Thus, spinach Ca is much less readily available than milk Ca.

The presence of lactose (milk sugar), lactase (the enzyme that breaks down lactose), and the acidic amino acids, lysine and arginine, are all factors that improve calcium absorption.

Decreased absorption is associated with diets high in fiber. Foods such as whole grains and spinach are high in phytates and oxalates, compounds known to bind with calcium and reduce absorption.

Vitamin D intake is a second factor, as active calcium transport is directly and proportionally dependent on the presence in the intestinal cell of calbindin D_9k, the biosynthesis of which is totally vitamin D dependent. 

Absorption in jejunum and ileum is the major absorptive process when calcium intake is adequate. No more than 10% of total calcium absorption takes place in the large intestine.