Dietary antioxidants are important for pets
Oxygen gives pets life but may behave aggressively to try to shorten it.
Oxygen gives pets life but may behave aggressively to try to shorten it.
Without sufficient oxygen, a pet's tissues are in immediate jeopardy. Ironically, excess oxygen can poison the cells of a pet's body via free-radical formation-what a dichotomy!
Normal compounds stabilize their chemical bonds by sharing electrons, one from each atom. Highly unstable free radicals steal an electron from normal compounds (typically oxygen), producing a stable compound by pairing their unpaired electron. In doing so they alter the stable compound, creating another free radical, again short one electron, continuing the chain. This chain reaction creates havoc in the cell.
Free radicals are commonly produced as part of normal cell metabolism, but also can become excessive following injury, disease, or due to environmental pollutants such as UV radiation, cigarette smoke or smog.
Excessive free radical production or oxidative stress results when the formation of free radicals overwhelms the body's defense system against them producing an imbalance between production and removal of free radicals. Oxidative stress can overpower the pet's ability to fight back and may result in cell and tissue damage, thus shortening its life, just like oxygen can brown an apple or banana.
Uncontrolled free radicals may run amok throughout a pet's body, doing considerable damage to cells.
They alter the structure of cell membranes, and create havoc to polyunsaturated fats, cell proteins and cell DNA. The more active the cell, the greater the potential risk of tissue damage. In people, this damage has recently been linked to degenerative diseases such as rheumatoid arthritis, cancer, cardiovascular disease, inflammatory bowel disease, renal disease, Parkinson's Disease, cataracts, and may have a deleterious affect on aging. It is thought that free radical damage may play a similar role in certain diseases of pets and wreak havoc on the aging process (Freeman, et al 1999; Toll et al, 2001).
Figure 1: Vitamin E Protection of Cell Membranes
Antioxidants are the pet's major defense system against the scourge of free radicals and oxidative stress, keeping their damage to a minimum.
Antioxidants to the rescue
Dietary antioxidants, like vitamin E, scavenge and convert free radicals to relatively stable compounds and stop or prevent the chain reaction of free radical damage. Antioxidants are therefore important to protect pets from tissue damage, disease and may in the process, enhance immunity.
Endogenous enzymatic antioxidants, including superoxide dismutase and glutathione peroxidase, are one line of defense against free radicals.
Dietary antioxidants include vitamin E, vitamin C, taurine, and the carotenoids - beta-carotene, lutein, and lycopene and certain trace minerals. These antioxidants gobble up free radicals, stopping them in their tracks.
Antioxidants keep the pet's cells healthy, including lungs, heart, blood cells, muscles, nerves, GI tract, and reproductive organs. It was recently demonstrated that a cocktail of such antioxidants limited cell damage in dogs and cats (Heaton et al, 2001).
Currently work is being done to show the benefits of antioxidants to older pets' ability to learn, to modify certain behavior problems and function better in their geriatric years (Milgram et al, 2001).
Figure 2: Vitamin E Absorption, Transport, Utilization
The potential role of dietary antioxidant therapy to benefit diabetes, renal disease and feline asthma has been proposed (Harper 2000). It has been suggested that the addition of various antioxidants improves the antioxidant status of elderly dogs. (Zicker, 2001).
Vitamin C (ascorbic acid) is a powerful water-soluble antioxidant.
It regenerates vitamin E, glutathione, and flavonoids; reduces free radicals in cells; and works with minerals in their role as antioxidants.
It is also thought to destroy free radicals involved in carcinogen metabolism. High activity dogs, those competing in flyball, agility, racing and high-demand rescue work are thought to require increased amounts of vitamin C due to the increased demand of oxidative stress (Baskin et al, 2000; Piercy et al, 2000).
Although the dog can synthesize vitamin C, racing and other forms of high-energy exercise are thought to increase the demand for all antioxidants to combat increased oxidative stress with increased body use of oxygen (Marshall et al, 2001). Unlike humans and guinea pigs, the dietary vitamin C requirement of cats is negligible, because they can synthesize it, though their need for vitamin C, as an antioxidant is unexplored. Surely the benefit of vitamin C as an antioxidant is feasible.
Carotenoids are red, yellow, and orange fat-soluble pigments found in plant foods such as carrots and tomatoes. Though the dog and cat are predominantly carnivores, the value of these powerful antioxidants could be important to assist them. We don't yet know all the benefits of these compounds to dogs and cats to prevent degenerative diseases, cancer and combat conditions of aging. Studies have shown that cats do readily absorb beta-carotene across the intestinal mucosa (Chew et al, 2000).
Figure 3: Natural Vitamin E Production
It has been shown that dietary lutein stimulated both cell-mediated and humoral immune responses in dogs (Kim et al, 2000). Since dogs and cats are living longer today, carotenoids may benefit in ways yet undiscovered.
Taurine, an amino acid, is a critically important nutrient for cats. Its benefit is for successful reproduction, healthy eyesight and heart function.
Though important to cats, taurine is not similarly essential to dogs. For dogs and cats taurine is beneficial as an antioxidant, known to protect cell membranes from damage.
