Rabbits have a very round-about, complicated way of getting their food digested, and they do not process all foods equally well. For instance, the ability of the post-weaning rabbit’s small intestine to digest fructose (fruit sugar) increases, while the ability to digest all other sugars decreases (Buddington 1990). So, a mature rabbit can easily digest (and get fat on) the sugar in fruit, whereas the sugar in a candy bar can make the same rabbit sick. This is because sugars and starches that the small intestine can’t digest will wind up in the cecum. If they arrive in large quantities in the cecum, they encourage the overgrowth of toxin-producing bacteria with consequent illness to the rabbit.Adult rabbits absorb protein in the small intestine (up to 90%), but this depends on the source. The protein in soybean meal is very digestible, but a high portion of the protein in alfalfa (which is largely bound to the plant cell wall) is indigestible to rabbits. Rabbits digest cellulose poorly (Fraga 1990). This seems a paradox for an animal who lives naturally on vegetation. But this is part of the plan. The low digestibility of fiber and rapid elimination of large, hard-to-digest particles enable the rabbit to “maintain a higher level of feed intake than it would other wise be able to” (Sakaguchi 1992).
Particle Sizes and Transit Times
Indigestible fiber is indigestible whether it’s pulled off the bark of a tree or blended into a feed, yet the size and type of the fiber can affect the length of time it takes to pass through the GI (gastro-intestinal) tract.
Contrary to what you might expect, the large particles don’t get stuck inside the rabbit while the small ones exit easily. It’s the other way around.
Transit times of particles moving through the GI tract have been measured, in several scientific studies, by placing markers in the feed. High-fiber alfalfa meal, pelleted into large particles (up to 3mm) moved through the digestive system in 14.1 hours in one study. When same high-fiber feed was ground to a finer size (1mm), it took 15.9 hours to pass through the digestive tract.
Markers in a pelleted low-fiber high-starch feed passed in 20.1 hours (Gidenne 1992). Why are smaller fibers and high-starch feeds slower in getting through? Because small particles and excess starch are sent to the cecum for fermentation, and this takes extra time. Fluids and small particles are separated in the colon and moved backwards into the cecum (Cheeke 1987), while large particles are passed quickly through the colon.
One study used particles up to 5mm in marked feed, which passed in 5 hours (Sakaguchi 1992). This may closely approximate the size of chewed hay. I can say with strong certainty–from caring for disabled, diapered rabbits on a monitored diet with strictly scheduled feeding times–that the oat hay I give my rabbits in the morning is passed by the afternoon (4-5 hours).
Fast and Slow
So which is more desirable, fast moving or slow moving particles? Some of both are needed: a sizable quantity of coarse indigestible organic material to keep the gut working at an optimum rate and enough (but not too much) digestible material to be absorbed in the small intestine and cecum.
How do you know when there is optimum movement? Try listening. You should hear normal “gut sounds” caused by the fluid ingesta mixing with gas as it moves along. Too much noise may indicate an irritated intestine, while no sound at all indicates stasis.
What Is a Balanced Diet?
A balanced diet for a rabbit is a nutritional and a time balance. It includes protein, starch, carbohydrate, vitamins, and minerals in slower moving digestible material that can be absorbed. It also includes large amounts of fast-moving indigestible organic material. The next thing to learn is specific quantities of digestible and indigestible material that are appropriate for your rabbit.
Marinell Harriman in consultation with Carolynn Harvey, DVM
House Rabbit Journal Volume III, Number 3, Summer/Fall 1994
1 Swallowed plant food is passed to the STOMACH from the PHARYNX through the ESOPHAGUS.
2 Muscular contractions squeeze and churn the food in a .circular path, separating food particles and mixing them with the gastric fluid of the stomach.
The stomach has two regions. The fundic region is lined with layers that are nonsecretory. Secretions from glands in the lining of the pyloric region include hydrocloric acid, digestive enzymes and mucus. “The stomach serves as a storage organ metering ingesta into the small intestine.” (Cheeke 1987)
3 As the food particles exit the stomach enzymes produced in the pancreas and bile produced by the liver are secreted into the small intestine at the duodenum.
4 The major part of digestion takes place during passage through the small intestine. Fructose, most starches, and up to 90% of all protein are absorbed, while cellulose is not efficiently digested.
The lining of the small intestine has numerous tiny projections called villi which greatly increase the surface area for absorption. The small intestine has three functional areas:
The duodenum is the primary area for neutralizing stomach acids and mixing of digesta. It receives pancreatic enzymes, involved in carbohydrate, protein and fat digestion and liver bile acids, important in vitamin and fat absorption. The jejunum is the major site of digestion and absorption. The ileum is the remaining portion and is characterized by lymph nodules in the submucosa. (McLaughlin 1990)
5 Fiber is separated into large and .small particles. Larger particles are eliminated; smaller particles are sent to the cecum for fermentation.
Ingesta is discharged fairly uniformly from the ileo-cecocolonic junction, at the end of the ileum. Fine particulate materials are sel- ectively channeled to the cecum by haustrae. Circular muscle fibers “move material in a retrograde manner to the cecum,” (Cheeke 1987) while “larger particulate material is passed through the large intestine as fecal pellets.” (Percy 1993).
6 Bacteria digest cellulose (small particles), most sugars, excess starch, and any remaining protein not digested in the small intestine; B-complex vitamins are produced along with volatile fatty acids, which are absorbed directly into the bloodstream. (Evans 1991)
The cecum is the blind end of the colon, with a capacity of two to three times that of the stomach. Continuously in motion, it mixes its contents by rapid contractions back and forth along its length. “There is a continual flux of material between the cecum and proximal colon.” (Cheeke 1987) “The cecum contains symbiotic microorganisms that produce cellulase to break down the cellulose walls of plant cells.” (McLaughlin 1990)
The appendix secretes an alkaline fluid buffer to volatile fatty acids produced during cecal fermentation. (Cheeke 1987)
7 Undigested fiber and waste (hard fecal pellets) pass through the large intestine along with vitamin-rich cecotropes (soft cecal pellets), which are formed from fermented cecal material. Cecotropes are higher in protein and water and lower in fiber than fecal pellets. (Cheeke 1987)
Colon/large intestine. The proximal colon winds forward toward the stomach. Segmental contractions separate the digesta into fecal pellets.The distal colon, at the level of the stomach, loops caudally to the short straight rectum. (McLaughlin 1990)
8 Cecotropes are consumed directly from the anus and returned to the digestive system (cecotrophy).
9 Protected in mucous membrane packages, cecotropes continue to ferment in the stomach for several hours, until they pass to the small intestine where the nutrients are absorbed. (Cheeke 1987) *
Cheeke, P. R. 1987. Digestive Physiology. Pp 20-32 in Rabbit Feeding and
Nutrition. Orlando, FL: Academic Press
Evans, R., consultant. 1991. House Rabbit Handbook. Alameda, CA: Drollery Press.
Harvey, C. 1993. Necropsy photos from Bay Area Pet Hospital archive.
Oakland, CA: Unpublished.
McLaughlin, C. A. and R. B. Chiasson. 1990. Pp 59-64 in Laboratory
Anatomy of the Rabbit. 3d ed. Dubuque, IA: William C. Brown
Percy, D.H. and S.W. Barthold. 1993. Pp. 179-80 in Pathology of Laboratory Rodents and Rabbits. Ames, IA: Iowa State University Press.