Keywords

FormalPara Key Points

  • Gestational Diabetes Mellitus (GDM) is the carbohydrate intolerance that results from maternal inability to cope with increased insulin resistance associated with pregnancy.

  • GDM management aims to achieve glycaemic control and promote adequate weight gain in the mother and also improve foetal outcomes.

  • Diet is the cornerstone of GDM management.

  • Low Glycaemic Index (GI) or glycaemic load (GL) diets by preventing postprandial glycaemic and insulinaemic peaks, attenuate cardiovascular risks; especially in subjects with obesity, insulin resistance or hyperinsulinaemia.

  • Low-GI diets are beneficial only when they comply with current dietary guidelines and therefore require appropriate dietetic supervision.

  • GDM subjects on low-GI diets have lower spikes in post-meal glycaemia and are less likely to require the initiation of insulin therapy when compared to those receiving standard diets with higher GI.

  • Low-GI diets in GDM may also reduce central adiposity in the foetus.

  • Low-GI diets are also likely to benefit GDM women in managing their glycaemia and body weight post-delivery.

  • Current evidence raises no safety issues in using low-GI/GL diets in GDM management.

  • However, further evidence is required to lend unequivocal support for the benefit of low-GI/GL diets in GDM treatment.

Introduction

Gestational Diabetes mellitus (GDM) is defined as the ‘glucose intolerance first recognised during pregnancy’ [1]. All pregnancies are accompanied by metabolic changes that promote adipose tissue accumulation in early gestation, followed by an increase in insulin resistance to provide adequate nourishment to the foetus [2]. The insulin resistance is accompanied by increased pancreatic insulin secretion to maintain maternal euglycaemia as the pregnancy progresses [2]. Hyperglycaemia results when the maternal insulin secretion is unable to meet the increased insulin demand [1, 3]. Therefore, the pathophysiology of GDM is similar to that of type 2 diabetes mellitus (T2DM); namely, marked insulin resistance and impairment of insulin secretion [4] and associated dyslipidaemia [5]. Thus management of postprandial glycaemia and insulin demand are essential targets for GDM management.

Diet is the cornerstone of GDM management [1, 6]. Dietary management for GDM has the following maternal goals: achieving glycaemic control, ensuring adequate weight gain and appropriate nutritional status. Achieving these goals ensures maternal and foetal health. More intensive medical management and increased surveillance are instituted in women who fail to respond adequately to diet therapy and increases treatment costs [7]. Most importantly, GDM increases long-term health risks for the mother and her offspring [1, 7] posing greater demand on healthcare resources.

Carbohydrates predominantly influence postprandial glycaemic response [8]. Therefore, carbohydrate restriction has historically been the prime focus of dietary management for GDM [1]. Restricting carbohydrates to provide around 45% of the energy is safe in GDM pregnancies [9], though evidence from randomised-controlled trials (RCTs) support the use of diets with reasonably high amount of complex carbohydrates [1]. Low-carbohydrate diets that are high in protein and fat intake, may increase risk for diabetes specifically among pregnant women [6] and can compromise foetal outcomes [7]. In the absence of concrete evidence to favour any particular diet, consensus panels for GDM have no specific recommendation but encourage the adoption of conventional healthy diets [1, 5].

As the role for low-carbohydrate diets is limited by their health concerns, the effect of carbohydrate quality (type) on glucose metabolism and insulin resistance has gathered interest [10]. Emerging evidence suggests that deterioration of glucose homeostasis can be prevented by monitoring both carbohydrate quantity and quality [10]. Concepts of glycaemic index (GI) and glycaemic load (GL) were born out of this need to describe the quality or type of carbohydrate foods.

This review aims to assess the current evidence for the treatment of GDM with low-GI/GL diets. The objective of this review is in-line with the professional societies’ repeated calls for the consolidation of current evidence and efforts to bridge the knowledge deficits in this area to identify optimal diets for GDM women [5, 7, 11]. This is especially important because GDM affects a significant proportion of pregnant women globally, and alarmingly its prevalence is increasing [11].

Glycaemic Index (GI)

Carbohydrate foods even when consumed in equal amounts differ in their glycaemic effect. Hence physiological effects of carbohydrates are better described by their in vivo ability to raise blood glucose [12]. GI is such a physiological classification of carbohydrates [12], that ranks them on a scale of 0–100, in accordance to their postprandial glycaemic effect [13]. GI, therefore, reflects the rate of conversion of a carbohydrate into glucose [13]. Higher the GI value of a food, greater the postprandial glycaemic response it elicits [14] (Fig. 19.1).

