An iguana’s teeth are designed to deal with tree leaves. These are tough fibrous plant materials, with sturdy veins and rugged cell walls made of cellulose. The teeth are designed to clip tree leaves from the stems, then quickly swallow them virtually whole to be stored in the capacious stomach. In order to minimize their exposure to predators, iguanas often hurriedly gulp down a large amount of food before retreating to a safe spot to bask. The stomach therefore performs very little actual digestion, and serves largely as a temporary storage pit.
Since much of the material in mature tree leaves is indigestible, iguanas have symbiotic bacteria living in their intestines, which break down the cellulose through fermentation and convert it to components that the iguana can use. These bacteria are found in folds inside the intestine, where they enter the food as it passes through. It can take as much as 24 hours for food to be broken down, and for the waste products to be passed out of the digestive tract.
Iguanas do not have gizzards, and do not use gravel or grit in their stomachs to help grind up food, like a bird does. All the work of digestion is done by the bacteria living in the hindgut, and any foreign substances in the digestive tract, like sand, dirt or gravel, can clog up the intestines and prevent food and wastes from being carried through the intestine, leading to a rapid death.
Phosphorus/Calcium Ratio
We have already seen that the interaction between calcium and phosphorus is an important part of the iguana’s biochemistry. This balance is partly regulated by exposure to ultraviolet wavelengths in sunlight. The iguana’s diet, however, also has a large impact on its blood chemistry, and it is important to consider this when formulating a proper diet for your lizard. Both calcium and phosphorus are extracted from the iguana’s food. In the lizard’s body, these are combined to form calcium phosphate, which is used to build bones and teeth. Ideally, the ratio should be two parts of calcium taken in for every part of phosphorus in the diet. If the diet contains too much calcium, the excess will be deposited in the iguana’s internal organs as calcite crystals, which can cause internal damage. If there is more phosphorus than calcium, the excess phosphorus will chemically combine with the calcium in the iguana’s bones, carrying it out of the body and weakening the bone (this is known as "metabolic bone disease", and it is a leading killer of captive iguanas).
Your iguana’s diet, then, must be carefully formulated to avoid an excess or deficiency of either calcium or phosphorus. Foods that are either too low in calcium or too high in phosphorus must be avoided. The process is made even more complex by the fact that some foods contains chemical components that can alter the calcium/phosphorus ratio. Some vegetables and fruits, for instance, contain a class of chemicals called "oxalates", which bond chemically to calcium atoms. This calcium-binding process makes large amounts of calcium unusable to the iguana, and alters the proper calcium/phosphorus ratio. Over the long term, this imbalance can kill your iguana.
Other foods present different potential problems. Some vegetables contain chemicals which bind to iodine, making it unusable by the iguana. These chemicals are known as "goitrogens". Iodine is vital for proper functioning of the thyroid gland, which monitors several metabolic functions. Iodine deficiency caused by ingesting these iodine-binders produces goiter, a serious disease that can be fatal. For this reason, vegetables or fruits that are goitrogenic must be excluded from the iguana’s diet.
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