Compare Active Ingredients to Citracal® +D Maximum? Calcium Supplement Highly Absorbable to Support Bone Health Laboratory Tested - Guaranteed Quality Gluten Free Calcium Citrate is a highly absorbable form of Calcium, an important mineral for supporting bone health. Calcium also works to support muscle and nerve function. Vitamin D3 is added to assist with Calcium absorption and as support for healthy immune function.21st Century