The Metabolic Effects of a Hyperglycaemic Meal in Lean and Obese Individuals Using a [14C]-Glucose Microtracer Approach

Obesity is associated with substantial metabolic dysregulation, including impaired glucose handling, increased oxidative stress, and altered nutrient partitioning. A metabolic pathway of particular interest in this context is the polyol pathway, in which glucose is converted to sorbitol by aldose reductase and subsequently to fructose. Preclinical studies suggest that flux through this pathway increases when intracellular glucose concentrations rise, such as during hyperglycaemia or insulin resistance. Greater activity of this pathway has been linked to the formation of advanced glycation end products, oxidative stress, and stimulation of de novo lipogenesis. These mechanisms have been proposed as contributors to metabolic complications commonly observed in individuals with obesity. Despite these findings from animal and in vitro studies, polyol pathway activity and its regulation by dietary glycaemic load have not been systematically quantified in humans. This clinical trial addresses this gap by applying a highly sensitive [14C]-glucose microtracer approach to measure the metabolic fate of glucose following ingestion of meals with differing glycaemic properties in lean individuals and individuals with obesity.

The study makes use of uniformly labelled [14C]-glucose, which allows tracing of glucose-derived carbon into metabolic intermediates, expired CO₂, urine, faeces, and lipids using Accelerator Mass Spectrometry. This technology enables quantification of metabolic products with extremely small isotope doses, resulting in radiation exposure far below natural background levels. Through this approach, the study can directly assess the extent to which ingested glucose is oxidized, converted into polyol pathway intermediates, incorporated into lipids, or excreted. The microtracer method provides a level of mechanistic resolution that cannot be achieved with stable isotopes or traditional metabolic tests.

The study design includes a single 72-hour metabolic test period during which participants consume either a high- or low-glycaemic breakfast depending on group allocation. The subsequent ingestion of [14C]-glucose allows tracking of postprandial metabolic routing under these two dietary conditions. Lean participants are randomized to either glycaemic condition, whereas individuals with obesity receive the high-glycaemic meal to address the study's main objective of comparing pathway activity between lean and obese phenotypes under hyperglycaemic challenge. Although the protocol includes multiple laboratory measurements, the aim of this Detailed Description is not to reproduce the procedure schedule, but to summarize the scientific characteristics of the design. In general terms, the study integrates whole-body, biochemical, and tissue-level metabolic assessments to characterize glucose metabolism in vivo.

Whole-body energy expenditure and substrate oxidation are measured repeatedly through indirect calorimetry to determine the proportion of glucose that is oxidized versus stored or redirected into other metabolic pathways. Breath samples are collected to quantify 14CO₂ production, which provides a sensitive measure of glucose oxidation and contributes to mass balance calculations. Serial blood sampling enables the measurement of plasma glucose, insulin, and the appearance of 14C-labelled metabolites, providing insight into the dynamics of glucose disposal and conversion to sorbitol, fructose, and downstream metabolites.

A distinctive feature of this study is the assessment of forearm arteriovenous metabolite balance, obtained from arterialized and deep-venous blood sampling combined with Doppler ultrasound measurement of forearm blood flow. This technique allows calculation of tissue-specific uptake and release of glucose and glucose-derived metabolites across skeletal muscle, a major site of postprandial glucose disposal. These measurements offer a physiologically meaningful index of muscle insulin sensitivity and provide additional perspective on how glycaemic load and obesity influence metabolic flux at the tissue level.