Fig. 19.1
figure 1

Comparison of blood glucose curves after consumption of low- and high-GI foods. Legend GI: glycaemic index

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The GI of a food is measured as ‘the incremental area under the blood glucose response curve of a 50 g available carbohydrate portion of the food expressed as a percentage of the response after 50 g of glucose taken by the same subject’ [14]. To simplify interpretation, foods are often classified into three categories based on their GI: high (GI >70), intermediate (GI between 55 and 70) and low (GI <55) [15].

Factors Affecting Glycaemic Index

The differences in GI of food depend on the type of sugar and or starch it contains [16], the extent of processing it has undergone [15] and the presence of factors that determine the rate of carbohydrate absorption [16]. Low-GI recommendations utilise these determinants to lower postprandial glucose responses.

Foods with a high content of fructose (fruits), and galactose (milk products) provide lower amounts of absorbable glucose, and thus have lower GI [16]. Beans and seeds have fibrous coats that slow down the access of enzymes to the starch inside [17, 18]. Beans and rolled oats are also rich in viscous fibre that delays gastric emptying [19], enzymatic starch hydrolysis [17] and consequently delay glucose absorption [19]. Basmati rice and legumes also contain a greater amylose: amylopectin ratio that slows down the rate of starch hydrolysis and glucose absorption [16]. The presence of organic acids in oranges [20] and legumes [18] reduce the rate of starch digestion and thereby elicit lower glycaemic responses. These foods are therefore recommended in low-GI diets. Small amounts of acetic acid (vinegar) when consumed along with the meal, reduces postprandial hyperglycaemia by 20% due to delayed gastric emptying and inhibition of digestive enzymes [21] and is a probable strategy to reduce meal GI. Furthermore, gelatinization of starch during heat treatment increases its availability to amylases and its GI [22]. Therefore, low-GI recommendations emphasise on the need to prevent overcooking of cereal foods like spaghetti and oatmeal.

Glycaemic Index of Mixed Meals

GI of individual foods in a meal has shown to predict the glycaemic response when eaten together [16] in different environments and for different cuisines [23]. The GI of a mixed meal is calculated as the sum of the proportional GI contributions of each carbohydrate component of the meal [16]. Daily diet GI is similarly calculated as the mean GI of meals consumed during the day [18].

A 15% reduction in dietary GI (~10 GI units for most populations) is thought to confer clinically significant health benefits [24, 25]. Given that staple cereals predominantly determine dietary GI, a 10 unit GI reduction is achieved by substituting usual high-GI staples with lower GI alternatives, while maintaining their prescribed serving size [26]. Another practical strategy to efficiently lower GI is to include one low-GI food in each meal, since GI works through the principle of averages [24]. A sample of dietary recommendations used to lower the GI of healthy diets is provided in Table 19.1.

Table 19.1 Dietary recommendations use to lower dietary GI without causing major macronutrient changes
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Glycaemic Load (GL)

Due to its methodology of determination, GI may not reflect the glycaemic effect of a typical carbohydrate serving [27]. The glycaemic load (GL) concept was therefore invented to quantify the overall glycaemic effect of a portion of food [28]. The GL of a typical serving of food is the product of the amount of available carbohydrate it contains and its GI value [8]. GL of a serving is thus a measure of both carbohydrate quality and quantity [8] and accurately predicts postprandial glycaemia [29]. Accordingly, GL of a meal can be reduced either by reducing the amount of carbohydrate in diet, selecting foods that have lower GI or a combination of both [29] (Fig. 19.2).

