Question

Why is vitamin D important?

The best understood role for 1,25 (OH)2D (calcitriol) is in the control of how the body uses calcium and phosphorus to make strong bones.1 The body continually works to maintain serum calcium levels within tight margins.2 In a vitamin D insufficient state, the body becomes inefficient at absorbing intestinal calcium, affecting the balance of serum calcium levels. In an attempt to restore serum calcium levels, the body enhances tubular reabsorption of calcium in the kidneys and mobilisation of the calcium stores from the skeleton.2 Pre-osteoclasts stimulate the formation of multi-nucleated osteoclasts to dissolve the bone collagen matrix and release calcium stores into the circulation.2

The increased activity of osteoclasts and loss of calcium from the bone matrix results in destruction of the skeleton leading to rickets in children, and osteomalacia and osteoporosis in adults, increasing the risk of fractures in these patients.1,2

References 

1. DeLuca H. Overview of general physiologic features and functions of vitamin. D. Am J Clin Nutr, 2004; 80:1689--96.
2. Holick M. Vitamin D: a D-Lightful health perspective. Nutrition Reviews, 2008; 66(2):182--194.