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Adaptation to the Diet

Pet food nutritionists believe that given enough time, dogs (and to some extent cats) adapt to any foodstuffs in commercial pet foods.¹ Because dogs’ feeding behavior, digestive processes, and metabolism adapt easily, the pet food industry can develop many commercial dog foods. Some types vary tremendously from others. Experts believe that changing from one type to another requires no more than a few days to adapt. Abrupt changes often cause inappetence, vomiting, diarrhea, and flatus. Short-term adaptability depends on adjusting digestion to assimilate diets with different nutrient (carbohydrate, protein, and fat) compositions. An abrupt change to a high fat or high carbohydrate diet, for example, usually causes digestive upset. Within a few days gastrointestinal functions adapt and signs of indigestion disappear.

Dogs and cats do not invariably adapt to eating commercial pet foods. The requirement to adapt begins with the first feeding of commercial food. The age of a puppy or kitten decides whether the pet can adapt. If adaptation does not occur, a lifelong problem with gastrointestinal upsets begins. More common problems occur in animals adapting to commercial diets when they are young but then losing the adaptation later. They most often lose adaptation following acute gastrointestinal disease, due to a variety of causes; the pet loses its ability to consume and "tolerate" any previously fed commercial diets.

Adaptation to Nutrient Excess
A cat or dog usually can adapt to commercial diets containing excess nutrients. Long-term adaptability is possible for diets with relative excesses of either carbohydrates, fats, or proteins. Excesses of many vitamins cause few if any problems. Excess vitamins A or vitamin D can cause life-threatening disturbances. Vitamin A acting with some toxins (often found in the gastrointestinal tract at very low concentrations) can damage the liver. Vitamin D excess can kill an animal by stimulating calcium absorption so hypercalcemia results. Commercial pet foods can also contain excess trace minerals iron and copper. After absorption both elements are stored in the liver and they are not easily eliminated from the body. Accumulated iron or copper are hepatotoxic.

Nutrient Deficiency
Adaptation to commercial pet foods deficient in one or more nutrients is not possible when no other foods are fed. Vitamin and mineral deficiency will eventually cause signs of disease, which may take months or years, especially if the deficiency is marginal. To avoid deficiencies manufacturers supplement diets with excess vitamins and minerals. Sometimes the nutritional requirement for a nutrient is an estimate. In such cases the nutrient is often added in even greater excess to minimize a possibility for deficiency.

The Critical Adaptation―Weaning^2-4

The first and most significant adaptation to a new food begins when a puppy or kitten is weaned from nursing, as the sole source of nutrients, to eating a variety of other foods. During nursing, young animals receive milk designed to meet specific needs. The composition of maternal milk varies greatly between species. Milk from a different species substituted for its own disrupts a nursing animal's growth and normal gastrointestinal tract function. That kind of change can interfere with the neonate's ability to develop normal gastrointestinal tract function for months or years to come. It is important to appreciate all the changes occurring in the neonate during the development of normal gastrointestinal tract function.

Exposure to Foreign Substances Begins
When a puppy or kitten begins eating foods other than its mother's milk, it consumes something "foreign." The gastrointestinal tract must develop so it protects the body against anything foreign passing through. If protective mechanisms are inadequate it is likely an adverse reaction to food will appear. An adverse reaction can be food intolerance (a reaction not involving the immune system by which an allergy or hypersensitivity develops) or a food allergy (an immunologic or hypersensitivity reaction). During its lifetime a dog or cat will consume hundreds of pounds of foreign foods to which it must adapt; without adaptation an adverse reaction to food will appear.

Protection Against Foreign Substances
The normal process of adaptation begins even before the neonate begins to eat foods other than maternal milk. Shortly after birth the gastrointestinal tract encounters other foreign substances such as bacteria, viruses, and parasites. The neonate's adaptation is the development of a protective barrier, the mucosal epithelial surface of the gastrointestinal tract. This mucosal barrier protects against entry by foreign substances whether they are food, bacteria, viruses, parasites, or even the many foreign substances that are added to commercial pet foods. Timing of the protective mucosal barrier’s development is critical to when a young puppy or kitten can be weaned.

