Effect of Dietary Macronutrients on Postprandial Glucagon and Insulin Release in Obese and Normal-Weight Women

Hepatic Metabolic Memory Triggered by AM Exposure to Glucagon Alters Afternoon Glucose Metabolism

The second meal effect describes an improved glycemic response observed after consuming a second identical meal. We previously showed that morning (AM) exposure to hyperinsulinemia primes the liver for enhanced hepatic glucose uptake and glycogen storage in the afternoon (PM), with no significant effect on PM non-hepatic glucose uptake. Given that meals often trigger both insulin and glucagon secretion, we aimed to determine if AM hyperglucagonemia alters the priming effect of AM hyperinsulinemia on PM hepatic glucose metabolism. To test this, dogs were exposed to a 4h AM hyperinsulinemic-euglycemic clamp, with insulin delivered in a pattern mimicking the insulin profile observed earlier during a 4h AM duodenal glucose infusion. This period of hyperinsulinemia was paired with either basal (Prime, n=8) or elevated (Prime + ↑GGN, n=8) glucagon, maintaining a consistent insulin-to-glucagon molar ratio throughout the AM clamp. After a 1.5h rest period, the dogs underwent a 2.5h PM hyperinsulinemic-hyperglycemic clamp, during which glucose, insulin, and glucagon levels, along with the artery-to-portal vein glucose gradient, were carefully controlled to replicate postprandial conditions. During the PM clamp, the mean net hepatic glucose uptake (NHGU) in the Prime + ↑GGN group was only 59% of that in the Prime group (3.6±0.4 vs. 6.1±0.6 mg/kg/min, P<0.0027, respectively). Additionally, PM direct glycogen synthesis was two-fold greater in the Prime group compared to the Prime + ↑GGN group (3.2±0.7 vs. 1.5±0.2 mg/kg/min, P<0.0014, respectively). The observed difference in PM NHGU between the groups was not due to enhanced PM hepatic glucose uptake (HGU), which was similar in both groups (5.7±0.5 mg/kg/min in the Prime group vs. 5.2±0.3 mg/kg/min in the Prime + ↑GGN group), but rather a prolonged effect of AM hyperglucagonemia on PM hepatic glucose production (HGP) (-0.3±0.3 mg/kg/min in the Prime group vs. 1.7±0.4 mg/kg/min in the Prime + ↑GGN group, P<0.0072). This increase in PM HGP in the Prime + ↑GGN group was not driven by differences in PM gluconeogenic flux but by futile glucose cycling between glucose and glucose-6-phosphate, as well as hepatic glycogen storage and breakdown. In summary, these findings suggest that morning exposure to elevated glucagon shifts the insulin-driven priming effect on afternoon hepatic glucose metabolism by promoting sustained glucose cycling at the expense of glycogen synthesis and glycolysis, leading to persistent HGP despite identical PM insulin, glucose, and glucagon levels.

On the pathogenesis of obesity: causal models and missing pieces of the puzzle

Application of the physical laws of energy and mass conservation at the whole-body level is not necessarily informative about causal mechanisms of weight gain and the development of obesity. The energy balance model (EBM) and the carbohydrate-insulin model (CIM) are two plausible theories, among several others, attempting to explain why obesity develops within an overall common physiological framework of regulation of human energy metabolism. These models have been used to explain the pathogenesis of obesity in individuals as well as the dramatic increases in the prevalence of obesity worldwide over the past half century. Here, we summarize outcomes of a recent workshop in Copenhagen that brought together obesity experts from around the world to discuss causal models of obesity pathogenesis. These discussions helped to operationally define commonly used terms; delineate the structure of each model, particularly focussing on areas of overlap and divergence; challenge ideas about the importance of purported causal factors for weight gain; and brainstorm on the key scientific questions that need to be answered. We hope that more experimental research in nutrition and other related fields, and more testing of the models and their predictions will pave the way and provide more answers about the pathogenesis of obesity than those currently available.

Substitution of Carbohydrates for Fats and Risk of Type 2 Diabetes among Korean Middle-Aged Adults: Findings from the Korean Genome and Epidemiology Study

Using data from a 16 year follow-up cohort of the Korean Genome Epidemiology Study, this study assessed the effects of carbohydrate intake on incident diabetes, including replacement of fats or proteins with carbohydrates. In addition, this study evaluated modification effects based on 24 genetic variants associated with type 2 diabetes. For the daily intake of macronutrients, the energy-adjusted intake and percentage of total energy intake were calculated. The effects were assessed using a Cox proportional hazards model; results were presented as hazard ratios with 95% confidence intervals (CIs). Among the 7413 participants considered to be diabetes-free at baseline, 1193 individuals were considered to have incident diabetes. The risk of incident diabetes was found to be high at both extremes of carbohydrate intake, with the lowest risk at 78 E%. The replacement of 5 E% intake from fats with isocaloric carbohydrates showed an 11% increase in the risk of diabetes (95% CI: 1.01−1.21), which was significant in men, participants >50 years of age, and participants with a high educational level. Regarding gene−environment interactions, the relationship between carbohydrate intake and incident diabetes was not dependent on genetic variants. A nonlinear relationship was observed between carbohydrate intake and incident diabetes. The substitution of carbohydrates for fats was also associated with an increased risk of incident diabetes.