Certain minerals, especially zinc (Zn), copper (Cu) and selenium (Se) and manganese (Mn) are antioxidant minerals.
They occur in the structure of specific enzymes that are integral parts of various enzymatic antioxidants in the pet's body.
Zinc and copper are in superoxide dismutase as well as other enzymes, and selenium is in glutathione peroxidase. These enzymatic antioxidants are equally important as dietary antioxidants consumed by pets.
Vitamin E is unique among vitamins. As alpha-tocopherol, vitamin E is notably essential for the proper function of the reproductive, muscular, nervous, circulatory and immune system. Its antioxidant property is its prime function. Vitamin E, which can't be synthesized in a pet's body, is possibly the most important and essential antioxidant, protecting cell membranes from free radical damage by preventing lipid peroxidation of unsaturated fatty acids (Figure 1).
Vitamin E (alpha-tocopherol)
Figure 4: Vitamin E (alpha tocopherol) structure
Vitamin E is especially necessary to newborn kittens and puppies, since placental transfer is poor, first milk, colostrum is important to provide adequate vitamin E. Vitamin E works in concert with other antioxidants, including vitamin C, carotenoids, and mineral-containing enzymatic antioxidants.
Vitamin E may also work synergistically with taurine (Carnevale, Hintz, and Schryver, 1988, personal communication).
Vitamin E absorption, transport, and utilization begin with its uptake in the small intestine (Figure 2.)
Vitamin E is absorbed with a mixture of dietary fat sources, primarily triglycerides, through the small intestine, with the assistance of bile and pancreatic fluids. It is moved via the lymphatic pathway to the liver in association with chylomicrons. Once vitamin E reaches the liver, it is transported to peripheral tissues in the blood bound to lipoproteins (VLDL, LDL, HDL).
The majority of vitamin E is found in adipose tissue, the rest is primarily found in the liver and skeletal muscles. There is considerable vitamin E present in lung and spleen.
Usually synthetic vitamin sources are, for the most part, equal in efficacy and structure to the natural form of the vitamin, such is the case of vitamin C. Not so for vitamin E. The source of vitamin E with the highest biological activity is natural vitamin E (d-alpha-tocopherol) isolated from seed oils. Synthetic vitamin E (dl-alpha-tocopherol) is made from petrochemicals (Figure 3, p. 24S). The body preferentially transports and incorporates natural vitamin E. Synthetic vitamin E is not as biologically potent in comparison. The difference between natural and synthetic is in the chemical structures of the two.
Natural E differs from synthetic E
The tail structure is critical to vitamin E's absorption, transport and tissue retention. Within the tail, each of the 2, 4, and 8 numbered carbon atoms have methyl groups attached at a 'right' or 'left' configuration. Natural vitamin E is a single entity, one isomer. It has each of these positions in the "right" configuration, 'RRR', and therefore the greatest biological potency.
Study results (Ingold, et al, 1987) have shown preferential uptake of natural vitamin E by the lung, red blood cells, blood plasma, and brain. Synthetic vitamin E, also known as racemic, or "all-rac," on the other hand, consists of eight stereoisomers, i.e. RRS, RSR, etc, with only 12.5 percent identical to the natural RRR isomer.
The structure of natural tocopherol sources (Figure 4, p. 24S) provides clues to their variation in biological potency.
The number of methyl groups attached to the ring structure dictates whether the tocopherol structure is alpha, beta, gamma, or delta-tocopherol. With 3 methyl groups, alpha-tocopherol has by far the greatest vitamin E activity. In fact, it is the only tocopherol to be recognized to have vitamin E activity (1.49 I.U. per mg.) The other three tocopherols do possess antioxidant activity but essentially no vitamin E activity. Beta- (2 methyl groups), gamma- (2 methyl groups), and delta- (1 methyl group) tocopherols do serve an antioxidant function primarily within food prior to consumption by the pet that will be discussed later in this article.
For pets, natural vitamin E is better than synthetic. It functions better to provide their enhanced antioxidant needs.
Natural better than synthetic
From mice to elephants it has been shown that natural vitamin E has more punch, more 'bounce per ounce' than synthetic. It has recently been shown (Stone et al , 2003) that human infants similarly discriminate between natural and synthetic vitamin E.
There are physiological differences in its absorption, transport, utilization and tissue retention. Though past research has shown that natural vitamin E is 36 percent better than synthetic, current research in several species, including humans, shows a 200-300 percent potency of natural versus synthetic vitamin E.
Seed oils such as soy, safflower and sunflower seed and organ meats such as liver and spleen are higher in vitamin E. In the wild, cats and dogs may naturally obtain vitamin E from organ meats. Today's pet foods are supplemented with vitamin E.
Need for vitamin E supplementation
Most critically for pet diets is the increase in vitamin E requirement with an increase in dietary polyunsaturated fats. Cat foods high in fish oils and other unsaturated fats require an increased need for vitamin E.
