By Nichole F. Huntley and John F. Patience, Iowa State University Department of Animal Science
The previous two columns (March 1 and Feb. 1) of our series were on fiber, what it is and its abundance and influence in swine diets. To continue this discussion, this column will dive a little bit further into the components or building blocks of fiber and why they are becoming more relevant and important to understand in swine nutrition. Increasing the fiber level in the diet can decrease digestion and the utilization of individual nutrients and the diet overall. In turn, feed efficiency can be hurt. Generally, when we feed high-fiber ingredients, we either accept poorer feed conversion and probably — but not always — slower growth, or we add fat to the diet to maintain the energy level of the diet, and hopefully also maintain growth rate.
Enzymes, such as xylanase, can be included in high-fiber diets to combat this issue. Enzymes work, in part, by degrading the fiber and in doing so, release nutrients, such as starch and protein that are otherwise trapped within the rigid structure of the grain cell wall. This allows the pig to utilize nutrients in the diet that might otherwise not be digested.
As the enzymes break up the fiber, they can also release the smaller component pieces of fiber into the small intestine. Interestingly, fiber is essentially long chains of sugars bound together in different combinations and arrangements. So, when xylanase breaks up the fiber, the individual component sugars, such as xylose, can be released and absorbed in the small intestine. We are still learning a great deal about the use of xylose in pig diets. One frustration is that xylanase does not always provide benefit to the pig, at least regarding growth rate or feed efficiency. One branch of our research program at Iowa State University is to understand better exactly how xylanase works in the gut of the pig so that we can achieve greater and more consistent responses.
When many people think of the word “sugar,” they think of the sweet compound used in cakes and cookies and other similar products. This household table sugar is known as sucrose and is itself actually made up of two sugars, called glucose and fructose. Glucose is one of the most widely used sugars in the plant and animal kingdom. Glucose is found not only as a part of sucrose, or table sugar, but it is also present in starch, which is found in all cereal grains. It is how plants store energy. But the glucose in starch is not sweet like it is in household sugar, because the fructose molecule is not present. This is a good example of how different sugars can be, depending on how they are linked together. Table sugar is sweet and easily digested, and if consumed in excess tends to make us gain weight; while fiber, made up of different sugars and linked in different ways, is hard to digest and tends to help us lose weight.
So, glucose is a very common sugar in the plant kingdom, but it is not the only one. Xylose is another sugar found only in the fiber fractions of swine diets. Unlike sugars such as glucose, xylose is not normally absorbed in the small intestine and utilized by pigs because, without supplemented enzymes, fiber is not digested. The bacteria that live in the gut of the pig can break down fiber through a process called fermentation, but this occurs mainly in the cecum and large intestine and results in the release of volatile fatty acids, not the free xylose sugar. Therefore, prevailing thought has been that pigs do not utilize xylose well and it is an inefficient energy source.
But this theory is based on only a few studies from the ’90s and earlier which fed pigs diets containing 5-40% xylose. They concluded that a majority of dietary xylose is absorbed, but most are later excreted in the urine. This means that xylose does not contribute energy to the pig as efficiently as glucose. However, typical swine diets contain a total of only 3-5% xylose and not all of this will be released from the fiber structure with xylanase supplementation. This is why part of my Ph.D. research is evaluating how xylose is utilized and its value to the pig as an energy source.
In this study, 48 pigs were fed either a xylose-free control diet (0% treatment) or the control diet with 2, 4 or 8% added xylose. Diet digestibility, urine xylose concentrations, and energy balance were measured three separate times to study if pigs can adapt over time to use xylose more efficiently. So far, the data indicate that almost all of the dietary xylose was absorbed in the small intestine and nutrient digestibility was similar across all treatments. Our results agree with previous studies that concluded that a large portion of xylose is excreted in the urine, but our results add to this knowledge that xylose utilization efficiency decreased as the dietary xylose concentration increased. For example, in the 2% xylose treatment, about 60% of the consumed xylose was retained in the body, whereas this value decreased to 47% and 41% in the 4% and 8%-xylose treatments, respectively. Furthermore, the pigs seemed to adapt to utilize xylose more efficiently after at least 12 days of adaptation to the xylose-containing diets.
It is important to quantify how much xylose is retained versus excreted because xylose in the urine represents energy that is not available to the pig for growth. As xylose consumption increases, the amount of energy excreted in the urine increases which decreases the amount of energy available to the pig for maintenance and growth. Nutritionists formulate diets to precisely balance the amount and concentrations of energy and nutrients the diets provide. This means that to effectively utilize enzymes to improve pigs’ performance on high-fiber diets, we must understand how enzymes like xylanase impact dietary energy availability. We hope our research on how pigs utilize xylose will help nutritionists better understand xylanase effects on dietary energy value.