Final answer:
When dietary calcium is low, the body increases calcium absorption through the secretion of parathyroid hormone, which activates vitamin D to enhance intestinal calcium uptake.
Step-by-step explanation:
The statement, 'When dietary intake of calcium is low, calcium absorption increases,' is generally considered true. This is because the body has mechanisms to maintain calcium homeostasis. When dietary calcium is insufficient, the parathyroid glands secrete parathyroid hormone (PTH), which stimulates the conversion of vitamin D into its active form.
Activated vitamin D enhances the absorption of calcium from the intestine, maintaining normal calcium concentration in the blood. PTH also signals the release of calcium from bone matrices and increases the reabsorption of calcium by the kidneys.
Additionally, high levels of dietary proteins increase the absorption of calcium, and several factors such as a deficiency in vitamin D, excess of dietary phytates, or an imbalance in the calcium to phosphorus ratio can adversely affect calcium absorption.