The primary focus of the small intestinal phase of meal assimilation is on the bile acids, which are crucial constituents of bile. Bile acids form micelles, which act as protective shields for the hydrophobic products of lipid digestion in the watery environment of the lumen. These bile acids serve as biological detergents and are required in significant amounts (1 to 2 g/day) for optimal lipid absorption. The majority of bile acids are recycled from the intestine back to the liver through the enterohepatic circulation. To ensure their retention in the lumen for lipid assimilation, bile acids are synthesized in a conjugated form that restricts their passive crossing of the intestinal epithelium. However, once the meal contents reach the terminal ileum, where lipid absorption is completed, conjugated bile acids are reabsorbed by the apical Na+-dependent bile acid transporter (ASBT) in association with sodium ions. Only a small portion of the bile acid pool is left to enter the colon, where they become deconjugated and are passively reabsorbed. This process allows for the cycling of the majority of the bile acid pool between the liver and intestine on a daily basis, influenced by signals generated in the postprandial period. Bile acids also have additional biological functions beyond their role as detergents, as they bind to cell surface and nuclear receptors in various cell types throughout the body, regulating their own synthesis and other metabolic processes.
The primary physiological function of the small intestine is the absorption of nutrients derived from the digestion of ingested macronutrients, namely carbohydrates, proteins, and lipids. The small intestine is vital not only for the uptake of nutrients into the body but also for their final digestion into simpler molecules that can be transported across the intestinal epithelium. Each nutrient's assimilation process will be discussed individually, starting with carbohydrates. Carbohydrate digestion occurs in two phases: within the intestinal lumen and on the surface of enterocytes, known as brush border digestion. The latter is especially important as it generates easily absorbable sugars at the site of absorption, thereby limiting their availability to the small number of bacteria present in the small intestinal lumen that could utilize these sugars as nutrients.
Carbohydrates found in the diet consist of various molecular classes. Starch, the initial class, is a mixture of both straight-chain and branched-chain polymers of glucose. The straight-chain polymers are referred to as amylose, while the branched-chain molecules are known as amylopectin.
When considering the small intestinal phase of meal assimilation, the bile constituents we are most concerned with are the bile acids. These form structures known as micelles that serve to shield the hydrophobic products of lipid digestion from the aqueous environment of the lumen. Bile acids are in essence biological detergents, and large quantities are needed on a daily basis for optimal lipid absorption—as much as 1 to 2 g/day. The majority of the bile acid pool is recycled from the intestine back to the liver after each meal via the enterohepatic circulation (Fig.
30.7 ). Thus bile acids are synthesized in a conjugated form that limits their ability to passively cross the epithelium •Fig. 30.7 Enterohepaticcirculationofbileacids.Activeuptakeofconjugatedbileacidsoccursviatheapicalsodium-dependentbileacidtransporter(asbt).
lining the intestine so that they are retained in the lumen to participate in lipid assimilation (discussed in detail under that heading). However, when the meal contents reach the terminal ileum, after lipid absorption has been completed, the conjugated bile acids are reabsorbed by a symporter, the apical Na+-dependent bile acid transporter (asbt), that specifically takes up conjugated bile acids in association with sodium ions. Only a minor portion of the bile acid pool is left to spill over into the colon in health, and here bile acids become deconjugated and subject to passive reabsorption (see
Fig. 30.7).
The net effect is to cycle the majority of the bile acid pool between the liver and intestine on a daily basis, coincident with signals arising in the postprandial period. Bile acids also exert biological actions beyond their role as detergents by binding to both cell surface and nuclear receptors in a variety of cell types throughout the body. In this way they regulate their own synthesis as well as other metabolic processes.
The most important physiological function of the small intestine is to take up the products of digestion of ingested nutrients. Quantitatively the most significant nutrients (macronutrients) fall into three classes: carbohydrates, proteins, and lipids. The small intestine is critical not only
The Small Intestinal Phase of the Integrated Response to a Meal for absorption of nutrients into the body but also for the final stages of their digestion into molecules that are simple enough to be transported across the intestinal epithelium. We will consider the processes involved in assimilation of each of these nutrients in turn, beginning with carbohydrates. Carbohydrate digestion occurs in two phases: in the lumen of the intestine and then on the surface of enterocytes in a process known as brush border digestion. The latter is important in generating simple absorbable sugars only at the point where they can be absorbed. This may therefore limit their exposure to the small number of bacteria present in the small intestinal lumen that might otherwise use these sugars as nutrients.
Digestion of Carbohydrates
Dietary carbohydrates are composed of several different molecular classes. Starch, the first of these, is a mixture of both straightand branched-chain polymers of glucose. The straight-chain polymers are called amylose and the branched-chain molecules are called amylopectin (Fig.