Following massive intestinal resection, three distinct clinical stages have been described . The first stage is characterized by large fluid and electrolyte losses and generally occurs over the first few weeks following resection. In the second stage, a shift of emphasis to nutritional support is the primary concern. This stage may last for up to 2 years, and it is during this time that most adaptation and most PN weaning occur. Stage three is considered a homeostatic phase where no further adaptive changes occur.
Intestinal adaptation refers to a process following intestinal resection in which the remaining bowel undergoes a variety of macroscopic and microscopic changes in response to a variety of stimuli (Table 8.2), in order to increase its ability to absorb fluid and nutrients [29, 30]. Both morphological (i.e., structural) and functional intestinal adaptive changes can occur depending upon the extent and site of the intestine removed and the nutrient components of the diet. Changes described in structural adaptation include an increase in villus height, crypt cell depth and enterocyte number, while functional adaptive changes include modifications of the brush border membrane fluidity and permeability, up- or down-regulation of carrier-mediated transport and a slowing in the rate of transit allowing increased time for absorption to occur .
It is important to recognize that most investigations on intestinal adaptation following bowel resection have utilized animal models, and, importantly, few studies have confirmed these adaptive responses in humans. In addition, most studies investigating the process of adaptation have utilized animal models with a jejuno-ileal anastomosis; therefore, the physiological and structural changes that occur are of unclear clinical relevance to humans with SBS who uncommonly have this bowel anatomy. Furthermore, while animal intestinal
Table 8.2 Factors Affecting Intestinal Adaptation
Remaining bowel anatomy Colon present Ileum present Luminal factors Nutrients
Hormones/growth factors Trophic Antimotility adaptation is characterized by epithelial hyperplasia, human intestinal adaptation appears to be primarily associated with an increase in the absorptive function of the enterocyte irrespective of morphological changes [32-34].
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