“You can take your egg shells, clean them, roast them and dry them in the oven for about 20 minutes on 350. And you're left with what? Calcium carbonate. Crush them into small little fine pieces, and you give your cat about 1/4 teaspoon for 10 pounds of body weight daily. The additional Calcium, it binds additional phosphorus in the diet.…”

Calcium carbonate is a genuine, mainstream intestinal phosphate binder in cats and dogs. The 2016 ISFM guidelines list calcium carbonate in their table of common oral phosphate binders at an initial total daily dose of 90 mg/kg, with hypercalcaemia named as the possible adverse effect. Kidder and Chew give 30 mg/kg every 8 hours or 45 mg/kg every 12 hours, given with a meal. A 2025 Feline VMA clinical handout gives 60 to 90 mg/kg/day and states the binder should be titrated to effect while avoiding signs of toxicity. A veterinary product combining chitosan and calcium carbonate (Epakitin) is labelled at 1 g per 5 kg twice daily, which delivers 40 mg/kg/day of calcium carbonate. The guidelines place binders second, not first. The 2026 iCatCare guidelines state that feeding a phosphate-restricted renal diet is the initial treatment of choice to keep serum phosphate within the IRIS stage-specific target range, and that a binder can be considered only if the renal diet has not brought serum phosphate into that range after 4 to 6 weeks. The same guidelines state that binders should be thoroughly mixed into all meals fed, with the total daily dose divided between the number of meals, because efficacy requires the phosphate and the binding compound to be present at the same time. On calcium-containing binders specifically, the 2026 iCatCare guidelines state that calcium carbonate is effective and relatively cheap but should be avoided in hypercalcaemic cats, and list non-calcium alternatives. The 2016 ISFM guidelines state that if calcium-containing binders are used, monitoring of serum calcium, ideally ionised, is recommended because hypercalcaemia is occasionally seen as an adverse event. The 2026 guidelines also report that approximately half of all cats with azotaemic CKD show increasing calcium concentrations over time after transition onto a phosphate-restricted renal diet, and that the consequence of ionised hypercalcaemia in cats with CKD is an increased risk of nephrocalcinosis, which appears to be associated with more progressive CKD. The IRIS 2023 dog recommendations likewise specify serial monitoring of serum calcium alongside phosphate binder treatment. A Today's Veterinary Practice IRIS CKD review advises avoiding calcium-containing enteric phosphate binders in hypercalcaemic patients or monitoring closely. On the outcome claim, Elliott et al. 2000 followed 50 cats with naturally occurring chronic renal failure. Cats that accepted a veterinary diet restricted in phosphorus and protein had a median survival of 633 days versus 264 days for those that did not. The intervention tested was the therapeutic diet, with binders added in a subset where phosphate remained high, not eggshell powder and not a binder alone. The WSAVA 2016 proceedings state that hyperphosphataemia contributes to renal secondary hyperparathyroidism, tissue mineralisation and progression of CKD. On eggshell powder specifically, no feeding study, clinical trial or dose-finding study of eggshell powder as a phosphate binder in cats or dogs was located. The only retrieved record of its use is Caney's 2017 owner survey, in which eggshell powder was the binder chosen by 1 owner, 0.31% of those who proceeded with a binder. On composition and amount, eggshell is roughly 94 to 95% calcium carbonate, around 380 mg of elemental calcium per gram. Homemade-diet references commonly state that one large eggshell yields about one level teaspoon weighing 5.5 g and providing roughly 2,000 mg of calcium, so 1/4 teaspoon is about 500 mg calcium, equivalent to about 1,250 mg calcium carbonate. Other references put a teaspoon at about 2.5 g, giving about 225 mg calcium or 560 mg calcium carbonate per 1/4 teaspoon. For a 10 pound (4.5 kg) animal, the claimed dose therefore delivers roughly 125 to 275 mg/kg/day of calcium carbonate.