Low-Carbohydrate Diets in Gestational Diabetes: A Summary of the Evidence for Dietitians

Gestational diabetes mellitus (GDM) is a form of glucose intolerance first diagnosed during pregnancy. GDM develops due to increasing insulin resistance in the second and third trimesters, driven by the “contra-insulin” effects of placental hormones. These hormones, including human placental lactogen (hPL), cortisol, and estrogen, interfere with insulin function, reducing glucose uptake by maternal cells.

GDM is associated with adverse short- and long-term outcomes for both mothers and their infants, including increased likelihood of preeclampsia, cesarean section, and progression to type 2 diabetes in the mother and macrosomia and neonatal hypoglycemia in the infant. 

Current strategies to manage GDM include dietary interventions, physical activity when appropriate, metformin and insulin. Many individuals diagnosed with GDM are closely monitored by an interdisciplinary GDM clinic. 

Low-carbohydrate diets (LCDs) have been proposed by some as a dietary strategy for managing GDM, but their safety and efficacy remain under scrutiny. This article explores the impact of LCDs on maternal and fetal health, current guidelines, potential risks, research limitations, and practical recommendations for Registered Dietitians.

Current Guidelines on Carbohydrate Intake in GDM

Some restriction of carbohydrate intake is recommended to help support glycemic control. Health organizations do not currently recommend low or very low carbohydrate diets for GDM management. 

• Diabetes Canada: Recommends a minimum of 175 g/day of carbohydrate distributed over 3 moderate-sized meals and 2 or more snacks 

• The National Academy of Medicine (formerly Institute of Medicine) and the American Diabetes Association: Recommend a minimum intake of 175 g/day of carbohydrates during pregnancy to ensure adequate fetal development

  
  

What is considered a Low-Carbohydrate Diet in Pregnancy?

There is no single definition of low-carbohydrate diets (LCDs) in pregnancy, and various studies have used different thresholds such as:

• Moderately low-carbohydrate: 135–165 g/day or 33–40% of total energy intake

• Low-carbohydrate: <130 g/day

• Very low-carbohydrate (ketogenic): <50 g/day 

Effects of Low Carbohydrate Diets on Maternal and Fetal Outcomes

Glycemic Control and Insulin Requirements

GDM management primarily focuses on glycemic control to minimize complications. LCDs are thought to reduce postprandial glucose levels by limiting carbohydrate intake, but studies have shown mixed results.

• One randomized controlled trial compared a modestly lower carbohydrate (MLC) diet (135 g/day) to routine care (200 g/day) in 46 women with GDM. They found that the LCD did not significantly improve glycemic control, nor did it reduce insulin requirements compared to the control group. Furthermore, compliance was a challenge, as only 20% of participants met the prescribed carbohydrate target​.

• Another study compared a low-carbohydrate diet (40% of total energy intake) with a higher-carbohydrate diet (55%) in 152 women with GDM. They found no significant differences in insulin use between the groups (both around 55%) and reported similar pregnancy outcomes​.

• A review of recent studies on LCDs in pregnancy found that carbohydrate intake below 175 g/day was associated with lower insulin use in some cases but did not universally improve glucose tolerance​.

• One small randomized control trial reported that women following an LCD (45% of total energy) had lower postprandial glucose levels compared to those consuming a high-carbohydrate diet (65%). However, fasting glucose levels remained unchanged​.

These findings indicate that while LCDs may help regulate postprandial glucose levels, they do not necessarily reduce the need for insulin in GDM patients.

Long-Term Risk of Type 2 Diabetes

Women with GDM are at increased risk of developing T2DM postpartum. The long-term impact of LCDs on this risk is not well-established.

One prospective study that followed 4,502 women from the Nurses' Health Study II (NHSII) over 20 years. They found that a low-carbohydrate diet high in animal fats and proteins increased the risk of developing T2DM. In contrast, plant-based LCDs did not significantly increase T2DM risk​. These findings were also confirmed by a more recent study. 

Micronutrient Deficiencies

Carbohydrate rich foods are also a source of micronutrients that are important for pregnancy.

• Folate: Low carbohydrate intake may reduce folate consumption as grains are fortified a source of folic acid in many countries 

• Iodine: LCDs often limit iodine-rich foods like fortified bread or dairy, raising concerns about fetal thyroid function

• Iron: LCDs can result in lower iron intake due to the reduction of fortified grains and legumes, common sources of dietary iron

• Fibre: LCDs frequently lack fiber, leading to gastrointestinal issues like constipation

Glucose Availability for the Fetal Brain

Glucose is the primary energy source for fetal brain development, transported across the placenta via facilitated diffusion, driven by the maternal-fetal glucose gradient. The Institute of Medicine (IOM) recommends ≥175 g/day of carbohydrates during pregnancy, with at least 33-35 g/day allocated for fetal brain growth.

