My Greens and Cyclura were on a 100% Mazuri Tortoise food diet.......Alot of other Iguana keepers do the same........It eliminates the guesswork involved in a supermarket diet........
As a certified animal nutrition counselor, I'd rather go with the "guesswork."
Mazuri's iguana and tortoise diets have really horrid ingredients for an iguana to be eating, and increasingly we find in animal nutrition that processed diets can have profound, even if not immediately obvious, affects on their
long-termhealth. The context & interaction of nutrients is equally as important as what percentages they are in, or such is the direction nutritional science is moving in humans and animals.
Much as we are seeing increased incidence of diabetes and food allergies in companion animals fed a 100% processed diet (as opposed to in the past when animals like cats in particular would hunt for some of their own food), or changes in gut health in livestock fed processed feeds (as opposed to natural forage and graze), I suspect we'll see major issues over time in exotics fed processed diets. We already see some issues in some species; I know that there is at least anectdotal evidence of problems in amphibians, hermit crabs, and turtles fed a 100% or predominantly processed diet.
To give you an idea of what's found in Mazuri's foods for iguanas and tortoises, they are basically soy, corn, wheat, oat, and alfalfa. About the only thing on that list an iguana should potentially eat is alfalfa, and not even really since they are more eaters of leafy vegetation in the wild than grasses. IMO if you wouldn't feed an iguana soybeans, corn, wheat, and oats, why would you feed them a processed diet made up of those ingredients? Sure the %'s are where they belong, but what is that doing to your iggy's gut flora and overall health over time?
It's a valid point that produce isn't a completely natural diet, but I don't think it's a leap across a logical chasm to suggest that leafy greens from this area are closer to leafy greens from south america than corn, soy, wheat, and oats will ever be. Edible leafy greens everywhere have pretty much the same basic properties in terms of their effect on the GI and basic nutrient content, with some variability obviously based on species, soil quality and so fourth.
The intensive nutrition needed for a malnourished animal is different than the basic nutrition needed for a healthy animal. I don't doubt that Mazuri gets a sickly ig back on its feet quickly, but I don't think that means it is the best source of life-long nutrition for a healthy animal. We give nutrient-dense slurries overloaded with vitamins and minerals to sick dogs, bulk-building feed to emaciated horses, and electrolyte solutions to dehydrated baby rabbits. It doesn't mean a healthy dog, horse, and rabbit needs those things, nor that such things would even be good for them as their primary/sole source of nutrition long-term.
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On the matter of spinach: to be fair, feeding it once in a blue moon won't kill your ig or make it deficient. It does have some good properties as a green, and some wild greens an iguana would eat also have fairly high oxalete. The key is providing as great a variety of greens as possible as the staple. This not only pretty much assures an appropriate balance of nutrients, but also prevents problems that may stem from certain properties of a given variety, oxaletes being a perfect example. Any veg and fruit offered should also be fed in variety. This paired with supplementation and UVB should result in a healthy animal.