Fig. 19.2
figure 2

Postulated practical application of GL concept. Legend GI: glycaemic index, GL: glycaemic load, CHO: carbohydrate amount (g), PPG: postprandial glycaemic response. Top panel shows that a similar portion of a lower GI option (wholegrain bread) versus a higher GI option (wholemeal bread) will reduce GL and hence result in lower postprandial glycaemic response. Bottom panel shows that theoretically a smaller serving size of a higher GI option (high-GI rice: e.g. glutinous rice) can have a similar GL as a slightly larger serving size of a lower GI option (low-GI rice: e.g. Basmati rice). However the increase in total calories as the number of carbohydrate exchange increases should be considered

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While dietary GL can be reduced by different methods (Table 19.2) , efforts that lower risks for T2DM [28] and cardiovascular disease (CVD) [3], reduce GL predominantly by lowering dietary GI, with minimal reduction to carbohydrate (compensated by slightly higher protein) content [29]. Thus, healthy low-GI/GL diets are essentially matched for calories, macronutrient distribution and other aspects of nutritional adequacy afforded by conventional healthy diets. The difference remains in the source of carbohydrates, primarily with respect to staples.

Table 19.2 Comparing options to lower dietary GL of a sample 1800 kcal diet
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Possible Benefits of Low Glycaemic Index Diets in GDM Management

The advocacy for low-GI foods in promoting health draws from its ability to lower postprandial glycaemic and insulinaemic responses [17]. Chronic consumption of high-GI foods results in marked rise in glycaemia [17], and demands more insulin. This demand is initially compensated by increased insulin secretion [12]. This increased insulin demand exacerbates insulin resistance [12]. Hyperinsulinaemia and insulin resistance that are central to GDM pregnancies [2], eventually lead to β-cell fatigue and increased cardiovascular risks [12] as shown in Fig. 19.3.

Fig. 19.3
figure 3

Potential mechanisms of low-GI diets in the management of glucose homeostasis and cardiovascular risks. Legend: ↑: increase; ↓: decrease, GI glycaemic index

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In contrast, when compared with high-GI diets, low-GI diets show a slower and more sustained glycaemic response [30]. They prevent exaggerated postprandial glycaemic excursions during pregnancy [31]. Additionally, low-GI meals diminish glycaemic response to the subsequent meal [32]. Besides improving glycaemic control [33], low-GI diets improve insulin sensitivity [34] and increase β-cell function in individuals with impaired glucose tolerance [35, 36] and T2DM [37]. These actions prevent the degeneration of the glucose tolerance [16] and suggest the potential benefit of low-GI diet in GDM management.

Low-GI foods also lead to the increased secretion of anorexic gut hormones which induce satiety and suppress appetite [39, 40]. Therefore voluntary energy intake is reduced for the rest of the day after a low-GI meal is consumed [41]. Moreover, low-GI diets prevent decreases in fat oxidation induced by hyperglycaemia and hyperinsulinaemia; and increases lipolysis [38, 42]. The postulated mechanisms of action of dietary GI in modulating fat oxidation and body weight gain [42,43,44,45,46,47] are compiled in Fig. 19.4. Furthermore, low-GI diets increase protein retention in both normal and hyperinsulinaemic men [48] and favour lean body mass retention [38]. Whether these mechanisms can further optimise body weight management in GDM women, who are more likely to be obese and gain more weight during and after pregnancy [1], remains to be established.

Fig. 19.4
figure 4

GI and fat oxidation. Legend: ↑: increase; ↓: decrease, GI glycaemic index. High-GI foods reduce hepatic carnitine palmitoyltransferase-1(CPT-1) messenger RNA (mRNA) expression [42]. CPT-1 transport fatty acids into mitochondria for oxidation. High-GI foods concomitantly increase hepatic acetyl-CoA carboxylase (ACC) mRNA expression. ACC catalyses the formation of malonyl-CoA. Malonyl-CoA is a potent inhibitor of CPT-1, resulting in decreased fatty acid oxidation [42, 43]. Thus, high-GI foods lower lipolysis and facilitate fat storage

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Additionally, low-GI diets by virtue of increased production of short chain fatty acids (SCFA) from colonic fermentation [49], decrease the colonic luminal pH and stimulate the absorption of minerals such as calcium, potassium, magnesium, copper, zinc and selenium [50]. Colonic fermentation also increases folate availability and promotes normal homocysteine concentrations [16, 51]. Colonic fermentation moreover reduces inflammation by altering the bacterial species in the colon [52]. These effects of low-GI diets need to be verified in GDM women.

Interestingly, low-GI diets are especially beneficial to those with central obesity, insulin resistance, hyperinsulinaemia, diabetes, hypertension and metabolic syndrome [24, 53,54,55,56]. In such populations, low-GI/GL diet favours weight reduction, glycaemic control and CVD risk reduction, suggesting its potential success in the management of GDM, a condition that shares many of these risks.

