Eggshells Are Stone Not Appropriate for Raw Diet
No Bones About It: Calcium From Eggshell?
Here on Pack Lunch we've talked about bones quite a bit. (Notably in the article Terminology: Raw Meaty Bone.) There's a reason: Bones are integral in a raw diet. This goes both for bones that are actually eaten and bones that are not. A lot of folks out there don't want to feed bones or feel they can't feed bones, though. Often the recommendation for these people is to use eggshell "for calcium". Eggshell is cheap and easy to source, and there's no argument that eggshell has calcium. In fact, eggshells (all "eggshell" referred to here are bird eggs — chicken eggs are, of course, the most familiar and common eggs) are almost all calcium — upwards of 95%! The rest is another form of calcium (calcium phosphate), magnesium carbonate (a form of magnesium that has low bioavailability), and some really minute amounts of other stuff. (cite 1) Following is a discussion on using eggshell in place of bone. Note that this does not apply to feeding whole eggs with the shells on them, or even using eggshell is a super-temporary substitute. This is about the idea that you can just replace bone with eggshell, and why you might not actually want to do that.
First and foremost to consider is why you're not going to feed bone. Bone is the best source of calcium for a carnivore of the type that dogs, cats, and ferrets are. It's the only thing you can feed without getting math and biochemistry involved in your meal planning. Feeding bones is also a really important part of food and eating for carnivores. Aside from just the nutrition factor, pulling the meat from bones and eating bone-in items is needed physically to keep teeth and gums toned and to regulate and stimulate digestion. Eating meals with bones also provides mental stimulation and exercise for other muscle groups. Working on a nice big raw meaty bone is isometric exercise for the whole body. The importance of bone can't be overstated. If you have a healthy companion carnivore you really should feed raw bones as part of their diet for many reasons. Even pets with limited abilities (e.g., damaged or missing teeth) really should be working on and eating meaty bones. Most people would be really surprised at what a dog with even no teeth can accomplish! That's a discussion for another day, though. My main point here is that if it's you as the meal provider that is not comfortable with raw bones, then there's no time like the present to overcome that discomfort and learn that bones aren't really optional.
That said, if you want to use eggshell then you must take into consideration the following: Eggshell is 95%-99% (depends on who you ask and presumably the bird and egg laid) of a type of calcium called calcium carbonate. (Calcium doesn't really exist in nature alone. It's always bound up in different compounds, of which there are many.) The calcium in bone is within a crystalline structure calledhydroxyapatite. Additionally, this substance in bone "only" makes up like 65% of the overall bone. Bone also contains organic material — namely lots of collagen — and even water! (cite 2) Trying to figure out how much actual calcium is released for use in the body and how efficiently from eggshell vs. from bone is a problem so complex I think it's safe to say no one's really ever worked just that one part out. I'm sure there have been feeding trials performed where calcium carbonate is part of a food formula that's being tested, but that's just one part of a big whole. There are too many factors to consider for there to be just a formula for subbing amounts of calcium carbonate for hydroxyapatite (or calcium phosphate or calcium lactate or calcium citrate or calcium gluconate). If there is some table that's been produced for this purpose I'd be very wary and wanting to know how the source came up with the calculation!
We could probably argue the benefit (or lack-of) from powdered eggshell calcium 'til the cows come home. Some say it's great, a few say it's not, though most sources agree that it's pretty easily assimilated by the canine (and human) body. Whether that high 'usability' is good or bad is the point, though. Here's where I really take issue with the whole eggshell thing: Just because powdered eggshell calcium is highly "usable" in the body doesn't make it a good idea. More isn't better. You can't one-up nature by eliminating edible bone and replacing it with a form of calcium that's has a higher bioavailabilty than a dog's natural time-tested source and call it a day. Even ifa person were to figure out exactly how much calcium carbonate to use in place of bone, there's the issue of what replaces all the other stuff in bone in the diet/lifestyle, nevermind achieving a good balance of all the co-factors that need to work together to put calcium to use in the body. Most dog people are familiar with the importance of the calcium : phosphorus ratio, but how many of those people are equally as diligent about magnesium and vitamin D balances? You can't use calcium right without those in the right amounts, too. I'll note here that nutrition is way more complex than matching up, say, the NRC or AAFCO established requirements for a nutrient with the same amount of said nutrient. Nutrient synergy is way too complex to reduce it all to a catch-all mathematical formula.
Proponents of eggshell calcium use/supplementation point out that the body will "get rid of" excess calcium so you don't really have to worry about it — just feed "enough". While it's very true that the body is very good at maintaining homeostasis including the "right" level of calcium in the cells and blood given what it has to work with (i.e., what's eaten), I think it's worth considering what pressures and imbalances on and in the body that dealing with excess creates. Kidney stones are only one associated problem with the intake of calcium carbonate in excess of calcium demands. Another issue is that calcium carbonate dissolves in acid, and weakens the strength of stomach acids that are crucial for proper digestion. (TUMS™ antacid tablets are calcium carbonate.) These things might not actually turn up as problems for a long time, but once you get on that roller coaster it's hard to get off.
In conclusion, since there's no "easy" answer, it's my thought that sticking with the tried-and-true methods established and perpetuated in nature (eating bones) is a preferable bet to mucking with such a questionable substitute (powdered eggshell).
Website citations (please forgive the unorthodox method of citing these references):
(1) http://antoine.frostburg.edu/chem/senese/101/consumer/faq/eggshell-composition.shtml
(2) http://www.petmd.com/dog/nutrition/evr_nutritional_aspects_of_bone_composition?page=2
*** Please note a lot of the info contained on this webpage actually come from secondary sources cited through that page
Here's a few webpage links that contain information that's relevant, though not directly (in no particular order):
https://chriskresser.com/calcium-supplements-why-you-should-think-twice/
http://saveourbones.com/beware-these-two-supplements-can-hurt-more-than-help-your-bones/
https://en.wikipedia.org/wiki/Elemental_calcium
https://en.wikipedia.org/wiki/Calcium_carbonate
https://en.wikipedia.org/wiki/Bone_mineral
https://www.exploratorium.edu/cooking/eggs/eggcomposition.html
http://www.health.harvard.edu/family-health-guide/calcium-curious
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Eggshells Are Stone Not Appropriate for Raw Diet
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