Carbohydrate digestion and glucose supply in the gut of the ruminant.

The ruminant is able to digest and utilize a wider range of carbohydrates than most other mammals, yet the means whereby it carries out this digestion also ensures that it may, during periods of high productivity, suffer from a shortage of glucose. Indeed the plight of the high-producing ruminant is not so very different from that of the ancient Mariner who had ‘Water water every where. Nor any drop to drink.’ The basic reason for this shortage of glucose is that some 90% of the digestion of carbohydrates is by fermentation to short-chain fatty acids (volatile fatty acids (VFA)) by the bacteria and protozoa of the rumen and large intestine. Thus, only small amounts of glucose, derived mainly by hydrolysis of starch in the small intestine, are absorbed from the gut, and to meet its glucose requirements the ruminant relies heavily upon gluconeogenesis from the propionic acid and microbial protein produced by fermentation in the rumen. In the following discussion, those aspects of carbohydrate digestion recently reviewed for the Society by Armstrong & Beever (1969) will be covered only briefly.