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Last updated January 1, 2014

Evaluating Commercial Diets

Unfortunately, not all commercial diets are sound, nor are good products always fed to suitable animals, so room remains for improvement in the feeding and care of reptiles.

Susan Donoghue VMD MS DACVN and David Dzanis DVM, PhD, DACVN
Proceedings, ARAV Second Annual Conference, October 27-29, 1995. Sacramento CA. pp 74-79


Reptiles, and to a lesser extent amphibians, may be fed commercial diets formulated for the species or for domestic endotherms, such as dogs and cats. When formulated, manufactured, and fed appropriately, commercial diets offer sound and safe nutrition. Unfortunately, not all commercial diets are sound, nor are good products always fed to suitable animals, so room remains for improvement in the feeding and care of reptiles.

Regulations For Commercial Diets
Commercial animal foods are regulated at two levels, federal and state. At the federal level is the U.S. Food and Drug Administration (FDA), Center for Veterinary Medicine (CVM), operating under the authority of the Federal Food, Drug and Cosmetic Act.

Many state governments also regulate pet foods (including labels), usually through state departments of agriculture. The Association of American Feed Control Officials (AAFCO) is an advisory body for the states, publishing a model feed bill, pet food regulations, and feed ingredient definitions, all of which a state may adopt as part of its own state feed laws and regulations. The FDA-CVM offers scientific and regulatory support to AAFCO as well as to individual states on many pet food issues.

Much of what is on a pet food label is there by law. Much of what is NOT there is by law, too. For example, disease prevention claims identify the intent to offer products as "drugs." In order to be regulated only as "foods," such claims are not allowed on labels.

The parts of a pet food label of most use to veterinarians in reptile and amphibian practice may be the guaranteed analysis, list of ingredients, and address for the manufacturer. Next in usefulness (but not on the label) is the phone number for the state Department of Agriculture, where complaints and questions about pet foods are recorded. Also useful is the realization that due to resource limitations, regulating foods for non-domestic pets is a lower priority than foods for livestock, dogs, and cats. Many state departments may have neither the time nor manpower to routinely investigate commercial diets marketed for reptiles and amphibians. Because of this, don't assume that what is on the label of a product is reliable, that the diet inside is sound, that quality control was in place during manufacture, or that commercial diets provide a safe and efficient alternative for captive reptiles. Throughout this presentation we'll use commercial diets as examples. This in no way is intended to promote or disparage specific products. Because manufacturers often change formulations, a product that is considered unacceptable today may be acceptable next month.

Commercial Diet Assessment

Guaranteed analyses
A pet food label must state guarantees for minimum percentages of crude protein and crude fat, and maximum percentages of crude fiber and moisture. The term "crude" refers to a specific method of analysis, not to quality. The percentages are presented on an as-fed basis. Occasionally other guarantees are provided voluntarily, such as taurine and magnesium in cat foods. Unfortunately, nutritional data presented by a manufacturer in, say, brochures, are not guarantees that come under regulatory scrutiny as does the guaranteed analysis on a label. Only the guaranteed analysis on the label is subject to testing by feed control officials to ensure it conforms to the food inside the package.

Discrepancies between the guaranteed analysis and laboratory analyses are not uncommon in pet foods, where testing is more routine. Testing by states is rare for reptile diets. However, in one study evaluating commercial diets fed to juvenile green iguanas (Iguana iguana), a product tested twice in an independent laboratory was found to contain only 55% of the protein reported on the label guaranteed analysis. Let's look at the labels for two commercial diets marketed for carnivorous reptiles:


Product A

Product B

Crude Protein, min %



Crude fat, min %



Crude fiber, max. %



Moisture, max %



 Product A's label does not title the list as a "Guaranteed Analysis," but the feed control officials would most likely view it as constituting guarantees. It is canned, hence the high moisture content, while Product B is a dry extruded floating pellet. For canned products, the maximum allowable moisture is 78% (except for gravies and the like). Note that when comparing a canned and dry product, the levels of crude protein and other nutrients are lower for the canned food, because the moisture is higher. To make meaningful comparisons, the numbers have to be compared on an energy basis, or, if fat content (caloric density) is similar, a dry matter basis.

