43 - Methods of measuring nutrient substrate utilization using stable isotopes

Summary

Introduction

During fetal life a continuous supply of nutrients is transferred to the fetus via the placenta. Transplacental transported glucose is the major energy substrate for the fetus, which has glucose demands of 4–8 mg kg⁻¹ min⁻¹. The maternal–fetal transport of glucose occurs via passive diffusion facilitated by the glucose transporter system (specifically GLUT-1). Amino acids are transferred by active transport, and several classes of amino acid transporters have been identified in human placentas. Transplacental transfer of fatty acids is small. It is regulated by maternal fatty acid concentrations and is mediated by a number of different proteins. At birth, the transplacental supply of nutrients is abruptly interrupted and the newborn infant must first mobilize its own substrate stores and then rapidly adjust to enteral feedings to meet the metabolic needs.

Glucose is the primary substrate for brain metabolism, and the brain utilizes about 20 times more glucose than muscle and fat per gram tissue. Infants have a large brain to body weight ratio (12% in infants v. 2% in an adult). Thus, the glucose turnover rate on a per kg body weight basis in an infant is three times higher than that of an adult, ∼30 μmol kg⁻¹ min⁻¹ (∼6 mg kg⁻¹ min⁻¹) v. 11 μmol kg⁻¹ min⁻¹ (∼2 mg kg⁻¹ min⁻¹). As a result, ∼90% of total glucose utilization in an infant is by the brain.