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Why does calcium deficiency weaken bones?

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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.

Calcium deficiency weakens bones primarily because the body prioritizes maintaining specific levels of calcium in the blood and soft tissues to support vital functions, such as muscle contraction, nerve impulse transmission, and blood clotting.123

Because approximately 99 percent of the body's calcium is stored in the bones, the skeleton acts as a reservoir. When dietary intake is insufficient to meet the body's metabolic needs, hormonal mechanisms—specifically involving parathyroid hormone and vitamin D—stimulate osteoclasts to resorb, or break down, bone tissue. This process releases stored calcium into the bloodstream to maintain homeostasis.1234

Since the mechanical strength and rigidity of bone are directly proportional to its mineral content, the continuous removal of calcium to support other physiological processes leads to a reduction in bone density. Over time, this thinning of the bone matrix can result in conditions such as osteoporosis, making the skeleton more porous, fragile, and susceptible to fractures from minor stresses.2516