Protection by Gastric Acid and Pepsin
The gastrointestinal tract's protective functions begin with gastric acid and pepsinogen secretion. Acid alters dietary proteins so that they are less likely to be antigenic and they can be readily digested by pepsin. Acid also helps kill bacteria, viruses and some parasites entering the gastrointestinal tract. The protective function continues in the small intestine by pancreatic and intestinal digestive enzymes that degrade carbohydrates, fats, and proteins. As undigested foods they are foreign and can cause adverse reactions.

Regulation of Bacteria Populating the Gastrointestinal Tract
Very young puppies and kittens secrete little gastric acid. They readily digest and absorb maternal milk with no need for gastric acid. Acid protects against microorganisms entering the gastrointestinal tract but that is less important in nursing neonates. After birth, bacteria soon populate the gastrointestinal tract, with the largest number in the large intestine and smaller numbers in the small intestine. A more favorable population develops when young animals consume only maternal milk. When young animals begin to eat other food (including any other milk or formula), the bacterial population changes so that potential disease causing bacteria can now populate the gastrointestinal tract. That is an important reason why formula-fed orphans have so many more gastrointestinal tract problems than nursing puppies or kittens. Even when the neonate continues to nurse, the introduction of any new food changes the bacterial population of the gastrointestinal tract to that found in the adult animal. The change is irreversible. It is also a major change to which the puppy or kitten must adapt. If an animal is too young for making the appropriate adaptation, signs of gastrointestinal problems develop.

Digestion of Foreign Substances
The neonate requires weeks to develop its capacity for digesting carbohydrates, fats, and proteins. Lacking mature gastrointestinal functions, intact foods are absorbed from the intestine. Intact foods, especially proteins, are foreign and stimulate allergic or hypersensitivity responses when absorbed. Nursing animals consume milk protein that is digested and absorbed more completely than most other proteins. In contrast, an animal of the same age eating a commercial pet food must digest and absorb the poorly digested proteins found in commercial foods. That combined with a not-yet-mature digestive capacity for proteins increases the likelihood for absorption of intact proteins and the subsequent development of an allergy.

Permeability and Protection by Mucosal Barrier
The protective mucosal surface is also a physical barrier protecting against absorption or entry for intact particles. In neonates the barrier is much more permeable to entry of such particles than in more mature animals. Very young animals eating commercial pet food absorb undigested material that can cause allergy. Diets with many new and poorly digested ingredients should not be fed until animals mature. The gastrointestinal system needs time to mature. The mucosal physical barrier needs time to develop. Development of allergies or hypersensitivities following an acute gastrointestinal disorder is described later. Acute episodes disrupt the mucosal barrier so it becomes permeable to entry by substances in the intestine.

Intestinal Motility―Peristalsis
The protective barrier to entry by foreign substances includes normal peristaltic activity that prevents accumulation of contents in the intestine. A loss of normal peristalsis causes intestinal bacteria to increase in number. Drugs and disease impair peristalsis and cause this protective function to be lost.

Immune Barrier―Oral Tolerance^2-4
Complex interactions within the gastrointestinal tract and elements of the immune system are essential for protecting an animal against the foreign matter passing through the gut. The result of these interactions is called "oral tolerance." It is the ability of the animal to ignore the foreign matter that could stimulate an allergic response or hypersensitivity. This tolerance is essential to life. Without it animals would likely develop allergies to foreign material in the intestine. Animals are not born with oral tolerance; it develops at a young age. Tolerance can be subsequently lost with development of allergy or hypersensitivity. Oral tolerance is necessary because the nonimmunologic mucosal barrier is imperfect. Small but significant amounts of intact proteins and other allergy-promoting substances can penetrate the intestinal mucosa and enter the body. Oral tolerance consists of two parts. One is antibody made specifically against foreign matter. The antibody (IgA) is secreted onto the mucosal surface and covers it like an antiseptic paint. Antibody binds to foreign matter in the lumen which prevents its mucosal penetration. Impaired antibody production increases susceptibility for allergy. The second component to oral tolerance consists of immune cells, lymphocytes, that specifically suppress immune responses. With tolerance to a specific substance these lymphocytes are sensitized to suppress the ability of that substance to stimulate other parts of the immune system to cause an allergic response. These suppressor cells inhibit other lymphocytes that normally respond to a foreign substance by making antibodies or causing a hypersensitivity reaction. An animal needs both the ability to secrete the antibodies and the activity of suppressor cells to have tolerance for substances passing through the intestine. Oral tolerance is lost with development of food allergy, manifested by gastrointestinal signs. If the offending food is no longer consumed, it is possible to regain oral tolerance and signs of allergy will be gone. Some drugs interfere with the return of oral tolerance, however. Drugs suppressing immune system function can inhibit production of antibodies for secretion onto mucosal surfaces. Drugs can also inhibit suppressor cell function and prevent the regaining of oral tolerance. Corticosteroids are the most commonly used drugs to suppress the immune system.

