By Dr. Jean Hofve
Recently there has been renewed interest in fiber as a pet food ingredient. One pet food manufacturer published an article on the Internet condemning beet pulp in pet food as unhealthy if not downright dangerous, while others are touting the benefits of the latest fiber source, pecan shells. There is a lot of myth and misunderstanding concerning fiber, so let’s take a closer look at this controversial ingredient.
The term “fiber” (or “roughage”) applies to complex carbohydrates that are resistant to mammalian digestive enzymes, although certain bacteria possess the enzymes needed to break them down. Even ruminants, with their four-chambered stomachs and cud-chewing habits, must still rely on their symbiotic rumen bacteria to digest plant fiber. Fiber is found only in plants — hair, hooves, bones, fish scales and feathers do not contain any fiber. Fiber is composed of polysaccharides (complex sugars), and are found in plant cell walls, where they provide structural strength and rigidity. Sources of fiber commonly used in pet food include beet pulp, peanut shells, oat and other brans, tomato pomace, buckwheat and other grain hulls, psyllium, fruit pectin, guar gum and other gums, flaxseed, and powdered cellulose. This last one is defined as “purified, mechanically disintegrated cellulose . . . from fibrous plant materials.” I fondly refer to it as “sawdust,” which I believe is a fair description, since the raw material is, literally, wood pulp.)
Fiber necessity. The “wild” canid or felid diet contains very little fiber. Cats and dogs have no absolute physiologic need for it, although animals eating processed commercial foods appear to benefit from the addition of fiber.
Fiber functions. While fiber itself is indigestible and generally considered non-nutritive, it does contribute in many ways to the digestive process. Some fibers do contain nutrients, such as vitamins and minerals, which can be extracted during digestion, either by the mechanical grinding action of the stomach and intestines, or through bacterial fermentation in the colon.
The speed of passage of food through the digestive system is moderated by fiber. Fiber’s moisture-absorbing and lubricating actions can slow down peristalsis (the muscular contractions of the intestine that push food through the tract) in cases of diarrhea, or speed it up, in the case of constipation — in other words, fiber has a normalizing effect on the gut. Unfortunately, studies have produced conflicting results, depending on the fiber used and the composition of the diet, although research continues in this area. The presence of adequate fiber allows time for absorption of nutrients and water from the intestine into the blood. Some fibers also impart mucilaginous (slippery) qualities to the food, helping it “slide” along the gut walls. Certain fibers increase the rate of stomach emptying — this is one theory behind feline “hairball” diets — while others slow it down. Fiber binds some toxins in the gut and eliminates them in the stool.
Fiber is usually characterized by describing its solubility and fermentability. These terms are used for different properties, and any one fiber can be described in terms of either trait. Cellulose, for instance, is both insoluble and non-fermentable, while guar gum is soluble and fermentable. Others lie on a continuum between these two extremes. These terms evolved as the technology for analyzing fiber improved. However, the method used to assay “crude fiber” as stated on a pet food label is actually a very poor technique, and fails to detect most of the lignan, hemicellulose, and even some of the plain cellulose. Thus, the actual level of fiber in a pet food may be considerably underestimated by the outdated Crude Fiber method.
Soluble vs. insoluble. Soluble fibers are considered more digestible than insoluble fibers and will dissolve in water. Bacterial breakdown of guar gum in the colon produces several important short-chain fatty acids (SCFAs). These SCFAs can be absorbed by the animal and used as an energy source; acidify the colonic environment; and draw water into the stool by osmosis. Basically, they keep the colon bacteria happy, and this is the reasoning behind use of fructo-oligosaccharides found in such plants as chicory and yucca in pet food. Insoluble fiber tends to speed up gut motility. Soluble fiber dissolves in water to form a viscous gel, which may aid in food passage through the gut.
Fermentable vs. non-fermentable. Fermentable fibers are those that yield nutrients that can be used for energy by the body. Soluble fibers tend to be more fermentable than insoluble fibers. Short-chain fatty acids such as proprionate, acetate, and butyrate are produced by bacterial digestion of these fibers. Butyrate is thought to be beneficial to the cells lining the colon. Beet pulp contains about 18% fermentable fiber.
