Early Career Award Speaker: Short-chain fatty acid absorption across the ruminal epithelium: Current knowledge and strategies to modulate absorption

Microbial fermentation in the rumen yields short-chain fatty acids (SCFA) that serve as the primary energy substrate for ruminants. Past research has reported that factors causing a reduction in SCFA absorption predispose cattle to low ruminal pH, and sheep and cattle with greater rates of SCFA absorption are more able to maintain pH. Thus, understanding absorption of SCFA acids across the rumen epithelium can help to stabilize ruminal pH and promote greater energy supply. From a tissue level, SCFA absorption is known to occur via passive diffusion, anion-exchange through a bicarbonate-dependent process, through a nitrate-sensitive mechanism, and via a voltage gated channel. The relative reliance of individual SCFA for each transport pathway differs by chain length of the SCFA and on luminal conditions such as ruminal pH. For example, acetate uptake and flux are primarily mediated through bicarbonate-dependent anion exchange, while passive diffusion accounts for a significant proportion of the butyrate uptake and flux. Interestingly, the inhibitory potential of nitrate on the uptake of acetate and butyrate also differs with less of an inhibitory effect for butyrate. In the presence of low pH, the proportion of acetate transported via bicarbonate-dependent anion exchange increases. Dietary composition can modulate the pathway of SCFA absorption. Increasing dietary sugar with the inclusion of whey permeate has been reported to increase acetate and propionate absorption through bicarbonate-dependent anion exchange. In response to a moderate, but abrupt, decrease in the forage-to- concentrate ratio (92:8 vs. 50:50), uptake of acetate and butyrate via passive diffusion increased. As a strategy to modulate permeability of the ruminal epithelial membrane, we have reported that feeding a diet with a greater supply of saturated fatty acids increased the passive diffusive uptake of propionate and butyrate, but not acetate. Finally, SCFA absorption decreases in response to low feed intake and following induction of rumen acidosis. While the involved pathways responsible for these changes have not been evaluated, intervention strategies using functional nutrients may accelerate recovery of the ruminal epithelium and restore SCFA flux. Understanding the pathways and regulatory mechanisms for SCFA transport is one approach that can be used to develop management strategies to help stabilize ruminal pH and increase absorption of SCFA as an approach to improve the performance of ruminants.