ProteinA building block of the body composed of amino acids, protein, is perhaps the most vital nutrient after water. Amino acids are used to create new proteins to create everything from muscle, enzymes, and organs.
Protein is of special concern to reptiles that eat vegetables and fruit as a large portion of their diet. Well it is easy to achieve the right amino acid balance with a carnivorous or insectivorous reptile, more care has to be taken for something such as an Iguana or Uromastyx. Learn what these considerations are to have a healthy and happy animal! |
Amino Acids |
Although there are hundreds of types of amino acids found in food, there are only a few essential amino acids. That is, only a few are required in the diet. This list varies from species to species, but is generally about 10 amino acids that are vital to various reptiles and amphibians. These include arginine, histidine, isoleucine, leucine, tryptophan, lysine, methionine, phenylalanine, threonine and valine. In carnivorous and insectivorous species taurine and tyrosine may also be required in the diet.
The main reason amino acid requirements vary is that carnivores/insectivores may lose the ability to convert certain amino acids. If we use cats as an example, they have lost the ability to make taurine and tyrosine from other amino acids, so they require these in their diet. An additional reason for variation in these requirements has to do with microbial synthesis of the amino acids. Reptiles that have the ability for fermentation, generally all herbivores and some omnivores, can have microbes synthesize some of the amino acids for them. However, research has not been conducted to determine which amino acids they can synthesize. Generally the rabbit is used as a model animal to determine fermentation properties in reptiles, as the rabbit and herbivorous reptiles have a very similar digestive tract. |
Microbial Protein Absorption |
Most of the amino acids created by microbial fermentation in the hind gut are not absorbed unless the animal practices coprophagy (eating its own feces). This is common in young herbivorous lizards and should not be discouraged. Newly hatched iguanas have a strong instinct to seek out adult iguana fecal material in order to gain rare amino acids and seed their own gut micro biome.
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Selecting for Protein |
Wild herbivorous and omnivorous reptiles seek out sources of protein to fulfill their requirements for growth. They select younger plant leaves (which are higher in protein than mature growth) and plants species that have high protein contents. Omnivores will also rely more heavily on the insect based portion of their diet and begin to choose plants more often as they age.
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Excess Amino Acids and Gout |
If the body's amino acid requirement is met, the excess amino acids do not get used in protein formation. The liver catabolizes the amino acid to be used as energy. The amino group is separated from the carbon base. The carbon base is the structure that can be used as energy, while the amino group must be disposed of. In reptiles, the liver releases this as uric acid into the blood. The kidneys then filter uric acid from the blood, creating a chalky white waste product often seen in the cage with feces.
It is often thought that excess protein causes gout in reptiles, but there is simply not evidence to support this. Many species experience gout, from carnivorous alligators, insectivorous leopard geckos and herbivorous iguanas. An alligator is well adapted to using protein as an energy source, and thus creating uric acid. It is unlikely that "high" dietary protein is even a physiological possibility in alligators. Gout is the deposits of urate crystals on the surfaces of the joints and/or organs. It would stand to reason that excess protein causes excess uric acid in the blood, which causes excess urate to be deposited. This is partially true in that studies have shown in the green iguana that increasing or decreasing the protein content of a diet will increase or decrease serum uric acid levels, respectively. However, the study showed that the levels were all still within a low-normal range. The study also did not link the dietary changes to any occurrence of gout in the iguanas. As of this writing there are no studies that link increased protein intake to gout in reptiles. It is also known that an increase in potassium increases the amount of uric acid to be excreted. Sodium or potassium is required in reptiles in order to excrete uric acid. Any number of factors could be the cause of gout, chronic dehydration, low potassium intake, poor amino acid balance, low basking temperature and renal failure. Renal failure is the most simple cause of gout, but it is important to remember that dietary issues such as long term dehydration, high phosphorus and low potassium can cause renal failure. |
Feeding Protein |
Generally speaking, protein should not be a concern when feeding a mature animal. However, protein is the limiting factor in herbivorous reptile growth. It is best to feed young herbivorous reptiles as they would choose in the wild for protein, that is protein rich greens. These include mustard greens, kale and fresh alfalfa. Spinach should be avoided for its high oxalate content. Other good sources of plant protein are seeds and beans, but these can often be very high in phosphorus.
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Sources |
Allen, Mary E. and Olav T. Oftedale. Nutrition in Captivity. Biology, Husbandry and Medicine of the Green Iguana. 4:47-74 (2003).
Cheeke, Peter R. and Ellen D. Dierenfeld. Comparative Animal Nutrition and Metabolism. (2010). Donoghue, Susan and Julie Langenberg. Nutrition. Reptile Medicine And Surgery 1st Ed. 14:148-174 (1996) Nijboer, Joeke. Ed. Nutrition in Reptiles. Merck Veterinary Manual. 2015 |