Fact Vs. Fiction
Clarifying and verifying literature and conversations on the Internet
Susan Donoghue, VMD MS DACVN, Proceedings, Association of Reptilian & Amphibian Veterinarians, 1995
Perusal of advertisements for dietary supplements, articles in popular magazines, and overheard conversations in health food stores leave no doubts that nutrition lends itself to hype, quackery, and misinformation. Faulty information concerning reptile and amphibian nutrition hasn't reached the magnitude of that for humans, but misconceptions show up in articles and conversations. Some disturbing areas of misconception and misrepresentation are discussed here.
Fiction: Canned pet foods are well suited for herbivorous reptiles, because fat contents are low, with labels reporting only about 5% crude fat.
Fact: Nutrient contents reported in the guaranteed analysis on pet food labels are on an as fed basis which accounts for water content. For evaluation and comparison, nutrients should be considered on dry matter basis or energy basis.
As an example, I will use a food for humans. Milk is marketed as 4%, 2% and 1% fat. Have you ever wondered why a product containing only 4% fat is considered by dietitians to be a high fat food? These percentages of fat are reported on an as fed basis, just as the fat percentages on a package of pet food. A more important consideration than the "as fed" contents of nutrients is the fraction of calories arising from fat. So the 4% fat of whole milk needs to be converted to a dry matter percentage, then calculated on an energy basis, to understand how much fat the milk actually provides:
Note that 4% whole milk contains 30% fat on a dry basis and provides 50% of calories from fat. So, after the appropriate arithmetic, whole milk is revealed as a high fat food. Similarly, canned pet foods reporting 5% fat in the label guaranteed analysis provide more than 20% fat on a dry basis, and more than 30% fat on an energy basis. These products, too, are high fat foods unsuitable for hind gut fermenters.
Fiction: Feeding kale kills herbivorous reptiles.
Fact: Kale (Brassica oleracea, var. acephala) has been farmed since the time of the ancient Romans, first as an animal food, then, by the Middle Ages, as a human food. It is essentially the same as straighter leafed collards, which replaced kale as a major U.S. crop because kale is less heat tolerant. It is a source of iron, potassium, carotene, ascorbate, and has a Ca:P ratio of 3:1.
Kale contains a goitrogenic substance that may interfere with iodine metabolism by the thyroid gland. However kale has been a safe and effective food for dairy cows for many years, although in recent decades replaced by corn-, soy-, or alfalfa-based rations. Selection of foods for (amphibians and reptiles) should include risk/benefit assessment of safety. The risk of inducing goiter in reptiles fed mixed salads containing kale is exceedingly low. The benefit - a nutritious green that most herbivores love to eat seems greater than the risk.
Fiction: Hill's a/d® is the diet of choice for sick reptiles because it is low protein (thus preventing gout) and in a slurry form.
Fact: Diets for sick amphibians and reptiles should match their needs for fuel sources and contain no harmful substances. Hill's a/d® contains 10.5% protein on an as fed basis. This is more than 45% protein on a dry matter basis, 36% protein on an energy \par basis. In contrast, low protein diets contain about 10% protein (energy basis) for herbivores, 10-18% protein for omnivores, and 20% protein for strict carnivores. So, given these comparisons, Hill's a/d(r) should be regarded as a high protein food.
The Hill's a/d® ingredient list begins: water, liver, chicken, corn flour... Since ingredients are required to be listed in order of predominance by weight, liver is the primary ingredient, second only to water. Liver contains very high, potentially toxic, levels of purines (233 mg/100 g) and vitamin A (36,000 W/100 g). Depending on the source of liver, lead contents may be high enough to cause poisoning. Thus Hill's a/d® is contraindicated if one's goal is to restrict protein or purines. Indeed, few other diets would approach this product in purine content. Better suited for feeding sick reptiles are liquid enterals, or enterals blended into slurries with commercial pet foods, produce, or prey (see Nutrition Support above). Special nutritional products, such as this, have specific indications and, just as important, contraindications that call for warnings and disclaimers. Products rich in purines should be avoided because of risks regarding gout.
Fiction: Reptiles and amphibians synthesize vitamin C, hence need no dietary ascorbic acid.
