Changes

==Pathogenesis==

==Pathogenesis==

The normal physiologic nadir for rumen pH is 5.5. Excessive quantities of rapidly fermentable carbohydrates in the ruminant diet results in overproduction of volatile fatty acids by the rumen microflora, lowering rumen pH below its ideal range. Volatile fatty acids are a normal product of rumen fermentation and are readily used by tissues as an energy source. Tissues are capable of using excess volatile fatty acids (VFAs) produced from the fermentation of high levels of carbohydrate; however, the instability in rumen pH makes it difficult for these to be absorbed properly and hence put to good use.  

Excessive quantities of rapidly fermentable carbohydrates in the ruminant diet results in overproduction of volatile fatty acids by the rumen microflora, lowering rumen pH below its ideal range of pH 6-7. Volatile fatty acids are a normal product of rumen fermentation and are readily used by tissues as an energy source. Tissues are capable of utilising the excess volatile fatty acids (VFAs) produced from the fermentation of high levels of carbohydrate; however, the instability in rumen pH makes it difficult for these to be absorbed properly and hence put to good use. VFA are normally passively absorbed across the rumen wall via finger-like papillae. These papillae increase in length when ruminants are fed high-carbohydrate diets, presumably to enhance absorption and protect the animal from accumulation of acids in the rumen. Despite this, a diet too high in concentrates will actually impair the absorptive capacity of the rumen, and VFAs will acumulate without effective utilisation by tissues.

 

Unfortunately, lactate production at low ruminal pH can offset gains from VFA absorption. As pH drops, lactate-synthesizing bacteria such as Streptococcus bovis begin to ferment glucose to lactate instead of VFA. This is a dangerous situation, since lactate has a much lower pKa than VFA (3.9 vs 4.8) and lactate is 5.2 times less dissociated than VFA at pH 5.0. As a result, lactate stays in the rumen longer and contributes to the downward spiral in ruminal pH.

Unfortunately, lactate production at low ruminal pH can offset gains from VFA absorption. As pH drops, lactate-synthesizing bacteria such as Streptococcus bovis begin to ferment glucose to lactate instead of VFA. This is a dangerous situation, since lactate has a much lower pKa than VFA (3.9 vs 4.8) and lactate is 5.2 times less dissociated than VFA at pH 5.0. As a result, lactate stays in the rumen longer and contributes to the downward spiral in ruminal pH.

Additional adaptive responses are invoked if lactate production begins. Lactate-utilizing bacteria, such as Megasphaera elsdenii and Selenomonas ruminantium , begin to proliferate. These beneficial bacteria convert lactate to other VFA, which are then easily protonated and absorbed. However, the turnover time of lactate utilizers is much slower than that of lactate synthesizers. Thus, this mechanism may not be invoked quickly enough to fully stabilize ruminal pH. Periods of very high ruminal pH, as during feed deprivation, may inhibit populations of lactate utilizers (which are sensitive to higher ruminal pH) and leave them more susceptible to severe ruminal acidosis.

Additional adaptive responses are invoked if lactate production begins. Lactate-utilizing bacteria, such as Megasphaera elsdenii and Selenomonas ruminantium , begin to proliferate. These beneficial bacteria convert lactate to other VFA, which are then easily protonated and absorbed. However, the turnover time of lactate utilizers is much slower than that of lactate synthesizers. Thus, this mechanism may not be invoked quickly enough to fully stabilize ruminal pH. Periods of very high ruminal pH, as during feed deprivation, may inhibit populations of lactate utilizers (which are sensitive to higher ruminal pH) and leave them more susceptible to severe ruminal acidosis.