The reason many prefer to stay away from spinach is because of the oxalate content in it, which indeed binds to calcium. However, as I understand it, the oxalate situation is one of basic chemical reaction and not one of catalytic action, so one oxalate molecule binds to a certain amount of calcium and that's it. So with calcium supplementation, you shouldn't have a problem in this regard (that and the oxalate content really isn't that high compared to some fruits and veggies). But there is a warning I would give, and that is too high of oxalate content in the diet for extended periods of time, particularly with calcium supplementation can lead to calculi formation - so things like kidney stones and intestinal impaction can result. Spinach is healthy in low quantities, I wouldn't say you should avoid it.
As far as the acidic fruits go, I think this is a notion that was created by hobbyists and sounds "correct" because acid = bad in most people's minds. However, when you think about it, what happens when an animal eats something? It goes to the stomach where acids far, far more concentrated and of lower pH are secreted to chemically break down the food. If you took any chemistry in high school and understood it, you know that weak acid added to a stronger acid doesn't make an even stronger acid, it makes the stronger acid weaker. So the story of "staying away from acidic fruits" doesn't make logical sense in this aspect. About the only way acidic fruit would be potentially of concern than I could make out is that constant consumption of it will leading to weakening the enamel of the teeth. I doubt anyone would be feeding their tegus this much acid. So if your tegu enjoys citrus or pineapple, I don't see any reason to withhold it from them.
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