Fact: A fine study published several years ago showed that snakes tube- fed semi-purified diets containing either 0 or 500 mg/kg dry matter (DM) ascorbic acid for 76 days maintained total body concentrations of vitamin C. These data suggest that snakes which are unstressed need no supplementary ascorbate. The data may not hold true for other species, for rates of ascorbate synthesis between species is less an all or none phenomenon, and more a gradient or continuum. Some species synthesize ascorbate at greater rates than other species. Excretion rates for vitamin C increase with stress in endotherms, and a similar mechanism may exist in ectotherms. Data in racing sled dogs indicated that supplemental vitamin C (1 mg/kcal) helped lessen the stress of a long racing season, as measured by ACTH stimulation tests.
Thus we do not know ascorbate synthesis rates for most species of reptiles and amphibians, but there are likely species with lower rates than others, and we suspect increased ascorbate excretion (which could exceed rates of synthesis) when these animals are stressed. Since vitamin C is relatively innocuous (it causes diarrhea at very high doses, such as 5000 g or more per day in humans), supplementation with vitamin C at moderate levels (about 1 mg/kcal) may help amphibians and reptiles cope with stress.
Fiction: Fermentation of fiber in the lower bowel of herbivorous reptiles results in the production of protein and fat.
Fact: Fermentation of fiber in the lower bowel produces short-chain fatty acids, such as acetate, butyrate, and propionate, from carbohydrate , especially fiber. Short-chain fatty acids are used as fuel by enterocytes, and are absorbed into the circulation. Supplementation of patients (humans and rats) with butyrate following bowel surgery has aided healing and holds promise as an adjunct to nutrition support. Fermentation in the lower bowel is part of microbial metabolism and allows the proliferation of microbes. These form part of the feces, and their protein and fat is not available for absorption or use by the host animal.
Soluble sugars and starches that escape hydrolytic digestion in the small intestine are fermented to lactic acid in the lower bowel. This acid is not well absorbed, attracts water into the bowel, and may cause osmotic diarrhea. Fats that escape digestion in the small intestine tend to inhibit fermentation in the lower bowel. They are not absorbed and may be seen as greasy feces. Proteins that escape digestion in the small intestine may be putrefied by microbes in the large bowel. The odor of feces will be offensive. The lower bowel milieu of hind-gut fermenters also yields B vitamins, synthesized by microbes. The nutritional value of these vitamins formed at a postabsorptive site has been long debated.
Fiction: Spinach kills herbivorous reptiles.
Fact: Spinach (a s well as beet leaves, cabbage, peas, potatoes, and rhubarb) contain oxalates which limit the absorption of calcium and some trace minerals from the gut. Oxalates would likely clinically impact mineral metabolism only if mineral intakes were marginal.
Indeed, all plants contain substances termed "secondary plant compounds" which pose some risk when consumed. These non-nutritive substances include saponins, phenols, and alkaloids and serve as deterrents to bacteria, fungi, and herbivores. Spinach, as with kale, is a nutritious and tasty green that can be included as part of a mixed salad for herbivores. Just as I discourage feeding a diet of only kale, so I suggest avoiding diets of only spinach. However, spinach is safe to include as part of a balanced diet.
These few examples of commonly repeated nutritional misinformation represent a plethora. When reviewing nutritional information, I look for referenced data, reviewed articles, and replicated studies. I also discuss risks and benefits of food ingredients with owners, so that they can make informed choices regarding diets for their reptiles and amphibians.
Anonymous. 1993. The Key to Clinical Nutrition with Hill's Prescription Diet Products. Hill's Pet Nutrition, Inc. Topeka, Kansas.
Donoghue, S., D.S. Kronfeld, H.L. Dunlap, H.F. Schryver. 1993. Interet de Ia Supplementation vitaminique C chez le chien de traineau en situation de course ou de stress. Rec. Med. Vet. 169:773-777.
Ensminger, A.H., M.E. Ensminger, J.E. Konlande, J.R.K. Robson. 1994. Foods and Nutrition Encyclopedia. CRC Press, Boca Raton, Florida. 2nd Edition.
Pennington, J.A.T. 1994. Bowes and Church's Food Values of Portions Commonly Used. Lippincott, Philadelphia. 16th Edition.
Vosburgh, KM., P.S. Brady, D.E. UlIrey. 1982. Ascorbic acid requirements of Garter Snakes: plains (Thamnophis radix) and eastern (T. sirtalis sirtalis). J. Zoo. An. Med. 13:38-42.
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