In light of the pathophysiological similarities between GDM and T2DM [4], it is pertinent to note that data adds moderate to strong support for the use of low-GI diets in diabetes management [57,58,59,60]. American Diabetes Association grades the evidence to support the substitution of high-GL foods with those with lower GL, to modestly improve glycaemic control in diabetes at “level C” [61].

Current Evidence for the Role of Low Glycaemic Index and Glycaemic Load Diets in GDM Management

For Maintaining or Achieving Glycaemic Control During Pregnancy

Limited evidence supports the effectiveness of low-GI diet in maintaining a good glycaemic control in GDM pregnancies. Only three recent RCTs have investigated the impact of low-GI diet on blood glucose-related parameters [62,63,64].

Moses et al.’s intervention on women with GDM (n = 63) at 28 weeks of gestation, found only 29% (n = 9) of women receiving low-GI diets required insulin, as compared to 59% (n = 19) of women on a conventional-high-fibre-higher-GI-diet [62]. Eventually, 50% (n = 9) of these 19 women avoided insulin use after changing to a low-GI diet. However, the final GI of women in both groups were statistically similar and it was noted that increased fibre intake, reduction in carbohydrate intake and self-restriction of energy which occurred in both groups may have interfered with the study outcomes.

Grant and colleagues reported a pilot study (n = 47) on the feasibility and effectiveness of a low-GI diet on glycaemic control of GDM women [63]. In contrast to Moses et al. [62], Grant et al. reported lower dietary GI in the low-GI vs. the control group (49 vs. 58, p = 0.001). Improvements in glycaemic control in both groups were reported, but 58% of low-GI group had postprandial glucose within target as compared to 49% of control group (p < 0.001). This study was not powered to detect the small difference in self-monitored blood glucose (0.1–0.2 mmol/L) and postprandial blood glucose (1.2 mmol/L) observed between the study groups.

The most recent study by Hu et al. [64] was a relatively short 5-day intervention that compared the effectiveness of a low-GI staple versus a normal diabetic control diet among GDM women (n = 140) in Guangdong, China. Similar to the earlier studies, postprandial glucose levels were significantly reduced in both groups. However, post-intervention glucose levels taken after each meal were significantly reduced only in the low-GI group. There were also significantly greater reductions in glucose values from baseline in low-GI compared to the control group. The researchers observed a reduction in glucose parameters after breakfast in this group, though low-GI staple foods were only consumed at lunch and dinner. While the generalisability of the study findings may be limited to Asian women and the feasibility of adhering to a low-GI staple diet can be questioned due to a very short intervention period, the study nevertheless has set a good precedence for future exploration in this area.

Cumulatively these findings suggest that lowering the GI of standard diets by substituting high-GI staples with low-GI options may improve management of glycaemia in GDM women and reduce the likelihood of requiring insulin therapy. This interpretation is further supported by a meta-analysis [65] involving 257 participants that confirmed lesser use of insulin in the low-GI diet group (RR = 0.767, 95% CI = 0.597–0.986, p = 0.039) compared to those in control group. This translates into 13 out of 100 GDM women avoiding the use of insulin by adopting a low-GI diet during pregnancy [65].

Prevention of Complications in Pregnancy and Neonatal Outcomes

Foeto-placental overgrowth and higher infant body fat has been associated with high-GI intake during pregnancy, while low-GI diet reduces these tendencies [62, 66]. A small but intensive study by Moses and colleagues [62] showed that the consumption of low-GI diet in the second and third trimesters in normoglycaemic mothers reduced foetal birth weight, foetal percentile and Ponderal index. However PREGGIO, a similar but larger trial [67] found that an early intervention at 20 weeks of gestation did not result in significant differences in similar neonatal outcomes.

Accordingly, Louie et al. reported no significant difference in pregnancy outcomes such as birth weight, birth weight centile, prevalence of macrosomia, Ponderal index and adverse pregnancy outcomes after a low-GI dietary intervention that included a minimum of three face-to-face counselling sessions with a dietician [68]. The researchers postulated that a relative small five-point difference in GI between the study groups, early nutrition counselling for both groups, relatively lower GI than norm at the baseline, timing (third trimester) and short duration of the intervention (6–7 weeks) may have contributed to the findings. Another justification for the lack of difference may be the high proportion of participants with normal BMI (68%) and the researchers are now hypothesising low-GI diet to be more efficient among overweight and obese pregnant mothers who have higher level of insulin resistance and deficiency of β-cells [69]. However, it can be concluded that both low-GI and high fibre diet produce optimal pregnancy outcomes and this further strengthens the argument for safety of low-GI diet in the management of GDM.

