Calcium citrate, the type found in calcium-enriched orange juice, is less likely to cause stomach upset than other forms.
Calcium carbonate and calcium gluconate are the two forms that are most common in supplements; they both tend to be adequately absorbed. Calcium citrate is a good alternative, because it is very well absorbed and also enhances iron absorption, which may be inhibited by some other forms.
For all kinds of calcium supplements, it has been found that vitamin D, either in foods or supplements or manufactured by the body in the presence of sunlight, increases absorption of calcium.
The 1994 National Institutes of Health Consensus Development Conference statement on calcium says that gastrointestinal side effects of calcium supplements are usually observed at relatively high dosages.
Calcium ions stimulate gastrin secretion and gastric acid secretion, which can produce a rebound hyperacidity when calcium carbonate is used as an antacid.
These side effects should not be major problems with a modest increase in calcium intake, the statement said.
As with other nutrients, the conference emphasized: “The preferred approach to attaining optimal calcium intake is through dietary sources.”