Age for Development of Oral Tolerance
Animals must be able to develop oral tolerance when they are weaned and begin eating new foods. It is unknown when a puppy or kitten has the maturity to develop oral tolerance. It is estimated that they must be older than six weeks. If new foods are consumed before that age, it is likely that oral tolerance will not develop. Feeding a new food to which an animal has no oral tolerance is likely to result in allergy to that food. Many puppies and some kittens begin eating a variety of new foods long before they are six weeks old. When their eyes open at 10 to 14 days of age and they are able to crawl, young animals begin eating food available for adults. For many bitches and queens, food is always available and owners begin weaning at a very young age. This protects against marked deterioration of the mother's nutritional condition. The age at which a young puppy begins eating new foods is earlier for medium size to large breed dogs. A large litter of these puppies growing rapidly will quickly drain the mother of her nutrients. In consideration of the welfare of the mother and with the goal to rapidly restore her nutrition, so the owner can breed her again soon, puppies are weaned as early as possible. For larger breed dogs this objective is realistic and justifiable. Owners should not wean these puppies at a very young age, however, but at an age that is not likely to result in the puppy developing a food allergy.

Feeding Commercial Pet Foods and Food Allergies
Food allergies in dogs and cats are more common today than before 1960. Some may suggest that inbreeding of dogs and cats results in the increased prevalence of such allergies. Inbreeding has been a common practice for developing dog and cat breeds for centuries, however. The increase in food allergy prevalence is more likely a consequence of feeding commercially prepared pet foods over the last 50 years. Today most pet owners elect to feed commercial pet foods. They have little interest or knowledge for preparing their pet's diet. Before 1960 most people fed their pets leftovers from meals they consumed. (In Europe food allergies in dogs are common where people feed commercial pet foods. In Italy pet owners do not feed commercial pet foods and their dogs rarely have food allergies.)

The pet food industry markets products they claim to be nutritionally complete. The Industry argues that anyone feeding leftovers is likely to feed a nutritionally imbalanced diet, one that is likely to result in signs of a deficiency. Food allergies we see today are far more prevalent, troublesome, and important than deficiency from feeding a diet of human leftovers. Probably obesity is the greatest problem with people feeding leftovers. That is also a major problem with feeding commercial pet foods; the industry's chief objective is to provide the tastiest and most acceptable food possible. In both cases pets eat to satisfy tastes rather than to satisfy nutritional (caloric) needs.

From a Carnivore to a Grain-Eater
Quality of pet food ingredients is more important to a pet's well-being than the diet's nutritional "completeness." The list of ingredients on labels for dry food shows cereals as the most abundant (first on the list). Cereals as a least costly ingredient reduce production costs. Dogs and cats are not anatomically and physiologically designed to be vegetarians. Cereals are also not nutritionally adequate because they do not satisfy some essential amino acid requirements. Thus, dogs and cats are not designed to consume vegetable or cereal products as the most important part of their diet.

