The rate at which a meal enters the intestine is regulated by the rate of gastric emptying. Gastric emptying of liquids depends on their osmolarity. For digestible solids, the emptying is regulated by the particle size. However, intestinal contents entering the distal intestine inhibit gastric emptying (2). Gastric hypersecretion occurs after significant small bowel resection and reduces nutrient absorption by inactivating pancreatic enzymes.

Small bowel motility is three times slower in the ileum than in the jejunum (3). In addition, the ileocecal valve may slow transit, especially when the ileum has been resected (4).

The adult small bowel receives about 5 to 6 L of endogenous secretions and 2 to 3 L of exogenous fluids per day. It reabsorbs most of this volume in the small bowel. The amount reabsorbed in the small intestine depends on the nature of the meal (5). With a meat- and salad-based meal, most of the fluid is absorbed in the jejunum, whereas with a milk and doughnut meal, less is absorbed proximally and more distally. In addition, the absorptive processes are different in the jejunum as compared with the ileum. These differences depend partly on the nature of the electrolyte transport processes and partly on the permeability of the intercellular junctions. In general, water absorption is a passive process resulting from the active transport of nutrients and electrolytes. The transport of sodium creates an electrochemical gradient and also drives the uptake of carbohydrates and amino acids across the intestinal mucosa. In addition, the ileum has neutral sodium chloride absorption. However, the net absorption depends not only on these processes but also on the extent of back diffusion of the transported material into the intestinal lumen through leaky intercellular junctions. In the jejunum, these junctions are very leaky, and thus jejunal contents are always isotonic.

Fluid absorption in this region of the bowel is very inefficient when compared with the ileum. It has been estimated that the efficiency of water absorption is 44% and 70% of the ingested load in the jejunum and ileum, respectively. For sodium, the corresponding estimates are 13% and 72% (5). Hence, the ileum is important in the conservation of fluid and electrolytes.

The terminal ileum uniquely absorbs vitamin B₁₂ and bile salts. Bile salts are essential for the efficient absorption of fats and fat-soluble vitamins. Normally, the demand for bile salts imposed by fat absorption cannot be met by synthesis alone. This full need is met only by ileal resorption of bile salts, which are then recycled into the intestine. With ileal resection, the loss of bile salts increases and is not met by an increase in synthesis. The bile salt pool is depleted, and fat absorption is reduced. In addition, loss of bile salts into the colon affects the colonocytes and reduces the ability of the colon to reabsorb salt and water. The result is increased diarrhea. In the colon, bile salts are also dehydroxylated to deoxy bile salts that induce colonic water secretion.

The colon has the slowest transit, varying between 24 and 150 hours. The intercellular junctions are the tightest in this part of the bowel, and the efficiency of water and salt absorption in the colon exceeds 90% (6). In addition, carbohydrate is fermented in the colon to short-chain fatty acids (SCFAs) having two important actions. First SCFAs enhance salt and water absorption (7). Second, the energy content of malabsorbed carbohydrates is salvaged by being absorbed as SCFAs. Data suggest that in patients with short bowel syndrome, this salvage may be greater than in physiologically normal persons (8). Thus, the colon becomes an important organ for fluid and electrolyte conservation and for the salvage of malabsorbed energy substrates in patients with a short bowel.

Gastric motility is enhanced following small bowel resection (9). Whereas proximal resection does not increase the rate of intestinal transit, ileal resection significantly
accelerates intestinal transit (9, 10). In this situation, the colon aids in slowing intestinal transit so that in patients with a short bowel without a colon, a marker fed by mouth was completely excreted in a few hours (11).

The effect of intestinal resection depends on the extent and site of resection. Proximal resection results in no bowel disturbance because the ileum and colon absorb the increased fluid and electrolyte load efficiently. The remaining ileum continues to absorb bile salts, and thus there is little reaching the colon to impede salt and water resorption. In contrast, when the ileum is resected, the colon receives a much larger load of fluid and electrolytes and also receives bile salts that reduce its ability to absorb salt and water, with resulting diarrhea. In addition, if the colon is resected, the ability to maintain fluid and electrolyte homeostasis is severely impaired (12).

