Low calcium aggravates the consequences of vitamin D deficiency, this suggest an interaction between vitamin D and calcium intake.
The active interplay between calcium and vitamin D is needed to prevent skeletal disease. Clinical rickets or osteomalacia occurs more easily when vitamin D deficiency is coupled to low calcium intake or low calcium absorption.
Calcium is actively absorbed from the small intestine in the presence of vitamin D. Calcium and phosphorus form hydroxyapatite crystals to mineralize and strengthen bones. Thus, a diet containing both optimal vitamin D and calcium is important for proper mineralization of bone.
(Source of Information: tandfonline.com)