Molecular Nutrition: Carbohydrates

Vinood B. Patel

• Publisher: Academic Press
• Year: 2019
• Print ISBN: 9780128498866
• eBook ISBN: 9780128498743
• Topic: Technology & Engineering
• Subtopic: Food Science
• Index: Technology & Engineering

Molecular Nutrition: Carbohydrates presents the nutritional and molecular aspects of carbohydrates. As part of the Molecular Nutrition includes sections covering carbohydrate metabolism, carbohydrates in the diet, insulin resistance, dietary sugars, cardiometabolic risk, lipoproteins, low-carbohydrate diets, antioxidants, refined dietary sugars, fats, glucose transporters, glucose sensing, the role of phosphorylation, carbohydrate responsive binding protein, cyclic AMP, peroxisome proliferator-activated receptors, SIRT1, insulinotropic polypeptide (GIP) and GIP receptor (GIPR) genes rRNA and transcription, and more. In addition, the book addresses emerging fields of molecular biology and presents important discoveries relating to diet and nutritional health. - Summarizes molecular nutrition in health as related to carbohydrates - Addresses emerging fields of molecular biology and presents important discoveries relating to diet and nutritional health - Includes key facts, a mini dictionary of terms and summary points

Part 1

General and introductory aspects

Chapter 1

Glucose homeostasis and the gastrointestinal tract

Simon Veedfald^a,b; Nicolai J. Wewer Albrechtsen^a,c; Jens J. Holst^{a,d a} Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
^b Department of Endocrinology, Hvidovre Hospital, Copenhagen, Denmark
^c Department of Clinical Biochemistry, Rigshospitalet, Copenhagen, Denmark
^d Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen, Copenhagen, Denmark

Abstract

Carbohydrates are universal fuels for cellular energy metabolism. To reach the cells, carbohydrates in the form of monosachharides must be taken up from the external environment. The gastrointestinal tract houses the necessary processing apparatus to solve this problem.

The gastrointestinal tract is a complex multiorgan system that allows the timely assimilation of ingested foods. The uptake of nutrients across the extensive absorptive membrane of the intestine requires that foods are first broken down (digested) into absorbable moieties using mechanical implements and various chemicals and enzymes. The regulation of gastrointestinal motility and secretions is orchestrated by intricate neural networks within the gut wall, long nerve reflexes, and hormones released from cells nested in the inner epithelial lining of the gut. However, the influence of the gut on glucose homeostasis is not limited to digestion and absorption. Thus while nutrients are being taken up from the intestinal lumen, gut hormones direct the switch of metabolism from the fasted condition, where stored energy substrates are being mobilized, to the fed state where absorbed nutrients are deposited for later use. We here provide an overview of the ways in which the gut influences glucose homeostasis. After a brief introduction to the concept of glucose homeostasis and the general layout of a meal, we will introduce the most common dietary carbohydrates before following their fate from ingestion to the time when they are dispersed into the circulation. This we hope will provide the reader with a robust working knowledge of the various levels of glucose regulation. This is essential when trying to understand the consequences of dietary patterns and helping patients make better dietary choices.

Keywords

Gastrointestinal; Carbohydrate; Gastric emptying; Glucose; Gut hormones; Incretin

Outline

• Introduction
• The phases of a meal
• Dietary carbohydrate
• Carbohydrate handling in the alimentary tract: A physiological walkthrough
  • The oral cavity: Mechanical breakdown and enzymatic digestion
  • The stomach: Storage, digestion, and emptying
  • The small intestine: Digestion and absorption
  • Malabsorption
  • Postabsorptive disposition of glucose
• Signals from the gut
  • The roles of gut hormones
• The incretin effect and the incretin hormones
  • The incretin effect
  • The incretin hormones
• Harnessing the full potential of the gut
• Glossary
• References

Summary Points

• • Dietary carbohydrate (Fig. 2) is largely ingested as polysaccharides in the form of starch (amylopectin and amylose) with smaller contributions of disaccharides (sucrose, maltose, and lactose). All carbohydrates must be broken down into monosaccharide to allow absorption. Glucose is the major monosaccharide produced by the digestion of dietary carbohydrate.
• • The rate of carbohydrate delivery into the small intestine (i.e., gastric emptying rate) is a key determinant of blood glucose levels after a meal (postprandial glycemia). It influences the absorption rate of glucose and the secretion of gut peptides qualitatively and quantitatively.
• • The release of gut hormones depends on nutrient sensing. A major factor in nutrient sensing is the absorption of nutrients.
• • Gut hormones regulate appetite, control food intake, adjust gastrointestinal motility (including gastric emptying), regulate the secretion of digestive juices, and stimulate insulin secretion.
• • The gut peptides, glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), are responsible for the so-called incretin effect—the augmentation of glucose-stimulated insulin secretion after oral ingestion of nutrients.
• • Manipulation of gastrointestinal function by pharmacological, nutritional, and in particular surgical interventions has the capacity to change postprandial glucose handling and overall glucose homeostasis.

Key facts

• • Gastric emptying rate (GER) is the rate at which nutrients are dispensed into the small intestine from the stomach. The normal range spans 1–4 kcal/min.
• • Insulin is released together with C-peptide in equal (molar) amounts. Both peptides are produced from the same precursor peptide. While insulin is extracted by the liver (30%–50%), this is not the case for C-peptide. Therefore C-peptide has been used as a surrogate measure to better estimate the total amount of secreted insulin.

Introduction

Homeostasis is a term used to describe the maintenance of a stable inner environment by complex interactions between multiple organs (Bernard, 1974) ultimately serving to maintain the functions of the brain. Glucose is the preferred fuel of the brain, and a steady supply is critical. Glucose is a monosaccharide, a fundamental carbohydrate unit, and the key energy currency in the body. Glucose homeostasis is a term used to denote the maintenance of stable glucose concentrations in the blood stream within a narrow concentration range (euglycemia) while avoiding high glucose levels (hyperglycemia) and low glucose levels (hypoglycemia). During fasting, glucose is released from the liver to ensure a steady supply of glucose either by glycogenolysis (the release of glucose stored as glycogen in the liver) or by gluconeogenesis (the generation of glucose from other energy substrates).

The fasted state sets in after the complete digestion and absorption and storage of nutrients from preceding meals. Depending on the amount of food eaten during a given meal and the macronutrient composition of the meal, the fasted state may be reached after a few hours (dilute nutrient solutions) up to several hours (large, solid fatty meals). In the modern world, where the food supply is plentiful, it is not unusual for humans to enter the fasted state only during sleep because snacks between regular meals provide a more or less continuous supply of nutrients to the gastrointestinal tract.

Food intake breaks the fast. To dampen changes in glucose concentrations when food is ingested, a range of behavioral and gastrointestinal checks have evolved. Some are recruited concurrently, while others are engaged consecutively. Entry of nutrients into the circulation is steered by a system of reflexes and regulatory hormones; this includes various characteristic patterns of motility in the gastrointestinal tract, as well as the release of ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• Series Preface
• Preface
• Part 1: General and introductory aspects
• Part 2: Molecular biology of the cell
• Part 3: Genetic machinery and its function
• Index