One of the challenges in studying equine nutrition is to find out what is going on inside the horse's digestive tract. The gases that are produced by fermentation within the gut provide an indirect way of looking at the microbial activity. They are absorbed into the blood stream and then carried to the lungs, from which they are exhaled.
In the horse, hydrogen seems to be produced by fermentation mainly in the fore gut and to a lesser extent in the hind gut. This is unlike the situation in humans where hydrogen is produced only in the hind gut. The difference is thought to be because humans have very low bacterial counts in their small intestine. In contrast, horses have as many bacteria in the small intestine as they do in the large intestine.*
Methane, on the other hand, is produced primarily by bacteria in the hindgut of horses. So by comparing the production of hydrogen and methane it is possible to make assumptions about the bacterial activity in different parts of the digestive tract.
Researchers at the Institute for Animal Nutrition, in Hannover, Germany, have been using the measurement of exhaled hydrogen and methane to look at the response of the intestinal micro flora to different sources of carbohydrate in the diet. Professor Manfred Coenen described their findings at the Equine Nutrition Conference held in October.
Their study involved six healthy trotter geldings. Four test diets were chosen to represent different types of carbohydrate that the horses might eat:
* sugar beet pulp (a good source of pectin)
* grass meal (containing cellulose)
* oats (containing starch)
* Jerusalem artichoke (a good source of inulin, which is similar to fructan)
All horses received a basic diet of hay plus the test meal. On test days horses were starved for 12 hours before starting the test. The scientists collected exhaled breath samples every half an hour for ten hours after the horse had eaten the test meal\
With the exception of the Jerusalem artichoke, which was fed by naso-gastric tube direct into the stomach, all other feeds were fed normally. "We gave the Jerusalem artichoke by stomach tube because we were worried that some horses wouldn't have eaten it quickly enough" explained Dr Coenen.
They found that starch and inulin were fermented in both the foregut and the hind gut. When the horses were fed inulin, hydrogen was produced soon after the meal had started to leave the stomach. So the food was being fermented while it was still within the small intestine.
On the other hand, grass meal (cellulose) and sugar beet pulp (pectin) produced no increase in exhaled hydrogen in the exhaled air. They produced a increase in methane in the third part of the testing period.
Dr Coenen suggests that there may be a role for prececal fermentation in causing laminitis. Up to now, laminitis research has concentrated on the bacterial activity in the hindgut. "This work indicates that we should be looking at the foregut as well" he concluded.
For more details see: Hydrogen and methane exhalation after ingestion of different carbohydrates (starch, inulin, pectin and cellulose) in healthy horses. Anne Mösseler, Ingrid Vervuert, Manfred Coenen. Proceedings Equine Nutrition Conference: Pferdeheilkunde. (2005) 21, 73 - 74.
* A De Fombelle, M Varloud, AG Goachet, E Jacotot, C Philippeau, C Dragoul, V Julliand (2003) Characterization of the microbial and biochemical profile of the different sssegments of the digestive tract in horses given two distinct diets. A Sci, 77, 293 - 304.