Calcium in supplements is often found in combination with another substance, typically carbonate or citrate. The most common and least expensive type of supplemental calcium available is calcium carbonate. Calcium carbonate supplements tend to contain the highest amount of elemental calcium (about 40% by weight). Because calcium carbonate requires stomach acid for absorption, it's best taken with food. Most people tolerate calcium carbonate, but some people find they experience mild constipation or feel bloated. Calcium citrate supplements are absorbed more easily than calcium carbonate. Although the citrate form is a bit more expensive, it is more bioavailable, especially for older people who may have less stomach acid and digestive issues. They can take it on an empty stomach, as they are more readily absorbed by people who take acid-reducing medications for example. But because calcium citrate is only 21% calcium, you may need to take more tablets to get your daily requirement.