The goal of this clinical trial is to learn how a high-glycaemic meal affects the way the body processes glucose in healthy adults who are either lean or have obesity. The main questions it aims to answer are: Is the polyol pathway (conversion of glucose into sorbitol and fructose) more active after a hyperglycaemic meal? Is this pathway more active in individuals with obesity compared with lean individuals? We will compare people who eat a high-glycaemic meal with those who eat a low-glycaemic meal to see whether meal type changes how glucose is metabolized in the body. Participants will drink a small amount of 14C-labelled glucose so researchers can trace how the body uses glucose, spend one long study day (about 12 hours in the lab) plus short morning visits on days 2 to 4, and undergo repeated measurements, including blood sampling, breath sampling, indirect calorimetry, and complete urine and stool collection for 72 hours. This information will help us understand how glucose is processed in the body and whether people with obesity handle glucose differently than lean individuals.

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.

Collection of urine and faeces for 72 hours enables full recovery of the administered tracer, allowing detailed mass balance calculations. This information reveals how much of the ingested glucose is oxidized, excreted, or directed into biosynthetic pathways. By integrating data from breath, blood, urine, and faeces, the study can comprehensively map the metabolic fate of glucose and determine how this differs across physiological states.

The primary scientific questions addressed by this study are whether polyol pathway activity increases under hyperglycaemic conditions in humans and whether individuals with obesity demonstrate greater pathway activation than lean individuals. The study further explores the relationship between polyol pathway activation and de novo lipogenesis and evaluates whether the glycaemic load of a meal modulates these pathways. By combining microtracer-based flux analysis with whole-body and tissue-specific measurements, the study aims to provide mechanistic insight into early metabolic disturbances associated with obesity.

Overall, this trial will generate foundational human data on endogenous fructose production and glucose routing in response to dietary glycaemic load. These findings may contribute to improved understanding of how carbohydrate metabolism becomes dysregulated in obesity and may support the development of nutritional or therapeutic strategies targeting glucose-handling pathways. The study also demonstrates the potential of Accelerator Mass Spectrometry as a powerful tool for investigating nutrient metabolism in vivo with minimal participant burden and extremely low radiation exposure.

Inclusion Criteria:

• Healthy males and females using contraception during and for 3 months after the study.
• Aged from 18-65 years at the time of signing informed consent
• 18.5 < BMI < 25 kg·m2 or 30< BMI <35 kg·m2
• Must be willing and able to communicate and participate in the whole study, including consumption of 14C-glucose and meals offered during study conduct
• Must have regular bowel movements (i.e. average stool production of ≥1 and ≤3 stools per day)
• Must usually eat 3 meals per day (i.e. breakfast, lunch and dinner)

Exclusion Criteria:

• Diabetes (Type 1, Type 2, or genetic form of diabetes)
• Any diagnosed cardiovascular (heart) disease or high blood pressure (≥140 mmHg systolic and/or ≥90 mmHg diastolic)
• HbA1c higher than 53 mmol/mol
• History of clinically significant cardiovascular, renal, hepatic, chronic respiratory or gastro-intestinal disease, immunodeficiency, endocrine, neurological, or psychiatric disorders
• Any diagnosed respiratory disease, such as COPD or asthma
• Any previous motor disorders or disorders in muscle and/or lipid metabolism
• Known severe kidney problems
• Presence of an ulcer in the stomach or gut and/or strong history of indigestion
• Recent or chronic history of diarrhoea
• Known anaemia
• A personal or family history of thrombosis (clots), epilepsy, seizures, or schizophrenia.
• Regular use of dietary supplements (>3 times per week)
• Chronic use of any prescribed or over the counter pharmaceuticals (excluding oral contraceptives and contraceptive devices)
• History of any drug or alcohol abuse in the past two years
• A confirmed positive alcohol breath test at screening or admission
• Drug use
• Claustrophobia
• Subjects who are on a weight loss diet or following a high calorific/high protein diet to gain weight
• Subjects with functional constipation
• Any known food allergies or intolerances to the 14 major food allergens (celery, cereals containing gluten, crustaceans, eggs, fish, lupin, milk, molluscs, mustard, tree nuts, peanuts, sesame seeds, soybeans, sulphur dioxide and sulphites) or history of a malabsorption syndrome including coeliac disease
• Subjects who have regular gastrointestinal complaints including abdominal pain, stomach upsets and borborygmi or known or suspected irritable bowel syndrome
• Currently taking part in another scientific research
• Having received a product with 14C in the past 12 months
• Pregnant or breastfeeding
• Smoking or having used nicotine-containing products in the 6 months prior to the study.
• Subjects who have taken antibiotics within the 60 days prior to the adaptation period.
• Currently involved in a structured progressive resistance training programme (>3 times per week)
• Sedentary lifestyle as assessed using the International Physical Activity Questionnaire [IPAQ].
• Unable to give consent
• Employed or undertaking a thesis or internship at the department of Human and Animal Physiology

Individual participant data will not be shared outside the study team.