Hypophosphatemia is responsible for skeletal muscle weakness of vitamin D deficiency.

Arch Biochem Biophys. 2010 May 29. Schubert L, Deluca HF.
Department of Biochemistry, University of Wisconsin-Madison, Madison, WI, United States.

A deficiency of vitamin D results in muscle weakness as well as rickets in children and osteomalacia in the adult. To study the basis for this weakness, severe vitamin D deficiency was produced in rats as revealed by a low level or absence of 25-hydroxyvitamin D(3) in the serum. Vitamin D deficiency was achieved by feeding purified diets to weanlings for 16 weeks. Muscle force, peak contraction (P), time-to-half contraction (T(1/2)), time-to-peak contraction (T(P)), and time-to-half recovery (T(1/2r)) were measured.

A significant reduction in muscle force was found when vitamin D deficiency was accompanied by hypophosphatemia.

Within four days of correcting the hypophosphatemia, muscle strength was normalized. When serum calcium and serum phosphorus were maintained in the normal range in vitamin D-deficient rats, muscle weakness did not develop.

Further, hypocalcemia together with vitamin D deficiency did not produce muscle weakness. These results strongly suggest that muscle weakness noted in rachitic patients is the result of the hypophosphatemia of vitamin D deficiency. Copyright © 2010. Published by Elsevier Inc. PMID: 20515645