Absorption of nutrients occurs throughout the small bowel, and the removal of the jejunum alone causes the ileum to take over most of the lost function. In this situation, no significant malabsorption occurs (13). In contrast, even a loss of a 100 cm of ileum causes steatorrhea (14). The degree of malabsorption increases with the length of small bowel resection, and the variety of nutrients malabsorbed increases (15, 16). Balance studies of energy absorption showed that the absorption of fat and carbohydrate was equally reduced to between 50% and 75% of intake (17). However, nitrogen absorption was reduced to a lesser extent, namely, to 81% of intake (17). In the study of Ladefoged et al (16), the degrees of calcium, magnesium, zinc, and phosphorus absorption were reduced but did not correlate with the remaining length of bowel; and it was recommended that parenteral nutrition (PN) be mandatory in these patients. Other studies showed similar reduction in absorption, but only half the patients required parenteral replacement.

Data recalculated from Nightingale et al (18) in patients with a jejunostomy indicate that fluid balance can be maintained by the oral route if the remaining small bowel exceeds 110 cm (Fig. 77.1), but nutrient balance can be maintained even if the remaining bowel is as short as 60 cm (Fig. 77.2). The data taken as a whole suggest that it is easier to meet needs for energy and nitrogen by increasing oral intake than the needs for electrolytes and divalent ions. A review of the literature before the availability of PN showed that small bowel resections of up to 33% result in no malnutrition, and resections of up to 50% of normal length can be tolerated without special aids; however, patients with small bowel resection in excess of 75% of normal length require nutritional support to avoid severe malnutrition (19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29).

A major advance in the understanding of intestinal adaptation has evolved with studies of the role of intestinotrophic hormones. These include growth hormone (GH) (34), insulinlike growth factor-I (IGF-I) (35), epidermal growth factor (EGF) (36), transforming growth factor-α (TGF-α) (37), and glucagon-like peptide-2 (GLP-2) (38). In mice, Drucker et al (39) showed that GLP-2 modified to reduce degradation by dipeptidyl peptidase was the most potent intestinotrophic factor. Jeppesen et al (40) showed that in normal individuals, GLP-2 levels in blood rose with a meal. In contrast, there was an absence of such a response in patients with combined ileal and colonic resection. In contrast, patients who had an ileal resection but retained colon had elevated fasting and meal-induced GLP-2 levels (41). These studies show that ileocolonic resection markedly reduces the likelihood of intestinal adaptation and causes patients with a jejunostomy to continue to have severe malabsorption. In contrast, preservation of the colon allows the remaining jejunum to adapt and helps to explain why patients with a remaining colon can often avoid permanent PN. Finally, isolated jejunal resection leaves the hormonal machinery of the ileum and colon intact.

When food is excluded from the lumen, intestinal hypoplasia occurs in experimental animals. At the same time, body composition can be simultaneously maintained by the use of PN. These facts have been extensively documented, and the interested reader is referred to a review by Tappenden (42).

In growing or neonatal animals, PN and bowel rest maintained normal body growth but resulted in reduced bowel length and gastric and pancreatic hypoplasia (43, 44, 45, 46). Despite the occurrence of mucosal hypoplasia, the development of disaccharidase enzymes and glucose transport was accelerated, and mucosal levels of these enzymes increased in neonatal animals receiving PN (44, 46). Hypoplasia occurred mainly in the proximal small bowel and was less evident distally (45). In adult animals, the effect of PN and bowel rest diminished mucosal mass but stimulated glucose absorption per milligram of mucosal protein (47). In addition, PN and bowel rest increased intestinal permeability (48) and altered the response to endotoxin (49).

It is not simply the lack of food but also the nature of the diet that influences mucosal bulk. In neonatal studies, mother’s milk was no better than formula (45). However, refined intragastric liquid feeds caused relative hypoplasia as compared with a solid diet (43, 50).

In general, it appears that the decreased digestive and absorptive activities of the mucosa during bowel rest are the major reasons for hypoplasia. This concept is supported by the finding that simply increasing the tonicity of the bowel contents results in an increase of the mucosal mass (51). Absorption of amino acids leads to a nonspecific increase of mucosal function and mass (52). Finally, disaccharide hydrolysis followed by absorption stimulates mucosal growth to a greater extent than equivalent monosaccharide absorption (53).