The citrate form of calcium or magnesium is easier to absorb than other forms. It is estimated that 40% of post-menopausal women have significant stomach acid deficiencies and most of them do not know it. Stomach acid is a factor in optimal calcium absorption and the citrate form of calcium is able to compensate for diminished stomach acid. Generally speaking, people with a stomach acid deficiency absorb approximately 45% of the citrate form of calcium, compared to approximately 4% absorption of calcium carbonate. Adding vitamin D enhances the absorption process even more.