Oxalates will block the calcium in any food with which they are eaten. For example, if you drink tea with milk, the oxalates in the tea will block most of the calcium in the milk.
Yes, I read what the nutritionist at the Mayo clinic said about milk and tea, and I can't speak to that (but I don't feed my squirrels either milk or tea), but I researched and asked specifially about the oxalates in vegetables blocking calcium absorption, and from what I could find (and was told), it blocks the absoption of the calcium in the vegetable itself, not the calcium in any other food. That's why spinach, for example, is high in calcium but only 5% (I think) of it is absorbed.
I quote: "although spinach has a lot of calcium, it also contains a substance -- oxalic acid -- that binds up its calcium and prevents absorption of all but about 5 percent of it. However, the oxalic acid in spinach and foods like rhubarb does not interfere with absorption of calcium from other foods eaten at the same time."
I looked on several databases to find concrete evidence to the contrary and consulted a Phd with 35 years of researching and teaching nutrition, but if you have evidence to the contrary, I'll present it to her.
If one were attempting to feed a mix of high-calcium/low-phosphorus vegetables, adding spinach to the bowl would certainly work against that.
I don't understand this. Spinach has a 2.0 to 1 Calcium to Phosphorus ratio (99 mg Ca to 49 mg Phos per 100 gms). And if the oxalic acid affects only the calcium in the spinach, then for every 100 grams of spinach, the squirrels would still be getting 5 mg of calcium to the good.
I suppose one could advise folks to feed spinach as a separate meal, but that starts to get complicated. Plus, the concern about the formation of crystals or stones in the kidneys and bladder remains.
Yes, too much oxalate can cause kidney stones and crystals in the urine, but so can too much calcium. From what I've read, it's often the result of a Ph imbalance. Oxalic acid is an acid (obviously) and calcium is a base.
As for vitamin A, first of all, vitamin overdoses in humans other than through oversupplementation are rare. In small animals however, there is more danger: they eat 10 times more food relative to their own body weight than a human. Picture eating 15 pounds of healthy foods per day and you might start to get those vitamin levels up there.
Yes, but if you go on that logic, they're eating 10 times more of EVERYTHING--all vitamins, minerals, amino acids, fats (the idea that they consume their body weight a week is another question I want to research one day. I've read that it requires a pound of "mast" a week for them, but I haven't found evidence yet what percentage of that mast they actually eat). Obviously they are different than we are--their metabolisms and requirements. You could make that same argument for ANY nutritional element. The key word you used was "oversupplementation." I don't consider providing squirrels with a variety of veggies--some high in Vitamin A, some not--"supplementation." ADDING vitamins to a natural diet is supplementation--overdone or not.
However, my main concern with frank vitamin A toxicosis is, of course, with fortified foods and vitamin supplements designed for birds since they contain large amounts of vitamin A, and it's almost never the nontoxic, natural form (beta-carotene), but rather the much more toxic (and cheaper) retinoids.
I think any doctor or nutritionist would agree that the BEST way to get the necessary vitamins and minerals one needs is through natural foods. From what I've read, Vitamin A toxicosis has occurred primarily through gross oversupplementation. All blocks, including HHBs (and your blocks aren't the issue!), are supplemented with processed vitamins. Quite frankly, since so many foods have Vitamin A in them, and to use your argument, squirrels eat 10 times the food we do per body weight, I wonder why you supplement it (Vit A) at all if one feeds his or her squirrel a variety of vegetables (including the green, yellow, and orange) a week.
Although HHBs are made with mostly beta-carotene, other rodent blocks contain retinoids--often at very high doses. So the idea behind limiting high vitamin A veggies is simply to keep all the nutrients in balance, understanding that different brands of rodent block may be fed. And in any case, we certainly don't want our squirrels turning orange!
Ha! No orange squirrels for me! (But I do have three in my yard with orange tails--think there's a connection?). But again, the point was that they won't get Vitamin A toxicosis from the veggies. I'm sure you put the best quality/healthiest supplements in your HHBs, but again, why supplement something artificially that doesn't need supplementing if the squirrels can get it naturally?
That being said, the Healthy Diet is super-conservative, admittedly. And I'm sure some of the "rules" there could be broken without harm.
It's not the "rules" to be "broken" that's the issue, IMHO, but the possibility that the "rules" were made on the basis of inaccurate information"