Physiology of Intestinal Absorption and Secretion

doi:10.1016/j.bpg.2016.02.007

Best Practice & Research Clinical Gastroenterology

Volume 30, Issue 2, April 2016, Pages 145-159

https://doi.org/10.1016/j.bpg.2016.02.007 Get rights and content

Abstract

Virtually all nutrients from the diet are absorbed into blood across the highly polarized epithelial cell layer forming the small and large intestinal mucosa. Anatomical, histological, and functional specializations along the gastrointestinal tract are responsible for the effective and regulated nutrient transport via both passive and active mechanisms. In this chapter, we summarize the current state of knowledge regarding the mechanism of intestinal absorption of key nutrients such as sodium, anions (chloride, sulfate, oxalate), carbohydrates, amino acids and peptides, lipids, lipid- and water-soluble vitamins, as well as the major minerals and micronutrients. This outline, including the molecular identity, specificity, and coordinated activities of key transport proteins and genes involved, serves as the background for the following chapters focused on the pathophysiology of acquired and congenital intestinal malabsorption, as well as clinical tools to test and treat malabsorptive symptoms.

Introduction

This chapter emphasizes the physiology of intestinal absorption. The transport protein responsible for absorptive function of the gastrointestinal tract resides in the apical side of the villous structure, which is involved in facilitating the transport of nutrients across the length of the small intestine. Recent advances in molecular cloning have led to further understanding of these transport proteins and how they are altered in the settings of health and disease states. The chapter will also discuss the intestinal transport of ions, short chain fatty acids, sulfate, oxalate; as well as major nutrients such as carbohydrates, proteins, and fats. The chapter concludes with a discussion of the absorption of water-soluble vitamins, fat-soluble vitamins, minerals, and micro-nutrients.

Section snippets

Functional anatomy of the GI tract

The GI tract evolved to facilitate the transport of nutrients throughout its length. The small intestine measures approximately 6 m in length and 2.5–3.0 cm in diameter. The colon measures approximately 1.5 m in length with a diameter of 6–7.5 cm. The surface area of the small intestine is significantly enhanced by the presence of villi and microvilli, which increase the intestinal surface area by 30–600 fold, respectively. Villi are approximately 0.5–1.6 mm in length and are covered with

Intestinal Na⁺ absorption

Three mechanisms contribute to the apical Na⁺ transport in the small intestine: (a) nutrient-coupled Na⁺ absorption mediated by several families of Na⁺-dependent nutrient transporters such as sugar or amino acid transporters discussed separately in this chapter, (b) electroneutral NaCl absorption mediated primarily via the Na⁺/H⁺ exchange mechanism [1], and (c) colon-predominant electrogenic Na⁺ absorption by the epithelial Na⁺ channels (ENaC) [2]. Electroneutral NaCl absorption is attributed

Chloride absorption

Efficiently chloride absorption in the gut results in normal stool concentration of 10–15 mmoles/L. In the states of malabsorption, such as congenital Cl⁻ diarrhea, it can exceed 90 mmoles/L. Physiologically, Cl⁻ is absorbed from the intestinal lumen via three distinct mechanisms: (a) paracellular (passive) pathway; (b) electroneutral pathways which involves coupled Na⁺/H⁺ and Cl⁻/ ${HCO}_{3}^{-}$ exchange; and (c) ${HCO}_{3}^{-}$ -dependent Cl⁻ absorption. Passive pathway is predominant in the small intestinal

Sugar transport across the GI tract

The majority of consumed carbohydrates are derived from sugars and starches. Sugars include monosaccharides (e.g. glucose, galactose and fructose) and disaccharides (eg. lactose, sucrose, maltose and trehalose). Starches are the storage carbohydrates of plants and consist mainly of sugars linked together. Lactose and sucrose are hydrolyzed to monosaccharides via lactase and sucrase, respectively. Salivary and pancreatic amylase are responsible for the initial breaking down of starches, which

Amino acids and peptide absorption

The daily intake of protein in the western diet averages 100 g per day with an additional 35 g contributed by salivary and GI secretions. Low pH in the stomach activates pepsinogen to pepsin. Pepsin cleaves peptide bonds at the aromatic amino acids, and results in a mixture of intermediate protein moieties, peptides, and amino acids. These are then delivered to the duodenum, where pancreatic proenzymes (trypsinogen, chymotrypsinogen, proelastase, and procarboxypeptidase) become activated by

Lipid Absorption

The fat digestion process starts in the mouth with lingual lipase and continues in the stomach with the addition of gastric lipase produced by chief cells. However, the majority of fat digestion occurs in the duodenum by pancreatic lipase and colipase, with the assistance of emulsifying bile acids. The monoglycerides and long chain fatty acids resulting from lipolysis along with phospholipids, cholesterol, and fat-soluble vitamins, form micelles with bile salts. The components of the micelles

Water Soluble Vitamins

Water-soluble vitamins are essential for normal growth and development as they are involved in many metabolic processes. Humans cannot synthesize many of these vitamins and, therefore, have to rely heavily on their exogenous intake [20].

Vitamin A

Vitamin A in the diet exists in either retinol or ethyl esters (preformed) or as pro-vitamin A carotenoids. Most of these forms require metabolic conversion. Preformed vitamin A is consumed in the diet in a number of forms such as retinol vitamin A₁, retinol vitamin A₂, and retinol esters. Pro-vitamin A has a role in macula pigments and as a precursor for vitamin A. Retinoids have a role in the mammalian life cycle and are important for eye health and immune function. The intestinal absorption

Calcium (Ca²⁺)

Calcium is a major mineral required for a number of structural and physiological mechanisms related to health. The intestine, kidney, bone, and parathyroid glands work together to maintain serum Ca²⁺ within a narrow range, approx. 2.5 mM. The majority of circulating calcium is bound to albumin and the remaining fraction is ionized Ca²⁺. Major sources of calcium are dairy products, which account for approximately 75% of dietary calcium intake. The remaining 25 percent comes from cereals,

Zinc (Zn) absorption

Zinc is an essential nutrient for a number of cellular functions; including cell growth and development, the immune system, taste, and smell; and it is present in various enzymes required for important biological functions. The availability of Zn in the diet depends on the source of the diet. For example, Zn availability in red meat is approximately 55%, compared to high fiber cereal at approximately 15%. Zn bioavailability is greater from human milk than cow's milk. The amount of Zn in the

Conflict of interest

No conflict of interest has been declared by the authors.

Acknowledgments

We thank Mrs. Vera Madril and Mrs. Trudy Meckler for their editorial assistance.

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1Physiology of Intestinal Absorption and Secretion

Abstract

Introduction

Section snippets

Functional anatomy of the GI tract

Intestinal Na+ absorption

Chloride absorption

Sugar transport across the GI tract

Amino acids and peptide absorption

Lipid Absorption

Water Soluble Vitamins

Vitamin A

Calcium (Ca2+)

Zinc (Zn) absorption

Conflict of interest

Acknowledgments

Mol Aspects Med

Mol Aspects Med

Biochem Biophys Res Commun

J Biol Chem

Lab Invest

Subcloning, localization, and expression of the rat intestinal sodium-hydrogen exchanger isoform 8

Am J Physiol Gastrointest Liver Physiol

Functional and molecular characterization of NHE3 expression during ontogeny in rat jejunal epithelium

Am J Physiol