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Why is vitamin D needed for calcium absorption?

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USDA MyPlate dietary guidelines — human nutrition MyPlate, a revised set of dietary guidelines introduced by the U.S. Department of Agriculture (USDA) in 2011, divides the four basic food groups (fruits, grains, protein, and vegetables) into sections on a plate, the size of each section representing the relative dietary proportions of each food group. The small blue circle shown at the upper right illustrates the inclusion and recommended proportion of dairy products in the diet.

Vitamin D is essential for calcium absorption because it acts as a precursor to a potent hormone that regulates the body's ability to take up this mineral from the diet.12

The process functions as follows:
* Activation: Vitamin D is converted in the liver and kidneys into its most active form, known as calcitriol (1,25-dihydroxyvitamin D).13
* Intestinal Absorption: Calcitriol travels to the small intestine, where it increases the efficiency of dietary calcium absorption. It achieves this by influencing the permeability of the brush borders of the enterocytes (intestinal cells) to calcium.342
* Homeostasis: By enhancing this absorption, vitamin D helps maintain the necessary serum calcium concentrations required for vital physiological processes, such as neuromuscular function, blood coagulation, and bone mineralization.34

Without sufficient vitamin D, the body cannot efficiently absorb calcium, leading to low serum calcium levels. This triggers the release of parathyroid hormone, which may eventually result in poorly calcified bones, manifesting as rickets in children or osteomalacia in adults.13