Final answer:
Dietary calcium absorption from nuts and whole grains is mainly inhibited by phytates that cause formation of insoluble calcium salts. Vitamin D improves calcium absorption, whereas phytates, found in cereal grains, reduce it. The correct option is 1.
Step-by-step explanation:
Dietary calcium is not well absorbed from nuts and whole grains primarily because of the presence of phytates and oxalates, which form insoluble calcium salts. While other nutrients like magnesium and fiber can also affect calcium absorption, the specific interaction that inhibits calcium absorption in these foods is due to phytates found in cereal grains.
Additionally, calcium absorption is influenced by Vitamin D, but in a positive way, as it enhances the absorption of calcium from the intestine. Contrastingly, a deficiency of Vitamin D can lead to diminished calcium absorption and depletion of calcium from bones. Phosphates and excess iron in the diet can also contribute to the decreased absorbability of calcium, but the primary factor in nuts and whole grains are the phytates.