Re-Examining the Estimated Average Requirement for Carbohydrate Intake During Pregnancy: Adding Placental Glucose Consumption

Background: how first carbohydrate intake during pregnancy guidelines were set

The first guideline for carbohydrate intake during pregnancy was published by the Institute of Medicine in 2005 and has not been updated since. 

• Past recommendations for pregnancy focused on adequate protein and micronutrient intake
• The 2005 recommended dietary allowance (RDA), the estimate of carbohydrates needed for 97-98% of a population, is at least 175 g/d or 45% to 65% of total energy
• Dietary reference intake applies to all pregnant women
• Used to inform nutrition plans for preexisting and gestational diabetes

Diabetes during pregnancy – both preexisting and gestational – is on the rise globally.

• For more than 30 years, gestational diabetes mellitus treatment has focused on nutrition therapy to restrict carbohydrate consumption (<40% of energy intake).
• Nutrition therapy was designed to:
  • Meet maternal and fetal energy needs
  • Lessen post-meal maternal glycemia
  • Reduce fetal overgrowth

The mean population intake of carbohydrates is declining in some regions of the world, and many pregnant women are not consuming the recommended dietary allowance for carbohydrates.

• RDA for carbohydrate intake during pregnancy was created to meet maternal and fetal brain glucose requirements
• Placenta and the brain depend on glucose as the dominant energy substrate
• Concern that some pregnant individuals may not consume enough carbohydrates and/or will consume too much fat and protein

Researchers, Paul Rozance, MD, and Teri Hernandez, PhD, from Children’s Hospital Colorado and the University of Colorado School of Medicine and College of Nursing had two objectives for this paper:

• Review the physiologic rationale for the recommended dietary allowance for carbohydrate intake in pregnancy and re-evaluate appropriateness of the recommendation.
• Re-examine the recommended dietary allowance to consider placental glucose needs based on recently published in vivo human measurements of placental glucose consumption.

Study authors proposed placental glucose utilization should be an important consideration for nutrition guidelines during pregnancy.

Reviewing rationale for carbohydrate intake guidelines in pregnancy

The estimated average requirement for carbohydrate intake during pregnancy is determined by considering the brain's glucose requirements in non-pregnant individuals, with an additional component for fetal brain glucose needs.

• The brain is heavily reliant on glucose as its primary fuel substrate, supported by studies showing its high metabolic preference for glucose.
• The blood-brain barrier plays a key role in maintaining glucose supply to the brain while preventing the passage of neuroactive components from the blood.
• The brain can use ketoacids as a nutritional resource, but it prefers glucose, its most efficient fuel source.
• Zero carbohydrate intake during pregnancy may be possible, and studies in canines suggest adverse effects on offspring survival.

The estimated average requirement (covers the carbohydrate needs of ~50% of a population) and recommended dietary allowance (covers the needs of 97%–98% of a population) for carbohydrate aims to prevent the need for supplementation with alternate fuel substrates.

• In pregnancy, maternal physiology increasingly relies on fatty acids as gestation advances, making glucose critical for the fetal-placental unit.
• Both low and high protein intake extremes are associated with fetal growth restriction, which further complicates the need for protein in gluconeogenesis to support brain energy needs.

The estimated average requirement for carbohydrate intake in non-pregnant adults is 100 g/d. Physiologic factors considered include:

• Average adult brain weight
• Brain’s oxygen consumption rate
• Glucose requirement for cells that only metabolize glucose for energy, such as red blood cells
• Average brain glucose consumption rate without requiring alternative glucose supplementation

The fetal component of the estimated average requirement for pregnancy is 35 g/d. This takes into consideration:

• Average newborn brain weight
• Average brain glucose consumption rate at term without requiring alternative glucose supplementation
• Ability of the fetal brain to derive a small portion of energy from ketoacids when glucose is deficient
• Average maternal-fetal glucose transfer rate in sheep

The nutritional requirements for pregnancy aim to provide adequate glucose for both the maternal and fetal brain without resorting to alternative energy source.

• The maternal estimated average requirement for carbohydrate intake during pregnancy is 135 g/day (applies requirement for non-pregnant adults and fetal component of requirement for pregnancy).
• The recommended dietary allowance for pregnancy is ≥175 g/day (applies 15% interindividual coefficient of variation for rate of adult brain glucose utilization to both maternal and fetal estimates).

Is recommended dietary allowance for carbs during pregnancy still appropriate?

Estimated average requirement: adult brain

Recent data suggests the recommended dietary allowance of ≥175 g/day for carbohydrate intake during pregnancy remains appropriate.

• Newer brain measurement techniques support that the average brain's glucose consumption rate aligns with earlier recommendations, even though there's some variation among individuals.
• Interindividual coefficient of variation of 15% for the population mean continues to apply, suggesting the estimated average requirement of 100 g/day remains suitable

Estimated average requirement: fetal brain glucose consumption

The four-vessel sampling technique has allowed for in vivo studies of placental physiology and fetal substrate utilization during Cesarean section deliveries.

• ~30% of glucose extracted from maternal blood is consumed by the placenta, with the remaining 70% transferred to the fetus.
• ~10% of the glucose per liter of blood passing through the fetal-placental circulation is consumed by the fetus.

Study authors noted that the data do not provide an isolated estimate of fetal brain glucose utilization, but instead provide an estimate of whole-body glucose consumption. The in vivo data indicates the original estimated average requirement of 35 g/day for fetal brain glucose requirements remains appropriate with estimated average requirement of 100 g/day for maternal brain glucose needs, and interindividual coefficient of variation for adult brain glucose utilization.

The placenta is a glucose-consuming organ

The placenta plays a crucial role in transporting glucose from the mother to the fetus, relying on a maternal-fetal glucose concentration gradient.

• Like the brain, the placenta prefers glucose as its primary energy source and has similar specialized transport systems for glucose transfer.
  • Can use alternative fuels, but its capacity for gluconeogenesis remains uncertain
  • Highlights importance of maternal carbohydrate intake to meet placental glucose needs in addition to brain function during pregnancy.
• Using recent four-vessel data, the placental glucose consumption rate, adjusted for placental weight, suggests an estimated average requirement of 36 g/day
• If the placenta is included, recommended dietary allowance for carbohydrate intake during pregnancy would be adjusted to 220 g/day
  • Considers a conservative 15% coefficient of variation
  • Covers maternal, fetal, and placental glucose consumption

Discussion and conclusion: adjusting for placental glucose requirements

This study reevaluated the recommended dietary allowance for carbohydrate intake during pregnancy, considering various factors such as:

• Availability of four-vessel data on placental glucose utilization
• Popularity of low-carb diets in pregnancy
• Increase in maternal diabetes
• Existing recommendations were set almost two decades ago

While contemporary measurements support the original recommended dietary allowance for maternal and fetal brain glucose consumption (100 g/day and 35 g/day, respectively), placental glucose requirements have been overlooked.

The study authors propose placental glucose consumption be at the forefront of maternal nutrition considerations during pregnancy. Authors calculated:

• Potential new estimated average requirement of 171 g/day, encompassing maternal and fetal brain glucose needs as well as placental glucose needs
• Adjusted RDA of 220 g/day to supply adequate glucose without supplementation by alternative fuel

Study authors concluded by emphasized the importance of consuming high-quality carbohydrates over simple sugars and processed starches to meet recommended dietary allowance guidelines during pregnancy and that lower and upper safety thresholds for carbohydrate intake during pregnancy should be clarified.

This approach is globally relevant for pregnant women and their offspring and has specific implications for individuals with diabetes during pregnancy.