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 subjects. 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 the achlorhydric 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.