Among the eight studies investigated in this systematic review, four showed a protective association between low GI/GL and pregnancy-related outcomes, three showed no association, while one showed a potential increase in SGA risk. More studies are required to provide a convincing evidence base to support/reject the routine use of a low-GI diet in pregnancy. The current evidence suggests that the risk associated with a low-GI diet during pregnancy is minimal.
Traditionally, pregnancy diets recommended by health groups [24
] and government authorities [26
] focus on nutrient adequacy because the requirements for many nutrients increase during pregnancy [29
]. These recommendations, however, do not acknowledge any specific consideration of the glycemic potency of the foods in the diet. Many common staples such as rice, white bread, and potatoes, while nutritious, are high GI. The typical pregnancy diet is therefore of moderate to high GI [19
], depending on carbohydrate distribution and proportions of high-GI starchy foods versus low-GI foods such as fruit and dairy products.
Elevated maternal blood glucose levels are well recognised as contributing to excessive fetal growth [13
]. Among women with unrecognized maternal gestational diabetes mellitus (GDM), the prevalence of LGA infants is fivefold higher compared to nondiabetic controls and twofold higher compared to diet-controlled GDM women [30
]. The HAPO study also provided robust evidence that maternal hyperglycemia 1-hour after a 75
g oral glucose tolerance test (OGTT), even within the recommended ranges, increases adverse pregnancy outcomes [31
], and the risks increased further as the 1-hour postload blood glucose level rose. The 75
g OGTT can be regarded as a surrogate marker of meal postprandial glycemia. Therefore, maternal hyperglycemia (fasting, after a glucose load, and possibly postprandial) is likely to lead to adverse pregnancy outcomes [32
Interventions that reduce maternal postprandial blood glucose levels, including dietary strategies, have been found to be effective in reducing macrosomia (birth weight > 4
kg) and childhood obesity in diabetic pregnancies [12
]. Moderate carbohydrate restriction is the most straight forward and commonly used strategy to achieve this as carbohydrates are the main determinant of postprandial blood glucose level [33
]. However, a recent meta-analysis of randomized clinical trials among normal pregnant women showed lack of benefits of increasing protein intake in place of carbohydrate and the potential for increased risk for small-for-gestational-age (SGA) babies [34
]. For this reason, reduction of maternal postprandial glycaemia by substituting dietary carbohydrate with protein may not be recommended in healthy pregnancies at the present time.
On the other hand, postprandial glycemia can be reduced without carbohydrate restriction by slowing down the rate of carbohydrate digestion and absorption. Compared to moderate- or high-GI foods containing similar amount of carbohydrates, low-GI foods have been demonstrated to reduce postprandial spikes of blood glucose level in healthy individuals [35
]. A low-GI meal pattern therefore represents an alternative strategy for reducing postprandial glycemia in normal pregnancy without reducing the carbohydrate intake. The use of low-GI diets in normal pregnancy is controversial because any reduction in the rate of LGA may be matched by an increase in SGA, as has been shown in the epidemiological study by Scholl et al. [16
] which reported an alarming increase of 75% in SGA risk. However, the rationale for assignment of GI values in their food database was not described and may not have been accurate. Women in the lowest quintile of GI also ate more refined sugar. Hence, poor overall dietary intake in this low-income population may have contributed to a contradictory finding and limits generalisation. Indeed the two intervention studies (one in normal pregnancy and one in GDM pregnancy) by Moses et al. [19
] showed that there is no significant increase in SGA in subjects following a low-GI diet.
Because some low-GI foods have been associated with higher satiety [36
], a low-GI diet may also benefit pregnant women by reducing excessive maternal weight gain. High maternal weight gain has been linked to an increased risk of pregnancy complications [38
], excessive fetal growth [39
], and long-term adverse health outcomes for the mother-infant pair [41
]. The study by Deierlein et al. [17
], however, reported no association between GL and total gestational weight gain, but total carbohydrate intake and GI were not reported separately. It is possible that a high intake of high-GI carbohydrates has a detrimental effect while a high intake of low-GI carbohydrate may be neutral or protective, as demonstrated in recent studies on risk of cardiovascular disease [42
Intervention studies in normal pregnancy are more supportive. The study by Clapp [18
] was the first of its kind to investigate the effect of a low-GI diet on various pregnancy outcomes. While he reported results that favored the use of a low-GI diet during normal pregnancy, this study should be carefully interpreted. The number of subjects was small (6 in each group), the GI of the diets was not determined and the macronutrient proportions were not given. Differences in the amount of carbohydrate would also potentially affect outcomes. Expressed as a proportion of total energy intake, total carbohydrate intake has previously been shown to be associated with several pregnancy outcomes such as LGA and macrosomia, at least in pregnancy complicated with GDM [44
]. The more recent study by Moses et al. [19
] on the other hand, provided stronger evidence that a low-GI diet improves pregnancy outcomes which is consistent with the findings of Clapp [18
]. Unfortunately, the pregnant women in the study by Moses et al. reverted back to their baseline diet within 2 years [20
], suggesting that dietetic followup may benefit these women particularly if they plan to become pregnant again, as prepregnancy GI and GL has been linked to increased risk of developing GDM [21
It is now generally accepted that treating even mild GDM results in marked improvement in pregnancy outcomes. This view is supported by the large-scale Australian Carbohydrate Intolerance Study in Pregnant Women (ACHOIS) study [46
] in which women with mild GDM were either treated intensively or attended routine antenatal care for healthy pregnancies. Intensive treatment in mild GDM, compared to routine care, resulted in reduced risks of preeclampsia, perinatal morbidity (e.g., shoulder dystocia), as well as macrosomia. The recent Maternal-Fetal Medicine Unit (MFMU) Network study [47
] provided similar evidence. Even though lowering the dietary GI was not a specific aim of the dietary intervention in the ACHOIS study, it is likely that the GI was lowered because many of the healthy foods routinely recommended in pregnancy, such as fruit and dairy foods, are low GI. The dietary intervention in the MFMU study, on the other hand, may have incorporated low-GI foods as it was based on the American Diabetes Association position statement on “Nutrition Recommendation and Intervention for Diabetes
], which explicitly recommends “…low-glycemic index foods that are rich in fibre and other important nutrients are to be encouraged.
Postprandial glucose excursion has been associated with adverse pregnancy outcomes in women with GDM [49
]. Moderation of carbohydrate intake is usually recommended as the main and first-line strategy to achieve postprandial euglycemia [50
]. However, there is evidence to suggest that overrestriction of carbohydrate in pregnancy complicated by GDM may increase the risk of fetal macrosomia [45
], and therefore consideration to the glycemic potency of the carbohydrates in the diet is also important. By consuming low-GI carbohydrates one may achieve an adequate carbohydrate intake with lower postprandial blood glucose levels. The study by Moses et al. [23
] suggested that a low-GI diet in GDM pregnancy can effectively reduce the need for insulin for optimal blood glucose management.
Clearly there is a lack of research in this area despite growing interest from the medical and nutrition community. In 2008, Tieu et al. [51
] conducted a systematic review of dietary strategies for the prevention of GDM. They found only two trials [19
= 82 in total) comparing the effect of a low-GI versus high-GI diets on obstetric outcomes and concluded that the evidence to support the use of a low-GI diet during pregnancy was inadequate, mainly due to the small number and the high heterogeneity of the trials available. A larger, randomized controlled trial investigating the effect of a low-GI diet on outcomes in GDM pregnancy, such as birth weight z
-score, infant ponderal index, so forth, is currently underway [53
]. More studies, particularly those which intervene at an earlier stage of pregnancy, are warranted.