Ingredient lists:
Ingredients are required to be listed in order of predominance by weight. The weights are determined as they are added in the formulation, including water. So a moist ingredient, such as peas (90% water) or chicken (70% water), may be listed ahead of a dry ingredient, such as soybean meal (10% water), yet the soy actually contributes more solids to the diet. Ingredients provide insight into the likely digestibility of a product. Animal source ingredients for the most part are more digestible than plant source ingredients. So when evaluating a product for feeding a carnivorous monitor, for example, look for mostly animal source ingredients, say four of the first six on the label. For example, we compare here the first six ingredients of two commercial canned diets, one marketed for monitors (Product A) and one for cats (Product C):

Product A

Product C

Soybean Meal

Meat by-products



Corn meal

Poultry by-products

Wheat mill run


Kelp meal

Turkey giblets

Bone meal


Note that Product A contains only two animal source ingredients whereas Product C contains five animal source ingredients in the first six. The reptile product lists water as the last ingredient, below even the vitamins, which is hard to interpret. A canned product needs much water for processing, and it is likely that water is the first or second ingredient, if considered by weight. Since moisture is actually 78% for the reptile product (noted in the guaranteed analysis), water has to be its first or second ingredient. Because of its multiple animal source ingredients, the feline product will likely be more digestible and more palatable.

Given a label guaranteed analysis and list of ingredients, one can make rudimentary calculations of a product's calorie content, and proportions of fuels. Continuing with the example, the two products marketed for carnivores have these guaranteed analyses on their labels:


Product A

Product C

Crude Protein, min %



Crude fat, min %



Crude fiber, max. %



Moisture, max %



To better compare these products, the nutrient contents are converted to a dry matter basis, then an energy basis. The numbers are first divided by the dry matter (DM, 0.22) to calculate nutrients on a DM basis. Ash is estimated to be about 8% DM, then carbohydrate is found by difference [100 - (% protein + % fat + % fiber + % ash)].

To convert to an energy basis, DM percentages are multiplied by fuel factors - 4.0 kcal/g for protein and carbohydrate, and 9 kcal/g for fat. We then have the following estimates:


Product A

Product C

Energy, kcal/g



Protein, % kcal



Fat, % kcal



Carbohydrate, % kcal



Most likely, the factors are over-estimated for Product A because of the large number of less digestible plant based ingredients. Fuel factors for Product A may approach about 3.5 kcal/g for carbohydrate and protein and 8.4 kcal/g for fat. Between the two products, the feline diet appears to be better suited for feeding a carnivore because of its higher calorie density, better balance between protein and fat, lower carbohydrate, and greater palatability and digestibility.

This comparison on paper by calculation is valuable, but not a full substitute for a feeding trial. In a feeding trial, we measure body weight and condition, analyze samples of blood and excreta, and compare growth in the juveniles and reproductive efficiency in mature adults.


Nutritional Adequacy
Dog and cat foods may contain a label claim of "complete and balanced" because there are unbiased independent standards upon which to judge the nutritional adequacy of the products. Commercial diets for reptiles and amphibians cannot possibly make such claims because there is no body of scientific data upon which an independent authority can judge adequacy. Our diet formulations for reptiles take into consideration the (limited) scientific literature, our clinical experiences, and general guidelines for carnivorous, omnivorous and herbivorous species for which there are sound nutritional recommendations.

The animal feed industry balances economic pressures and nutritional value. If reptile enthusiasts want commercial reptile diets comparable in quality and safety to major petfoods, then they need to request:

  1. diets that truly meet the reptile's needs, verified by long-term feeding trials for growth and reproduction

  2. quality control in every step of the production process

  3. accuracy in labeling and provision, upon request, of detailed nutritional analyses

Progress may come about when competition between brands permits owners to observe for themselves which diets work best in their animals. In addition, veterinarians encountering diet related diseases in practice should provide impetus for improvements by reporting their findings.

Feeding trials:
The best assurance of completeness and balance of a diet is a long term feeding trial throughout the reproductive cycle then raising the young on the same diet. Such a trial could establish a diet for all stages of the life cycle (as for the best of the products marketed for dogs and cats). The major manufacturers of dog and cat foods maintain (or contract with laboratories that maintain) extensive colonies of these animals for feeding trials. In addition, they often fund research that tests their products in clinical academic situations. Once marketed, their diets are fed to millions of dogs and cats. And lastly regulatory agencies are ready to field complaints and queries about products.