Years ago, without this knowledge, diets high in PUFA's (fish, fish meal, fish oils), not supplemented with sufficient vitamin E, led to "yellow-fat" disease (steatitis)-rancidity and destruction of body fat, leading to death in kittens. With this knowledge, today's cat foods with increased PUFA's are now properly supplemented.
Similarly to cats, the dietary requirement of dogs for vitamin E is closely associated to the dietary concentration of PUFA's. Cordes & Mosher (1966) reported a similar condition to steatitis in cats, "brown bowel syndrome" in puppies. Hayes et al (1969) experimentally reproduced it in puppies fed varying amounts of PUFA's.
According to the National Research Council (NRC), the need for vitamin E in the diet of cats is markedly influenced by dietary composition.
Though 30 IU alpha-tocopherol per kilogram of diet is considered the minimum requirement for cats, 50 IU per kilogram of diet is probably better, considering the high PUFA's containing diets many pet cats eat. Actually, a high PUFA diet of tuna, or other fish probably requires three to four times the minimum.
Vitamin E deficiency dermatosis has been produced experimentally in dogs (Scott & Sheffy, 1987.) It was first reported (Elvehjem et al, 1944) that a smaller amount of vitamin E would not sustain reproduction in Fox Terriers fed unsweetened, irradiated evaporated milk, while a slightly larger amount would. However, one pup in four from a bitch receiving the higher level of vitamin E showed slight muscular dystrophy. In Beagle puppies 30 IU vitamin E per kilogram of diet prevented deficiency both clinically and histopathologically (Van Vleet, 1975). The possible benefit of vitamin E toward enhanced exercise performance in sled dogs is well studied (Piercy et al, 2001A; Piercy et al, 2001B; Baskin et al, 2000; Piercy et al, 2000.)
According to the NRC, the recommended allowance for maintenance, reproduction and growth of dogs, should be satisfied by 20 IU vitamin E/kg diet (3300 kcal ME), 1.1 IU/kg BW for pregnancy and 1.2 IU/kg BW for growth.
In order to assure animals are getting a guaranteed amount of vitamin E, pet foods are supplemented with a stabilized acetate-ester of vitamin E.
Vitamin E acetate from either natural or synthetic vitamin E is added to vitamin premixes in order to guarantee a certain level of vitamin E in the complete food.
For their own personal need, people prefer natural vitamin E. People search out natural-source vitamin E, d-alpha-tocopheryl acetate. It says "natural vitamin E" right on the label. They expect no less for their pets. Therefore, they should look for natural vitamin E in their pet's food. They should look for d-alpha-tocopheryl acetate, natural vitamin E in the ingredient list of dry dog or cat food. Similarly when people look for shelf life protection in their pet's food, they should look for mixed tocopherols, not BHA or ethyoxyquin.
Natural vitamin E in pet food
All dry pet foods are supplemented with preservatives to protect the ingredients from being oxidized.
Pet foods need
Dry foods are especially prone to fat rancidity resulting in reduced palatability. In order for dry foods to have sufficient shelf life, antioxidants (preservatives) are included to keep them stable and less prone to spoiling. In most of the premium pet foods, 'natural-source' antioxidants are used as preservatives instead of chemically synthesized preservatives, such as BHA, BHT, and/or ethoxyquin. Typically, the natural-source preservatives will always include mixed tocopherols often blended with other antioxidants such as rosemary extract, vitamin C and/or citric acid. The preservative is typically blended with the fat source prior to the pet food being manufactured and also at the time of pet food manufacture to help stabilize all ingredients in the pet foods.
Natural mixed tocopherols are composed of a blend of alpha-, beta, gamma, and delta-tocopherols (Figure 4) and do contain some vitamin E activity. However, due to the low quantity of alpha-tocopherol (usually 12-16 percent), mixed tocopherols are solely approved as a food preservative and not a source of vitamin E for pets. The intended function of the mixed tocopherols is to be an antioxidant in pet food, not for the animal. Some pet food manufacturers lead the pet owner to assume that the product is fortified with natural vitamin E, when in fact, synthetic vitamin E acetate is the source of vitamin E to guarantee a level of this key vitamin in the food. Although pet foods may be stabilized with mixed tocopherols, vitamin E activity is usually provided as synthetic-source vitamin E acetate. Very few companies actually use natural-source vitamin E acetate to guarantee vitamin E levels in pet foods.
Dietary antioxidants are important for pets.
It is important to make sure a pet's food contains the antioxidants he or she needs. With such interest in antioxidant vitamins in human nutrition, there is a keen interest in these vitamins in pet nutrition.
Of the antioxidant vitamins discussed, NRC has only published a vitamin E requirement for dogs and cats. Since the early 1990's, pet food manufacturers have used natural mixed-tocopherols and have avoided using synthetic preservatives in dry foods. Today, more and more pet food companies are also substituting natural vitamin E (d-alpha-tocopheryl acetate) for synthetic vitamin E to provide the vitamin E needs of pets.
Dr. Kane is a freelance writer for equine topics and senior nutritionist with Stuart Products, Inc., Bedford, Texas. He holds a Ph.D in equine physiology and nutrition from the University of Kentucky.