Iguanas are obligate low-nutrient eaters. Protein, chronic dehydration, and too much vit's and minerals can kill an iguana before it's fifth year. It also needs a well-check of an x-ray, fecal analysis, and blood test at least once per year (fairly cheap). A captive iguana is capable of living into it's twenties.
A well-check x-ray of one of my iguanas revealed a dime stuck to the side of it's stomach, causing a hiddden bout of zinc poisoning.