Contrast the situation with reptiles and amphibians. How many manufacturers maintain colonies of animals, publish results in reviewed journals, and fund extramural testing? How many times have you contacted a manufacturer for verification of nutrient content? How many complaints have reptile owners lodged with regulatory agencies?

Iguana feeding trial:
Uncertainties concerning commercial reptile diets prompted a feeding trial in juvenile green iguanas. Three commercial diets marketed for young iguanas were compared: a canned product, a gel, and a dry meal. A fourth diet was made from fresh romaine, dry clover and dandelion, soybean meal, dry egg, vitamins and minerals. Iguanas were fed each product for 6 weeks, then were accommodated for 2 weeks before another 6 week stint on another diet, and so on. Growth rates correlated with contents of protein and fiber. Iguanas fed the canned product (13% protein) lost weight, and those fed the gel product (14% protein) grew slightly, just 6% over 6 weeks. Iguanas fed the dry meal (22% protein) averaged 31% increase in body weight, while those fed the romaine-based diet (31% protein) increased their weight by 60%.


Choosing Diets
Even diets that appear to be complete and balanced may fail if inappropriate for a particular species. Just as cat food is too high in fat for, say, a rabbit or horse, so it is too high in fat for other hindgut fermenters such as green iguanas and tortoises. Likewise, fibrous foods such as alfalfa pellets, and plant based protein such as soybean meal, may have little place in the nutrition of strict carnivores such as snakes and many monitors. There are always exceptions to generalizations - just as the occasional dog gets by for years on an incomplete homemade diet, so a tortoise, for example, may appear to do well when fed canned dog food. These exceptions, however, should not be considered sound nutritional practice and most eventually fail.

Some of the trends prevalent in the USA animal feed industry today are likely to affect captive reptiles. For example, corn and soy are fed to many domestic species, carnivore and herbivore alike, because these ingredients are inexpensive sources of energy and protein in the USA. However, we are still learning how to feed diets rich in poorly hydrolyzed but rapidly fermented carbohydrates, such as oligogalactosides and resistant starches, to species that did not evolve on such diets.

Given this situation, how do we make choices among commercial pet foods? It is prudent to heed the species' nutritional heritage. For carnivorous reptiles, we select products with primarily animal based ingredients. Relatively high protein and fat are better accepted and improve digestibility and diet quality. Strict carnivores will likely have nutritional needs with a cat food, rather than dog food. Strict carnivores, like the cat, rely on meat based ingredients for such essential nutrients as taurine, arachidonic acid, niacin and vitamin A. We don't know if the strictly carnivorous reptiles share similar requirements, but there is no reason from what we know about their natural history and biochemistry, to suspect otherwise.

Similarly heeding evolution, when feeding commercial diets to herbivorous reptiles, we select products based on dried herbage (alfalfa and clover hays, for example, not meals and grain by-products). We try to supply long-stem fiber for hindgut fermenters. For example, alfalfa cubes provide longer fiber than pellets and can be crumbled and mixed into fresh produce. In our experience, clover and dandelion are as nutritious as alfalfa, and more palatable, but not as widely marketed.



Dzanis, D.A. 1994. Regulation of health claims for pet foods. Vet. Clin. Nutr. 1:5-11.

Dzanis, DA. 1994. The Association of American Feed Control Officials dog and cat food nutrient profiles: substantiation of nutritional adequacy of complete and balanced pet foods in the United States. J. Nutr. 124:2535S-2539S.

Donoghue, S. 1994. Growth of juvenile green iguanas (Iguana iguana) fed four diets. J. Nutr. 124:26265-2629S.


Susan Donoghue, V.M.D., M.S., D.A.C.V.N.
Nutrition Support Services, Inc.
Route 1. Box 186, Pembroke, VA 24136 USA

David A. Dzanis, D.V.M., Ph.D., D.A.C.V.N.
FDA - Center for Veterinary Medicine
7500 Standish Place, Rockville, MD 20855 USA

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