Description Calcium Citrate and Vitamin D3 are very popular among people who are looking for a calcium supplement that is easier to digest for enhanced absorption. Typical calcium supplements contain a form of calcium known as calcium carbonate that can be difficult for some people to digest. Calcium citrate is a form of calcium that generally requires less stomach acid to breakdown, which is why it is preferred by people who have digestive issues, gentle stomachs, or who experience constipation with standard calcium carbonate. Vitamin D3 is a critical nutrient for the metabolism and absorption of calcium in the body. Vitamin D3 may also have links to energy and a healthy immune system. Calcium is well-known for its key role in bone health. Calcium may also help maintain heart rhythm, muscle function, and more.