Dietary Allergy to Grains⁵
Dogs and cats are more likely to develop allergies to cereal products than to any other food. An allergy to a protein (gluten), found in wheat, barley, oats, and rye cereals, is a cause of chronic diarrhea and sometimes vomiting in dogs. The  allergy is gluten-induced enteropathy; in human beings the same problem is also called celiac disease. In dogs the allergy begins at the time of weaning when a puppy consumes a commercial pet food. The problem can be prevented in dogs. Studies on many human infants report that the incidence of celiac disease is much lower with two changes in the way they are fed.⁶ Fewer cases occur when breast feeding is the only source of nutrition and continues for a longer time. For example, fewer cases develop in infants nursing for six months. More cases of celiac disease occur in infants who begin eating other foods at one month of age. The second change in feeding practice that lowers the incidence of celiac disease is to feed infants gluten-free cereals. This information should be used to develop pet foods and feeding practices for dogs and cats. Scientific studies on nonhuman primates show that chronic diarrhea in mature individuals depends on how animals are fed at a very young age.⁷ Primate centers usually wean monkeys as early as possible. The protocols for weaning describe how commercial primate food should be soaked with milk and offered as the only food until the animal begins to eat it. They may eat a milk formula for the first two to four weeks of life and after that a change is made to monkey food. Monkey food is very different from what this primate eats in the wild. It is a cereal-based diet that is nutritionally balanced for the primate but not at all what this animal eats in its natural state. What is the result of these colony-raised monkeys being forced to eat monkey chow at a very early age? Primates weaned onto monkey chow at an early age have a much higher incidence of chronic diarrhea than a monkey allowed to nurse for at least three to four months.⁷ The early weaning practice causes monkeys to develop diarrhea which is their most common problem in captivity, whether in a primate colony or a zoological garden. Little is being done to prevent the problem by feeding these animals differently.

Puppies, especially of medium-size and larger breeds almost invariably eat cereal-based dry dog food by the time they are three weeks of age. Thus, they consume gluten-containing cereals, the most common food to which dogs are allergic, at an age when they are not old enough to develop oral tolerance. When a puppy becomes allergic to glutens, the allergy remains and worsens unless gluten-containing cereals are removed from the diet. If owners delay feeding such cereals until puppies can develop oral tolerance, no signs of gastrointestinal upset appear. When puppies eat the cereals at a very early age, signs of gastrointestinal upset are common. Diarrhea, vomiting, and sometimes weight loss continue as long as the cereal remains in the diet. If gluten-containing cereals are no longer fed, these signs of gastrointestinal upset disappear. Signs of gluten allergy disappear and they do not return months later on feeding gluten-containing cereals again. Allergy to glutens is then lost; oral tolerance to glutens has been regained.

A Weaning Protocol to Prevent Allergies
When should a puppy or kitten be weaned? What kind of diet should be fed at weaning time? Six weeks of age is close to an estimated ideal time for weaning. For other than small breed dogs and cats, that time is impractical for weaning. Effective recommendations can be made to prevent food allergies when an animal is too young for developing oral tolerance. Rather than emphasizing a time for weaning, one should know how to feed a young animal during weaning. This is based on knowing the conditions that make it more difficult for oral tolerance to develop.

Absorption of Intact Proteins to Cause Allergies⁸
Intestinal mucosa’s physical barrier is porous in very young animals. Its porosity facilitates absorption of colostral antibodies. Antibodies are large molecules compared to nutrients crossing intestinal mucosa. With time the mucosa becomes nonporous and prevents absorption of antibodies. Greater mucosal leakiness in very young animals permits absorption of intact food particles. Feeding commercial pet foods when the mucosa is leaky, increases the chances for developing allergies and oral tolerance not developing or being lost. As described later, any disease that increases mucosal permeability in adult animals allows easier absorption of intact foods. This often causes food food allergies to develop.

Mother's Role in Developing Allergies⁴
Besides antibodies in colostrum moving across the mucosa and entering the body, there can be entry of intact food particles in maternal milk. These food particles can be from bitchs’ or queens’ diet that are absorbed undigested and enter their milk. During nursing that milk can sensitize young animals so they become allergic to the food their mother consumes. Nursing young animals can also absorb antibodies against specific foods when mothers are allergic. If an allergy does not develop, the antibodies will at least prevent the nursing animal from developing oral tolerance against the foods. With persistence of the antibodies and, thus, without oral tolerance food allergy can develop when certain foods are consumed. Thus, what a nursing mother eats is important in preventing allergies in its young. Feeding commercial pet foods with their many ingredients to animals nursing their young contributes many of the factors necessary for allergy development.

Vaccines' Role in Developing Allergies
Young animals with antibodies against a certain food are not able to develop oral tolerance when they begin to eat that food. As stated above one way for a young animal to gain such antibodies is from its mother’s milk. Young animals can also develop antibodies against foods by vaccinations they receive for protection against a variety of infectious diseases. Drug companies prepare vaccines from viruses and bacteria cultured in broths containing common foodstuffs. On preparing vaccines the broth is removed but they are not completely clean because traces of broth remain. Beef blood plasma is one common food ingredient in vaccine broths. Inoculation with any impure vaccine stimulates  antibody production against beef. Beef is a source of meat protein in pet foods. Feeding such food to animals with antibodies against beef can result in 1) an inability of the animal to develop oral tolerance for beef protein and 2) a possibility for stimulating greater antibody production with the result being a beef allergy. Thus, indiscriminate administration of vaccines at an early age, when it is really not necessary, can result in young animals subsequently becoming allergic to foods they eat.

