Some studies have shown that calcium competes for absorption with a number of other minerals, while other studies have found no such competition. To be on the safe side, some doctors recommend that people taking calcium for long periods of time should also take a multimineral supplement.

One study has shown that taking calcium can interfere with the absorption of phosphorus, which, like calcium, is important for bone health.⁷. Although most western diets contain ample or even excessive amounts of phosphorus, older people who supplement with large amounts of calcium may be at risk of developing phosphorus deficiency. For this reason, the authors of this study recommend that, for elderly people, at least some of the supplemental calcium be taken in the form of tricalcium phosphate or some other phosphorus-containing preparation.

Vitamin D’s most important role is maintaining blood levels of calcium. Therefore, many doctors recommend that those supplementing with calcium also supplement with 400 IU of vitamin D per day.

Animal studies have shown that essential fatty acids (EFAs) increase calcium absorption from the gut, in part by enhancing the effects of vitamin D and reducing loss of calcium in the urine.⁸

Lysine supplementation increases the absorption of calcium and may reduce its excretion.⁹ As a result, some researchers believe that lysine may eventually be shown to have a role in the prevention and treatment of osteoporosis.¹⁰