Fiber and obesity. It has been thought for many years that high fiber diets are “more filling” and provide increased satiety. Thus, weight loss diets have traditionally included high fiber as well as less fat. However, a new study has reported that fiber appears to have no effect at all on a pet’s appetite. In one study, no matter how much fiber a food contained, all dogs were willing to eat a “challenge” meal given an hour later. Other researchers have suggested that the primary mechanism of weight loss produced by “light” diets may be decreased palatability.
Fiber and diabetes. Certain fibers’ ability to decrease intestinal transit time is the theory behind feeding high-fiber foods to diabetics. The addition of fiber slows absorption, resulting in a more stable blood glucose level over time.
Fiber and the stool. While different fibers behave differently depending on the composition of the diet and the individual metabolism of the pet, it seems clear that excessive fiber increases fecal bulk, frequency of defecation, and may produce loose stools and flatulence.
Dispelling the fiber myths. Beet pulp seems to be the current target for much of the misinformation flying around about fiber. Here are a couple of them:
“Kibble containing beet pulp swells up in the stomach and causes bloat.” This is based on the observation that extruded kibble that gets wet (i.e., dropped into the water bowl) will indeed expand, and any of us with sloppy or playful companions have seen the evidence with our own eyes. However, baked food does not swell, and several baked foods also contain beet pulp. The expansion of the kibble is mainly due to the air trapped in the pellet as it “pops” from the extruder. The other thing to notice about wet kibble is how easily it breaks apart. Far from forming “an indigestible mass” in the stomach, this property of kibble probably helps speed its passage from the stomach and its ultimate digestion.
“Beet pulp is full of sugar and can cause diabetes.” Pulp is what’s left over after the extraction of sugar from sugar beets. Sugar manufacturers want to extract every last little bit of sugar — it’s their livelihood. There is very little sugar left in the pulp when it’s dried for pet food, and the sum total of fiber in pet foods is generally less than 5%. You can see that this tiny potential bit of sugar is unlikely to be a factor in diabetes. Those nice, digestible, simple starches in corn and other cereal grains, however, do appear to be a major causative factor, at least in feline diabetes.
“The saponins in beet pulp cause bloat.” Saponins are basically soaps, which cause a decrease in surface tension of a liquid. While normal water forms bubbles, they burst rapidly. Soap allows the molecules to stick together longer. Saponins are found in beets, legumes like alfalfa and beans (including soybeans), and hundreds of other plants. Certain saponins are specifically associated with one type of bloat in ruminants, but this is unrelated to bloat in pets. Cows (and other ruminants) can actually get two separate kinds of bloat: gassy and frothy. Gassy bloat is similar to bloat in pets. Frothy bloat – the kind caused by saponins – is very different. In frothy bloat, gas normally produced by rumen bacteria becomes trapped inside millions of foamy bubbles. While the pressure of gassy bloat can be relieved by passing a tube or large-bore needle into the stomach to let the air out, frothy bloat’s foamy character makes this impossible, and thus it is more difficult to treat. Dogs and cats do not get frothy bloat.
“Beet pulp contains paralytic toxins (saponins) that cause bloat.” There are thousands of saponins; the soybean alone contains at least 5 different ones. The highly biased claim that all saponins (or even all soy and beet saponins) are toxic is ignorant and inaccurate. According to one expert, “From the biological point of view, saponins have diverse properties, some deleterious but many beneficial.” Medicinal herbs such as ginseng, licorice and alfalfa contain helpful bioactive saponins. Digitalis is a saponin that is extremely toxic in high doses, but has saved millions of lives as the basis for digoxin, a drug used to treat congestive heart failure. Saponins are largely destroyed by processing such as soaking and cooking; it is unlikely they have any biological effect when consumed in pet food. I spent hours searching the medical literature, yet found not a shred of evidence for one manufacturer’s statement that “saponins have caused bloat in every animal in which they have been tested, including dogs and cats.”
“Fiber has no nutritional value, is a filler, and is only added to slow down the digestion and make stools hard.” As we have seen, fiber has many functions. It doesn’t significantly contribute to whole body nutrition, but it can provide important nutrients of value to the cells and bacteria of the colon. Fiber may actually prevent absorption of some toxins, and is more likely to increase the speed of digestion than slow it. Excessive fiber increases stool volume, has a negative effect on firmness, and can irritate the intestinal lining.
S0 before giving fiber a bad rap, know your facts!