Final answer:
Dietary calcium absorption is limited by compounds that form insoluble salts such as phytates, oxalate, and iron, as well as by high dietary magnesium and excess fiber. Vitamin D is essential for calcium absorption, with its deficiency leading to poor absorption. Proteins and certain organic acids can enhance calcium absorption, while the optimal calcium to phosphorus ratio is also vital.
Step-by-step explanation:
The absorption of calcium from our diet is limited by various factors, including the presence of compounds such as phytates (found in cereal grains), oxalate, phosphate, and iron, which form insoluble calcium salts. Additionally, a high dietary concentration of magnesium (Mg) can decrease calcium absorption. An excess fiber intake can interfere with this process as well, along with the fact that a higher dietary intake of calcium can itself lead to a decreased percentage of absorption. Moreover, adequate levels of Vitamin D are crucial for calcium absorption, promoting the synthesis of specific calcium-binding proteins needed for efficient transport across the small intestine. A lack of Vitamin D, an excess of phytates, a low Ca/P ratio in the diet, an increased pH of the upper intestine, and malabsorption syndromes can all adversely affect the amount of calcium that is absorbed.
A high intake of dietary iron can disrupt this balance, as ferric phosphate is highly insoluble. This upset in the Ca:P ratio can be significant because when the ratio is not optimal (ideally around 1:1), calcium absorption is decreased due to the formation of insoluble calcium phosphate. Additionally, urinary excretion of calcium, often regulated by the kidneys, and any reduction in intestinal absorption can influence overall calcium levels in the body.