Why is vitamin D deficiency common in CKD and what is its effect on bone health?

• A. Vitamin D not affected in CKD
• B. Vitamin D deficiency due to excess sun exposure
• C. Vitamin D activation increases calcium absorption
• D. Kidney cannot activate vitamin D; leads to decreased calcium absorption and secondary hyperparathyroidism

Answer: (D)

In CKD the kidneys lose the ability to activate vitamin D because the enzyme that makes the active form (calcitriol) declines as nephron mass is lost. Without enough calcitriol, the intestine absorbs less calcium, and serum calcium falls. In response, the parathyroid glands secrete more parathyroid hormone, causing secondary hyperparathyroidism. The elevated PTH pulls calcium from bone to normalize blood levels, leading to increased bone turnover and mineralization problems (renal osteodystrophy) with bone pain and higher fracture risk. This is why vitamin D deficiency is common in CKD and why bone health suffers: the activation step is impaired, so calcium absorption drops and bone is mobilized to compensate. The other statements don’t fit CKD physiology—activation of vitamin D normally increases calcium absorption, which CKD cannot achieve, and excess sun exposure isn’t the cause of this deficiency in CKD.