The significant relationship between maternal glycaemic control and neonatal outcomes has been well-established [70,71,72,73]. Higher fasting glucose during initiation of diet therapy was associated with increased neonatal fat mass and elevated C-peptide among women treated for mild GDM in a multicentre RCT [71]. A higher prevalence of elevated C-peptide levels and neonatal outcomes such as macrosomia and large-for-gestational age babies were found among women with higher fasting blood glucose at the final two weeks of gestation. The findings were consistent with an earlier study which described fasting glucose levels to be associated with neonatal adiposity and increased skinfold thickness in neonates, regardless of whether maternal GDM was treated with diet or insulin [72]. Expectedly, secondary analysis of the ROLO study [70] found low-GI dietary intervention in pregnancy to have a beneficial effect on neonatal central adiposity, which was also positively associated with mother’s postprandial glucose. Although the study was conducted among normoglycaemic pregnant women, modest reductions in GI and GL were sufficient to lower neonatal waist: length ratio in the intervention group. This indicates that improved dietary carbohydrate quality may be associated with reduced neonatal central adiposity rather than birth weight. More importantly, epidemiologic studies among healthy pregnant women have found associations between high diet GI and congenital malformations such as neural tube defects, musculoskeletal and gastrointestinal defects [74].

These findings illustrate the importance of carbohydrate quality during pregnancy to promote neonatal well-being. However, while available data indicates the potential role of low-GI diets in reducing fat mass and central adiposity in neonates born to GDM mothers, there is insufficient evidence to establish the benefit of low-GI diets in preventing excessive maternal weight gain, foetal abnormalities, pregnancy complications or adverse pregnancy outcomes.

Prevention of GDM Recurrence and Overt Development of Diabetes

There is limited evidence relating low-GI diets and recurrence of GDM or development of T2DM among women with prior GDM. A recent Asian study [75] among women with prior GDM, compared the effectiveness of low-GI diet and conventional healthy dietary recommendations and reported improvements in glucose tolerance with low-GI educational intervention. The greatest improvement in glucose tolerance was observed among women with higher baseline insulin levels and in the lowest quartile of dietary GI at six months. The researchers also noted a significant reduction in 2-h post-75 g-oral glucose tolerance test (2HPP). In contrast, 2HPP levels increased in the comparison group, resulting in a significant difference in 2HPP changes between groups (Mean difference = 2.4 mmol/L, p = 0.004). It was suggested that a reduction of 2HPP by more than 0.84 mmol/L may halve the risk for T2DM and low-GI diet may be able to deliver that especially among women with a history of GDM and higher insulin levels.

In another distinctive study by Ostman and colleagues [76], seven women with impaired glucose tolerance and history of GDM were provided with either low-GI/high-fibre or high-GI/low-fibre bread products, during two consecutive 3-week periods, separated by a three-week washout period. The women receiving low-GI/high fibre bread had 35% lower insulin response to intravenous glucose challenge, though no effect was found on fasting glucose, insulin or lipid markers within the short 3 weeks of intervention. However, the sustainability of the effect remains to be established.

Concerns with the Use of Low-GI Diets

Since its inception, the utility of GI concept has been voraciously debated citing methodological issues and nutritional concerns [18].

Methodological Issues

Among the technical objections, the applicability of GI in mixed meals is predominantly questioned. However, studies reporting a lack of association between GI and glycaemic response when foods are taken as part of a mixed meal [77, 78] are thought to be methodologically flawed [14]. When analysed using standardised methods, the relative glycaemic impact of mixed meals is reportedly predicted by the amount of available carbohydrate they contain and the GI of their components [14].

