Bao SF, Windisch W. Kirchgessner M.: Calcium bioavailability of different organic dietary sources (citrate, lactate, acetate, oyster-shell, egg shell, emu egg shell, calcium phosphate), in Journal of Animal Physiology and Animal Nutrition, 78 (1997), 154 -160.

Abstract:
In a 2-week metabolic study with CA-Labeled growing rats, each of six groups of 10 animals were fed restrictively a semi-synthetic purified diet supplemented with calcium citrate, calcium lactate, calcium acetate, oyster-shell meal, egg shell meal, emu egg shell powder, or calcium phosphate. The total dietary contents of Ca, P and Mg were uniformly 2.8, 5.3, and 0.65g/kg. True absorption, intermediate utilization and bioavailability was uniform for Ca citrate, lactate and acetate (average 98.6, 97.9 and 96.5%). Oyster shell revealed only slightly lower values (97.7 97.6 and 95.2%) Egg shell and calcium phosphate were lower in bioavailability (93.8 and 92.0) because of a reduced true absorption (96.3 and 94.9%) and in the case of calcium phosphate also due to a slightly reduced intermediate utilization. (97.0%). Emu eggshell tests were done by an alternate source, (Regal Labs, Delta BC. 1998), to compare to information already published. No abstract is available, although the comparison looks very good.

In total, all tested calcium sources revealed an extremely high absorbability and utilizability. Thus, the chemical formulation of dietary calcium does not seem to be the primary factor of CA bioavailability in practical diets.