Calcium and Votamin D have synergistic relationship. They must be taken in conjunction to be efficie

Calcium should also be taken with magnesium in a 2:1 ratio to improve bone health

Calcium and Vitamin D work together to promote healthy bone density and a strong structure. Calcium 
cannot be efficiently absorbed without the presence of Vitamin D receptors to aid it. Calcium is an 
essential component of bone structure. Deficiency of Vitamin D can lead to rickets in children. Many 
people living in countries with inconsistent sunlight and working indoors are deficient in Vitamin D.
Sources of Calcium - milk, cream, cheese, butter, kale, nuts, flour, okra, collard, white beans, 
  soybeans, sardines, salmon, perch, calcium-fortified foods, cucumber, broccoli, boiled eggs, 
  bok choy, brazil nuts, berries, asparagus, almonds, cinnamon, olives, sesame, tofu, walnuts, 
  oranges, artichokes, kidney beans, and fennel etc.