The practical applicability of GI concept is also limited by the lack of a comprehensive GI database [24, 27]. While the international listing of GI and GL values is indeed comprehensive [13], determination of GI values of local foods is a work in progress in many countries. GI determination is cost and labour intensive and simplified methods have been devised to appropriately match foods and assign GI values to those with unknown values, till more local GI values become available [79]. As many factors affect the GI of a food, including its species, maturity (ripeness), storage time, processing and cooking method, [27], technical uncertainties exist in this estimation process. Estimation of diet GI and GL require in-depth knowledge of carbohydrate intake [16] and food composition tables lack the intricate detail necessary to accurately match foods and their glycaemic response [80]. However, this limitation affects not only the reliability of estimating dietary GI but of all other nutrients as well. Specific biomarkers to assess diet intake, including diet GI, need to be established and will improve the objectivity of dietary assessments.

The relationship between the dietary fibre content and the GI of a food despite being modest [81], confounds existing evidence for the health impact of low-GI diets which are also consistently higher in dietary fibre [27]. While the proponents of GI point to validity and reproducibility of GI values determined in standardised laboratories [29], phenotypic differences among populations in response to starch exist [82] and may limit the application of GI values as it is currently determined.

Nutritional Concerns

Nutritional concerns in using GI stem from the fear that it may incite public to consume foods low in GI but high in fat and sugars like ice-cream, cookies, etc. [18, 27, 83]. However, GI proponents argue that GI should be applied only to low-fat starchy foods [84]. GI was never meant to be used in “isolation”, but as an adjunct to other healthy eating principles [27]. Therefore the GI concept cannot replace, but should rather supplement existing nutritional strategies [18]. Perhaps the best approach to include GI education in diabetes counselling is to focus on individualisation [83] and this requires appropriate dietetic supervision.

Practical Issues with Implementation

The complexity of the GI/GL concepts make it difficult for patients to comprehend and implement the recommendations [18, 83, 85]. However, many low-GI diet books for weight control and wellness have been well-received in the West. Whether this acceptance can be extended to the other parts of the globe remains to be answered. Interestingly, various efforts at developing simplified GI-education modules have been successful. Categorising carbohydrates with simple terms like “gushers” and “tricklers” may ease patient comprehension of the concept [17]. Asian RCTs have shown that adults can be counselled to follow low-GI diets without having to memorise GI values [24, 86]. However, these are findings from clinical trials run by trained researchers and the practicality of providing GI-education in conventional healthcare settings remains to be proven.

Another concern with low-GI diets is that it can limit food choices and compromise nutritional adequacy [18, 83]. This may be especially important when dealing with pregnant women. Although traditional Indian and Greek cuisines include more low-GI foods than typical Western diets [83], adopting these food patterns may not be practical for all. Furthermore, food industries face challenges in producing palatable low-GI foods [84]. The issue is further compounded by the absence of a universally accepted logo that would facilitate consumer recognition of low-GI products.

While trials lasting a year or more show similar rates of adherence to low-GI and standard diets across continents [37, 87, 88], feasibility of long-term adherence to low-GI diets is unknown. While it may be possible to plan low-GI diets economically [86], its cost-effectiveness also remains to be established.

Recommendations

While emerging evidence suggests the possible benefit of low-GI diets in GDM management, there is urgent need for validation of the results. Optimising caloric intake to individual needs, restricting saturated fat, and distributing carbohydrates throughout the day will aid management of body weight and prevent of the degeneration of glucose tolerance in GDM women. Dietary recommendations should continue to encourage a moderate carbohydrate diet (45–50%), with adequate dietary fibre (25–30 g). Accordingly, dietary recommendations should encourage the inclusion of whole grains, beans, rolled oats, low-fat dairy and lean meat products, while being mindful of the daily energy needs. These strategies while in-line with conventional dietary goals, will also lower diet GI. Switching to low-GI-staples (such as whole grain breads, low-GI rice varieties and pasta) can be encouraged, taking cues from individual preferences. While adopting low-GI diets, there is a need to continue monitoring the portion sizes since postprandial glycaemia is affected by both the quantity and quality of carbohydrates. Low-GI diets that satisfy other nutritional considerations are acceptable in the treatment of GDM.

Conclusions

Existing evidence suggests that lowering the GI of conventional healthy diets may be beneficial in GDM treatment for managing maternal glycaemia and neonatal adiposity. However, a few practical issues in implementing low-GI dietary recommendations remain unresolved at present. There is an urgent need for adequately powered, well-controlled trials to further investigate the feasibility, acceptability, adherence, safety, clinical and cost effectiveness of low-GI dietary recommendations in GDM management.