in a Diabetes Centre where data has been collected on the outcomes of women with GDM managed by our service for the last 27 years. We recently published an analysis of 3281 pregnancies, where we found that women with EGWG by commencement of GDM management, were more likely to experience a number of adverse outcomes compared to those who did not.⁶ In this study EGWG was defined as exceeding maximum IOM weight gain targets by commencement of GDM management (see Table 1).

Women with EGWG, compared to those without, had higher mean oGTT fasting plasma glucose (FPG) after adjustment for confounders (5.2 mmol/L [95% CI 5.1-5.3] versus 5.0 mmol/L [95% CI 4.9-5.0], p<0.01), increased insulin therapy (47.0 vs 33.6%, p<0.0001) and a 40% higher likelihood of commencing insulin therapy compared to women that had not exceeded recommended weight gain at commencement of GDM treatment (p<0.01). We also then found that every additional 2 kg gained above the IOM guidelines during GDM management (cEGWG) was associated with a 30% greater likelihood of insulin therapy (p<0.001), an 8 unit increase in final daily insulin dosage [95% CI 5.4-11.0], p<0.0001, and a 40% greater likelihood of having a large for gestational age infant [95% CI 1.2-1.7], p<0.0001. Although this is an extreme definition of excessive maternal weight gain, it was surprisingly common in our service (one in four GDM pregnancies). A positive clinical message can be taken from these findings. In those with high amounts of weight gain before a GDM diagnosis, the ‘window of opportunity’ was still available to intervene positively through preventing or slowing further weight gain during GDM management.³⁰ Taken together, these findings suggest that outcomes of GDM pregnancies could be improved with a greater focus on gestational weight management.

Despite potential benefits of healthy GWG, published research suggests that few primary care physicians and antenatal services currently provide GWG advice in their services.⁶ Further, some antenatal services intentionally omit routine weighing for valid reasons.⁷ Firstly, there are concerns that weighing will cause psychological distress to pregnant women⁷ Secondly, for some time there was limited evidence of benefit.⁷ However, in line with a more recent growing body of evidence, the latest published National Pregnancy Guidelines (2019) in Australia recommend a return to routine weighing in pregnancy.⁷ It is important to note that routine weighing in pregnancy does not appear to be effective as a stand-alone intervention.³³ However, benefits have been shown in the context of healthy eating and/or physical activity interventions (n=6,920)⁷, and with provision of healthy gestational weight gain advice in accordance with IOM guidelines.^{26, 27} Women with GDM and at risk of GDM are particularly likely to benefit from healthy maternal weight gain advice and support from early in pregnancy.³⁰

So how much carbohydrate should a woman with GDM eat?

Although it would be clinically useful to have a simple answer to this question, in practice, a ‘one-size fits all’ approach cannot be applied in women with GDM. The American Diabetes Association stipulates that a low carbohydrate diet is not appropriate for pregnant or breastfeeding women.⁷  Further, the US A.N.D guidelines advises a minimum of 175 grams of carbohydrate (or 12 carbohydrate exchanges) per day.⁶ Beyond this minimum guide, the amount of carbohydrate required depends on multiple factors such physical activity levels (current and usual prior to pregnancy), appetite, pre-pregnancy BMI, maternal weight already gained and whether the woman has a singleton or multiple pregnancy. At initial education from a dietitian, women are usually advised to follow a healthy balanced diet, spreading higher fibre/lower GI carbohydrate across three main meals and three snacks. The typical carbohydrate content recommended by dietitians at initial education is 30-45 grams (two to three exchanges) for meals, and 15-30 grams (one to two 15 gram exchanges) for snacks (unpublished data). Some women will need significantly more carbohydrate than initially advised, whilst others may not. MNT needs to be individualised once a woman with GDM is in a routine of regular meals and SMBG levels. Women are often tempted to restrict their carbohydrate intake below minimum amounts, if their blood glucose levels are not achieving glycaemic targets. In practice, if minimum carbohydrate amounts do not achieve glycaemic targets once carbohydrate quality is also addressed, then insulin is likely to be required. Maternal weight gain tracking below IOM recommended targets can suggest undernutrition, and has been associated with a greater likelihood of small for gestational age (SGA) infants.²⁶

Implications for clinical practice

There is strong evidence for the benefit of individualised MNT for women with GDM.⁶ Rates of women diagnosed with GDM tripled in Australia from 2000/01 to 2016/17 (AIHW, 2019).⁷ However, this increase has not been met with a concurrent increase in funding for more dietetic hours in many diabetes services.  This makes it increasingly challenging for dietitians to provided individualised advice. As a result, more and more women receive group education only. Initial education should ideally never be ‘stand-alone’ dietary advice, as this would suggest a one-size fits all approach. Further, there is some evidence to suggest that less individualised advice is associated with greater insulin use in women with GDM.⁷ The RCT that validated the original American Dietetic Association guidelines for GDM stipulated a minimum of three dietitian visits.7 In addition, the most recently updated A.N.D GDM guidelines recommend ‘additional MNT visits should be scheduled every two to three weeks or as needed for the duration of the pregnancy.⁶ Although that level of individualised advice many not be feasible in many GDM clinics, this does highlight the need to provide at least some individualised advice to all women with GDM, and for vigilance in identifying those would benefit from further individualised MNT. Appropriate reasons for re-referral include multiple potentially diet-related glycemic excursions, excess hunger, and inadequate or excessive maternal weight gain according to IOM guidelines.