Protection in Mother's Milk²
Puppies and kittens receive protection from the mother as long as they continue to nurse. Children that are breast-fed are far less likely to develop inflammatory bowel diseases and diarrhea during infancy than bottle-fed infants.⁹ Antibodies in milk help prevent absorption of excess intact foods that could cause food allergies. Mother's milk also contains growth factors that modulate normal development of the gastrointestinal system. Lacking exposure to growth factors can prevent or delay complete and timely development. At weaning, complete withdrawal of mother's milk makes it more difficult for young animals to develop oral tolerance.

Oral Tolerance to Normal Gut Bacteria
Oral tolerance must enable animals to ignore non-food sources of proteins and carbohydrates in the gastrointestinal system. Non-food substances with ability to cause allergy include gastrointestinal bacteria. Antibodies are produced against these bacteria and secreted over mucosal surfaces to protect against bacteria penetration. Entry of large numbers of bacteria can stimulate antibody production so high circulating antibody levels result in allergy to bacteria. Chronic gastrointestinal disorders are, at least in part, perpetuated by a loss of tolerance for bacteria that normally live in the gastrointestinal tract. Thus, they become allergic to those bacteria and respond with signs of diarrhea and possibly weight loss and vomiting. When does the need arise for developing tolerance against these bacteria?

Populating the Gut with Bacteria¹⁰
At birth the gastrointestinal tract of all mammals is sterile; no bacteria or any other microorganisms are found. As neonates begin nursing, bacteria populate the tract. Bacteria numbers increase tremendously but their population consists of harmless species. That population persists and changes only when animals begin consuming different foods, such as during weaning. The changes include population by bacteria with the potential to cause problems. These new bacteria can produce toxins that under certain conditions cause severe gastrointestinal disorders and sometimes, death. When weaning neonates with maturity to develop oral tolerance, population by these potentially-pathogenic bacteria cause no problems because they live primarily in the large intestine. Changes during weaning also include increased numbers of bacteria (i.e., coliforms) that are pathogenic when their numbers increase. (These bacteria are an important source of endotoxins). Their numbers can increase 100 to 1000 times. Young animals must be mature enough to develop oral tolerance when these bacterial changes occur.

Altering the Gut's Bacterial Population
Weaning-induced changes in the gastrointestinal tract's bacterial population appear also when animals are formula fed. Feeding milk formulas changes intestinal bacterial populations even when nursing continues. Formula-fed neonates have more persistent gastrointestinal tract problems than nursing animals. This is recognized by veterinary students at the VMTH in Davis, California who conduct a program for raising orphan kittens. They formula‑feed kittens from birth. After gastrointestinal tract problems develop they cannot be reversed by a return to nursing and stopping all formula feeding. The intestinal bacterial population does not revert to its original state with the change in feeding. When a two-to-three-week old puppy or kitten begins eating commercial pet food, the gut's bacterial population also changes and cannot be reversed.

One of the most abundant of harmless bacteria populating the intestine of nursing animals after birth is Lactobacillus. It is similar to the Lactobacillus found in fermented forms of yogurt. Some people make claims of benefits for giving Lactobacillus cultures to young animals. (People make similar claims for the culture's use in animals with gastrointestinal problems.) There is no proven benefit in giving cultures of those bacteria to animals. Lactobacillus populates the gastrointestinal tract quickly after birth. There is no evidence that giving additional Lactobacillus is of any value. Sometimes manufacturers combine these cultures with other nutrients such as vitamins and trace minerals. These nutrient combinations are not necessary for a neonate and when given they are in great excess that could cause problems. These products should not be used.

