Postprandial Glucose Kinetics After Carbohydrate Restriction

This study will be a randomized, controlled, open‐label, two‐period crossover dietaryintervention designed to compare the effects of a low‐carbohydrate diet with those of anenergy‐matched Mediterranean diet on fasting and post‐load glucose metabolism. The studywill enroll adults with overweight or obesity and impaired glucose regulation who do nothave diabetes.The main objective will be to determine whether four weeks of carbohydrate restrictionwill modify the rate of appearance of orally ingested glucose, endogenous glucoseproduction, and glucose clearance compared with a Mediterranean diet prescribed with asimilar energy deficit. The study will also assess whether the two dietary interventionswill have different effects on glucose tolerance, beta‐cell function, insulin clearance,whole‐body and tissue‐specific insulin sensitivity, gastrointestinal hormones,non‐glucose substrates, and the fasting lipid profile.Participants will be randomly assigned in a 1:1 ratio to one of two interventionsequences. Participants assigned to the first sequence will receive the low‐carbohydratediet followed by the Mediterranean diet. Participants assigned to the second sequencewill receive the Mediterranean diet followed by the low‐carbohydrate diet. Each dietaryintervention will last four weeks. The two intervention periods will be separated by afour‐week washout period, during which participants will resume their habitual diet.Because of the nature of the dietary interventions, participants and study personnelresponsible for dietary counseling will not be blinded to treatment allocation.Both diets will be mildly hypocaloric and will provide an estimated energy restriction ofapproximately 200‐400 kcal/day relative to basal metabolic requirements calculated usingthe Harris‐Benedict equation. The low‐carbohydrate diet will be designed to provideapproximately 20% of total energy from carbohydrates, 55% from fats, and 25% fromproteins. The Mediterranean diet will be designed to provide approximately 50% of totalenergy from carbohydrates, 30% from fats, and 20% from proteins.Both dietary interventions will emphasize minimally processed foods, vegetables, fruits,legumes, fish, eggs, whole grains when compatible with the assigned macronutrientcomposition, and extra‐virgin olive oil. Consumption of red and processed meat,ultra‐processed foods, saturated fats, free sugars, alcohol, and salt will be limited. Aregistered dietitian will provide each participant with an individualized dietary planspecifying food choices, portion sizes, raw food weights, and possible substitutions withcomparable energy and macronutrient content. The dietary plans will be adapted toindividual food preferences when possible to facilitate adherence.Participants will complete three‐day quantitative food records during each week of bothintervention periods. The records will include the types and quantities of foods andbeverages consumed, condiments, and cooking methods. Dietary records will be analyzedusing the Italian Food Composition Tables. Dietary adherence, body weight, and otheranthropometric variables will be monitored during scheduled study visits. Adherence tothe low‐carbohydrate intervention will be considered inadequate when the estimatedcarbohydrate intake during that intervention is 40% or more of total daily energy intake.At the end of each dietary intervention, participants will undergo a frequently sampled75‐g oral glucose tolerance test using two stable glucose isotope tracers. The metabolicassessments will be performed after a 12‐hour overnight fast and after consumption of astandardized evening meal on the preceding day.A primed continuous intravenous infusion of [6,6‐²H₂]glucose will begin 120 minutesbefore ingestion of the oral glucose load and will continue throughout the test. At timezero, participants will ingest 75 g of glucose enriched with [U‐¹³C]glucose within fiveminutes. Blood samples will be collected before and at predefined time points for 180minutes after glucose ingestion.Plasma tracer enrichment and glucose concentrations will be used to estimate the systemicrate of appearance of orally ingested glucose, endogenous glucose production, and glucoseclearance. Fasting values, peak values, time to peak, and areas under the curve will becalculated, as appropriate. Glucose fluxes will also be examined during the early andlater phases of the oral glucose tolerance test.Plasma glucose, insulin, C‐peptide, glucagon, total glucagon‐like peptide‐1,glucose‐dependent insulinotropic polypeptide, non‐esterified fatty acids, andbeta‐hydroxybutyrate will be measured during the test. Fasting glycated hemoglobin, lipidconcentrations, and other routine biochemical variables will also be assessed.Insulin secretion rates will be estimated from plasma C‐peptide concentrations usingdeconvolution analysis. Mathematical modeling of glucose and insulin secretion will beused to quantify beta‐cell glucose sensitivity, beta‐cell rate sensitivity, insulinsecretion at a fixed glucose concentration, and the potentiation of insulin secretionduring the oral glucose tolerance test. Fasting and post‐load insulin clearance will beestimated from the relationship between insulin secretion rates and plasma insulinconcentrations.Whole‐body insulin sensitivity will be assessed using glucose clearance normalized tocirculating insulin concentrations and by the Matsuda index. Hepatic insulin resistancewill be estimated from the relationship between endogenous glucose production and insulinsecretion. Adipose tissue insulin resistance will be estimated from fastingnon‐esterified fatty acid and insulin concentrations.The two dietary interventions will be compared using paired statistical methods thataccount for the crossover design. Continuous variables will be evaluated for normalityand will be compared using paired parametric or nonparametric tests, as appropriate. Allstatistical tests will be two‐sided.