There was an article in The Horse and read:

Dietary fats are important components of performance horses' diets because they are calorie-dense and energy-rich.

Previous studies have shown, however, that diets high in soybean oil interfere with fiber digestion in trotters.

It is unclear whether feeding poorly digestible carbohydrates, like cornstarch, compounds this problem by overloading the cecum and altering its bacterial population.

Researchers at the Utrecht University in The Netherlands recently conducted a study to determine whether the inhibitory effect of soybean oil on fiber digestion is influenced by concurrent feeding of cornstarch.

Six trotters were fed three diets in three-week rotations. Each diet had one main source of energy--soybean oil, cornstarch, or glucose. The glucose diet served as a control measure, because glucose is almost completely digested in the small intestine, rarely entering the cecum or colon.

The results of the study suggest that the negative effect of soybean oil on fiber digestion in trotters is a specific effect, not influenced by cornstarch in the diet. This conclusion was drawn because the cornstarch and glucose diets produced the same effects on fiber digestion. It is unlikely, then, that cornstarch adversely effects fiber digestion.

What is still uncertain is how high-fat diets decrease fiber digestion. Previous studies have suggested that increased fat inhibits the activity of resident microbes in the cecum and colon. However, evidence of this action is lacking. Further studies are needed to clarify this unusual dose-effect relationship between fat and fiber in the horse.


**************************************************

OK, I have to ask, what in the world do they mean that the test was based on using soybean oil as the one main source of energy.

Are they saying that 50% or more of the diet was coming from FAT calories, because to the majority of the diet's energy source, wouldn't that have to be the case?

The original work that found a reduction in fiber digestibility with added fat from oil was done some years ago, I believe in England. The study found a statistically significant decrease in fiber digestibility, but statistically significant and biologically significant can be very different. In the case of added oil and fiber digestibility the main point the authors were making was that in racing or other high performance horses, where total daily calories must be maintained in order to maintain weight, if those horses are being fed fat that slightly more forage might need to be fed as calculations for calories from that forage might be a bit too low. That's really all that study said, but of course some people misinterpreted it to mean "don't feed fat it will interfere with fiber digestion." Not at all. As to this recent study, I read the original and although I don't remember the details the amount of fat fed per day was not excessive. What is meant here is that of the concentrated feed fed, the primary energy source was either fat, cornstarch, or glucose. They still had plenty of calories from forage!