Gastrointestinal adaptation: Rapid functional and retrogressive changes but slow proliferation

The gastrointestinal tract (GIT) is a key organ system facilitating, inter alia, movement of digesta, digestion, nutrient absorption, host-microbial communication, and must regulate permeability to control immunogenic exposure systemically. As a large consumer of maintenance energy, disruptions in nutrient supply can cause marked changes in GIT size and function. Although data on reticulo-ruminal responses are abundant, less information is available regarding postruminal regions. That said, greater nutrient supply increases the proliferative responses of the reticulo-rumen and intestinal regions. However, the increases in size and expansion of surface area are relatively slow suggesting these are longer-term adaptive responses. To compensate, cell activity, at least in the reticulo-rumen, increases rapidly to facilitate greater nutrient transport. On the other hand, feed deprivation, and periods of low feed intake markedly reduce absorptive surface area. Interestingly, the reduction in absorptive surface area through retrogressive adaptational responses is very rapid with greater than 50% reductions occurring within 5 d of low feed intake. Additionally, exposure to low feed intake decreases the rates of SCFA absorption and transiently increases paracellular permeability. Used as a common challenge model, ruminal acidosis also causes alterations throughout the GIT with reduced rates of short-chain fatty acid transport and altered permeability. Although paracellular permeability recovers rapidly following a challenge (low feed intake or ruminal acidosis), recovery responses for nutrient transport occur gradually, likely related to the gradual recovery for dry matter intake. The basal diet fed may also alter these responses as diets high in long-chain polyunsaturated fatty acids may decrease passive apical uptake of propionate and butyrate while increasing risk for paracellular permeability. Thus, the gastrointestinal tract epithelium adapts to modulate nutrient absorption and paracellular permeability using both short-term and long-term mechanisms.

Published in the proceedings of the 10th Symposium on Gut Health in Production of Food Animals 2022, St. Louis, USA.