Achlorhydria: A lack of hydrochloric acid in the digestive juices in the stomach (in technical terms, so that the pH of the stomach contents fails to fall below 4.0 under maximal stimulation). Hydrochloric acid helps digest food.
This is a study published by RR Recker in the New England Journal of Medicine comparing carbonate and citrate absorption. Volume 313:70-73, July 11th, 1985, Number 2
Defective absorption of calcium has been thought to exist in patients with achlorhydria. I compared absorption of calcium in its carbonate form with that in a pH-adjusted citrate form in a group of 11 fasting patients with achlorhydria and in 9 fasting normal patients. Fractional calcium absorption was measured by a modified double-isotope procedure with 0.25 g of calcium used as the carrier. Mean calcium absorption (+/- S.D) in the patients with achlorhydria was 0.452 +/- 0.125 for citrate and 0.042 +/- 0.021 for carbonate (P less than 0.0001). Fractional calcium absorption in the normal subjects was 0.243 +/- 0.049 for citrate and 0.225 +/- 0.108 for carbonate (not significant). Absorption of calcium from carbonate in patients with achlorhydria was significantly lower than in the normal subjects and was lower than absorption from citrate in either group; absorption from citrate in those with achlorhydria was significantly higher than in the normal subjects, as well as higher than absorption from carbonate in either group. Administration of calcium carbonate as part of a normal breakfast resulted in completely normal absorption in achlorhydria subjects. These results indicate that calcium absorption from carbonate is impaired in achlorhydria under fasting conditions. Since achlorhydria is common in older persons, calcium carbonate may not be the ideal dietary supplement.
What this means in plain English
There was a study containing people with normal stomach acid and those who have minimal stomach acid. They were given calcium citrate and carbonate that was radiologically changed into an isotope to track absorption. Folks with the lower stomach acid did not absorb the carbonate as well as the citrate. Folks with normal stomach acid absorbed the citrate better than the carbonate. Carbonate did absorb normally in pH neutral tummies when taken with a full meal. Citrate absorbs better in normal stomachs, as well as stomachs that have limited stomach acid. If one chooses to take carbonate, they should do so with a full meal.