A VRG member has osteoporosis and asked about the bioavailability of different vegan calcium sources such as fortified soymilk, vegan supplements, etc.

VRG Nutrition Advisor Reed Mangels, PhD, RD responds:

There’s not a lot of difference in terms of the percentage of calcium absorbed from different forms of calcium.

Here’s some information from FDA’s Office of Dietary Supplements: The two main forms of calcium dietary supplements are carbonate and citrate. Calcium carbonate is inexpensive, but is absorbed best when taken with food. Some over-the-counter antacid products, such as Tums® and Rolaids®, contain calcium carbonate. Each pill or chew provides 200-400 mg of calcium. Calcium citrate, a more expensive form of the supplement, is absorbed well on an empty or a full stomach. In addition, people with low levels of stomach acid (a condition more common in people older than 50) absorb calcium citrate more easily than calcium carbonate.

These two forms of calcium (calcium citrate and calcium carbonate) are well absorbed as is calcium phosphate.

Calcium absorption is best when a person consumes no more than 500 mg at one time. So a person who takes 1,000 mg/day of calcium from supplements, for example, should split the dose rather than take it all at once.

Calcium carbonate is the fortificant used in most soymilk sold in the U.S. As noted above, it is well absorbed. A variety of calcium salts are used to fortify orange juice. There has been little research on their relative bioavailability. One study found that calcium citrate malate was quite bioavailable.