Glucose Metabolism in the Fetus and Newborn, and Methods for Its Investigation

Isotopic Tracers

Beginning in the 1980s, improvements in the synthesis of metabolites labeled with stable isotopes and sensitive mass-spectrometric methods have allowed investigators to examine glucose metabolism in the human fetus and newborn. In addition, improved synthetic techniques have increased the number of isotopic tracers that can be used simultaneously, allowing investigators to answer more complex questions. By combining novel tracer methods with measurements of energy consumption, the metabolic fate of fuel substrates (e.g., glucose, amino acids, fatty acids) can be quantified along with their contribution to overall fuel economy. To help localize these processes, imaging techniques have been developed to interrogate fetal usage of fuel substrates in a visuospatial and quantitative manner.

Isotopic tracers have proved invaluable in understanding the details of fuel metabolism. They can be used in vivo to quantify rates of metabolic turnover including the appearance and disappearance of a substrate, to quantify the utilization and metabolic fate of a substrate, to determine the contribution of a substrate to another compound, and/or to determine tissue-specific aspects of metabolism. The isotopic labels employed in tracers can be either stable or radioactive, with each having differing merits. The obvious advantage of stable isotopic tracers is that they are nonradioactive. However, another major advantage is that they often can be designed to carry positional information that delineates how specific portions of a molecule are metabolically transformed. Mass spectrometry is used to detect stable isotopic tracers. One disadvantage of stable isotopes is that they are typically less sensitive than radioactive tracers, owing in part to a higher natural abundance, and thus greater amounts of tracer are often required than for radioactive tracers. This point highlights a major advantage of radioactive tracers, in that even very small amounts can be detected. Another advantage of certain radioactive tracers, especially those that emit positrons, is that they can be imaged to determine their location in the body in real time. An example of imaging a glucose uptake tracer during pregnancy in rats is shown in Fig. 36.1 (experimental details are described by Sawatzke and colleagues ). However, a disadvantage of radioactive tracers is that they do not provide molecular positional information as readily compared to stable isotope tracers. Additionally, their radioactivity limits their use in human studies. Isotopic tracer experimentation is not only a complex subject with various pitfalls but also an opportunity for savvy and informative designs as highlighted in recent reviews.

Indirect Calorimetry

Energy expenditure can be estimated from the rate of oxygen consumption and carbon dioxide production by an infant. In this system, a hood or canopy is placed over the subject’s head, and a pump is used to draw air through the hood. The air exiting from the hood is thoroughly mixed. The concentration of O ₂ and CO ₂ is measured in the mixed air as is the total flow of air through the system. From this information, energy expenditure can be estimated. This general approach has been used over many years in a large number of studies in adults, children, and newborn infants, ^, and its limitations have been examined. ^,

Doubly Labeled ( ² H ₂ ¹⁸ O) Water

Doubly labeled water contains the stable isotopes deuterium and ¹⁸ O, and can be used to measure energy expenditure by enabling estimation of the rate of CO ₂ elimination from the body. Its advantages include that it can be applied to free-living individuals, that CO ₂ elimination is measured over hours to days providing an integrated measure of energy consumption, and that the method is relatively simple and noninvasive. It requires administration of a single dose of doubly labeled H ₂ O and a few subsequent samples of urine, saliva, or blood to be obtained to measure the changes in isotopic enrichment over several days. The underlying general principle is that the deuterium in doubly labeled water is eliminated as water ( ² H ₂ O), whereas the oxygen ( ¹⁸ O) is eliminated with water (H ₂ ¹⁸ O) and with carbon dioxide (CO ¹⁸ O), owing to the rapid equilibrium between H ₂ O and CO ₂ in the body. Thus the difference in turnover rates between the ¹⁸ O and ² H of water represents the CO ₂ elimination rate. Use of doubly labeled water to study energy metabolism has been extensively evaluated in humans, both adults and newborns.