The concern is that low-carbohydrate diets (LCDs) may reduce maternal glucose levels, potentially compromising placental glucose transfer. One study found that while LCDs did not significantly alter ketone levels, neonates had smaller head circumferences, raising concerns about fetal brain development​. Similarly, a Tanzanian cohort consuming ≤100 g/day of carbohydrates had a 40% rate of neonatal microcephaly.

Fetal Exposure to Maternal Ketones 

Ketone exposure is another concern. Ketonemia is defined as elevated blood ketones. Studies show that severe carbohydrate and energy restriction (e.g. 50% reduction in energy) may lead to increased ketonemia and ketonuria. 

There are concerns that prolonged ketonemia may impact fetal brain development. Currently, no consistent association has been found between ketonemia or ketonuria and poor fetal outcomes.  However, one US cohort study found an inverse relationship between maternal ketone levels and offspring cognitive scores at ages 2-5 years, thus more research is needed to explore the potential risks of fetal ketone exposure​.

The exact level of carbohydrate restriction required to elevate ketone levels remains unclear. For instance, one study measuring blood ketones in women following a low-carbohydrate diet (~135 g/day) found no significant difference between the intervention and control groups. This lack of effect may be attributed to low adherence to the diet, as noted by the authors.

More research is needed to determine the specific carbohydrate threshold required to induce ketosis, and evaluate potential effects of ketonemia on fetal outcomes.

Macrosomia Risk 

A primary goal in GDM management is to prevent excessive fetal growth, which can lead to birth complications.

A low-carbohydrate diet in pregnancy may lead to higher maternal triglycerides (TG) and free fatty acids (FFA), potentially increasing fetal exposure to lipids. Some studies suggest that maternal lipids, particularly triglycerides and free fatty acids, may contribute to fetal overgrowth more than glucose alone. However, the role of LCDs in modulating these lipid levels remains unclear.

Concerns exist that higher fat intake in low-carb diets could heighten maternal insulin resistance, further increasing lipid transfer to the fetus. A secondary analysis of a multi-site RCT found that women in the lower carbohydrate group had higher fasting FFA and glucose, though only FFA remained elevated later in pregnancy. However, no significant associations were found between maternal lipids and fetal growth. More research is needed to clarify the impact of dietary fat intake on maternal lipid levels and fetal outcomes.

The glycemic index of the diet may be more predictive of macrosomia risk than carbohydrate load alone. One systematic review found no significant difference in birth weight or incidence of large-for-gestational-age (LGA) infants between LCDs and standard diets​ but found that lower glycemic index diets reduced macrosomia risk. 

Long-Term Metabolic Health in Offspring

Few studies have investigated the long-term metabolic effects of LCDs in offspring. Studies on fetal programming and long-term metabolic health are lacking, making it difficult to draw definitive conclusions​.

Limitations of Current Research

Despite growing interest in LCDs for GDM management, several limitations remain:

• Variability in Study Designs: LCD definitions vary, with carbohydrate intake ranging from 40% to <10% of total energy, making comparisons difficult 

• Short Study Durations: Most studies assess outcomes only until delivery, with limited data on long-term maternal and child health 

• Potential Bias in Dietary Assessment: Many studies rely on self-reported dietary intake, which may be subject to recall bias 

• Confounding Variables: Differences in physical activity, insulin use, and gestational weight gain complicate the isolation of LCD effects

Key Takeaways and Summary

• Health organizations recommend a minimum of 175 g/day of carbohydrates during pregnancy to support fetal development

  
  

• While some studies suggest that moderately low-carbohydrate diets (135-165 g/day) may help regulate postprandial glucose levels, they do not consistently reduce insulin use or improve overall pregnancy outcomes

  
  

• Low-carbohydrate diets may increase the risk of micronutrient deficiencies (folate, iodine, iron, fiber), reduce glucose availability for fetal brain development, and lead to higher maternal triglycerides, which could impact fetal growth. Some studies also raise concerns about fetal ketone exposure and its long-term effects on cognitive outcomes

  
  

• There is insufficient research on the long-term metabolic health of offspring exposed to low-carbohydrate diets during pregnancy. More high-quality, long-term randomized controlled trials are needed

  
  

• Given the inconsistent evidence and potential risks, a moderate carbohydrate intake with an emphasis on high-fiber, low-glycemic index foods and unsaturated fats is recommended

Conclusion

LCDs do not consistently reduce the need for insulin or improve pregnancy outcomes. 

A moderate reduction in carbohydrates (40-45% of total energy intake), with an emphasis on high-fiber, low-glycemic index sources, may be the most balanced approach.

Extreme carbohydrate restriction (<130 g/day) should be avoided due to potential risks, particularly for fetal development. More well-designed, long-term RCTs are needed to determine the optimal carbohydrate intake for women with GDM and to assess the metabolic health of offspring.