Feeding to Gain Oral Tolerance
Before the era of commercial pet foods, owners weaned puppies and kittens by feeding owner-prepared diets. They encountered few problems, even for puppies of large breeds. They began weaning young animals by feeding one new food at a time. The list of ingredients on a pet food label shows that feeding them commercial diets introduces many new foods and food additives at weaning. It is important to feed only one new food at a time. Feeding an animal one new food allows it to develop oral tolerance. If simultaneously or shortly after that a second or third new food is fed, the first new food fed stimulates the reticuloendothelial system to interfere with tolerance developing for the second and third foods. Thus, early feeding of a conglomerate of ingredients, such as in pet foods, results in oral tolerance developing for one but not for other ingredients. That makes it likely that allergy can develop for the other ingredients.

In the past dog owners and breeders weaned puppies by feeding specific foods that were highly digestible. Small amounts of intact or undigested food are always absorbed. Absorbed undigested foods are foreign and stimulate the immune system, something that often causes food allergies. Highly digestible foods leave little if any intact food for absorption and there is less likelihood for allergies. Feeding only small amounts of new foods helps the development of oral tolerance. Feeding large amounts of new foods prevents oral tolerance from developing. Thus, both digestibility and the amount fed determine whether oral tolerance develops for a new food.

Feeding a new food often as well as in small amounts makes it easier for oral tolerance to develop. Both together insure complete digestion of foods so lesser amounts of intact foods are available for absorption. Frequent feeding is probably more important than appreciated. For convenience, owners feed most puppies commercial pet foods at long intervals between meals.

Small animals poorly digest cereals in commercial pet foods. The digestibility for other ingredients is also poor. The digestibility for single foods is highest for cooked eggs, milk, meat protein, soy protein, and boiled rice. For the only cereal in this group, boiled rice has a digestibility of nearly 100 percent in dogs. The digestibilities of other cooked cereals such as corn, wheat, barley, and oats are poorer.

General nutrition is also important in developing oral tolerance. Feeding a poorly digested commercial pet food can result in protein deprivation. Oral tolerance is more difficult to establish in animals suffering from protein deficiency.

To insure the development of oral tolerance, feed puppies or kittens one highly digestible nutrient to begin the weaning process. With tolerance gained for one, gradually introduce a second highly digestible food to the diet. Following this protocol it is possible to develop a diet and wean the animal. Before this present age of scientific knowledge, common sense directed human mothers to follow these steps in weaning a human infant. They never thought of assaulting an infant with a barrage of new foods such as found in a commercial pet food.

With No Tolerance Allergies Develop^11,12
Once oral tolerance for a food fails to develop and allergy appears, allergy results from antibodies produced against the food. If the offending food is no longer fed, oral tolerance for that food cannot be regained until antibodies against the food disappear. Antibodies persisting against a food prevent redevelopment of oral tolerance when the food is eaten. Several months are needed before these antibodies disappear spontaneously and then if the food is eaten again, oral tolerance can develop. In essence, the animal can regain oral tolerance but only if it does not eat the offending food, even in small amounts, for many months.

Summary
Every kind of matter entering the gastrointestinal tract whether it is food or something else is foreign. The body must learn to ignore foreign matter; tolerance must develop for it. Without tolerance animals do not adapt to what they eat and food intolerance appears. Development of oral tolerance is crucial to good health. Animal care and feeding practices affect oral tolerance’s development and persistence throughout life. Defective oral tolerance causes signs of food intolerance.

References

1. Kronfeld, David S. 1972. Commercial Dog Foods. In Canine Nutrition. edited by David S. Kronfeld. from symposium on Canine Nutrition and Fluid Therapy at New Bolton Center, University of Pennsylvania, School of Veterinary Medicine.

2. Sampson, Hugh A. 1991. Immunologic Mechanisms in Adverse Reactions to Foods. Immunology and Allergy Clinics of North America 11(4):701-716.

3. Ferguson, Anne. 1994. Immunological functions of the gut in relation to nutritional state and mode of delivery of nutrients. Gut supplement 1:S10-S12.

4. Strobel, Stephan. 1992. Dietary manipulation and induction of tolerance. Journal Pediatrics 121:S74-S79.

5. Guilford, W. Grant. 1996. Adverse Reactions to Food. In Strombeck’s Small Animal Gastroenterology, edited by W. Grant Guilford, Sharon A. Center, Donald R. Strombeck, David A. Williams and Denny J. Meyer, 436-450. Philadelphia: W.B. Saunders.