Vitamin D is essential for intestinal calcium absorption. Without adequate Vitamin D, the gut absorbs only 10 to 15 percent of dietary calcium; with adequate levels, absorption rises to 30 to 40 percent. This relationship means that even a diet sufficient in calcium will not maintain bone density if Vitamin D status is poor. The UK's northern latitude and grey climate mean that the UVB exposure required for skin synthesis of Vitamin D is not available for the majority of the year. Government data consistently shows that over 40 percent of UK adults have suboptimal Vitamin D status in winter and spring.
What Optimal Looks Like
Serum 25-hydroxyvitamin D is the measurement used to assess Vitamin D status. The conventional sufficiency threshold is above 50 nmol/L. Many bone health specialists argue that optimal bone mineralisation and calcium absorption require levels above 75 to 100 nmol/L. At levels below 25 nmol/L, secondary hyperparathyroidism occurs: the parathyroid glands respond to low calcium absorption by raising PTH, which maintains blood calcium by drawing it from bone. Chronic secondary hyperparathyroidism is a direct driver of accelerated bone loss.
Muscle Function and Fall Prevention
Vitamin D's importance for bone health extends beyond calcium absorption. Muscle fibres contain Vitamin D receptors, and adequate Vitamin D is required for normal muscle contraction efficiency and strength. Deficiency is associated with muscle weakness, particularly in the proximal lower limb muscles, and significantly increased fall risk. Since approximately 90 percent of osteoporotic hip fractures are caused by falls, Vitamin D's effect on muscle function is as important as its effect on bone mineralisation.
Sources
- DIPART Group. (2010). Patient level pooled analysis of 68,500 patients from seven major vitamin D fracture trials in US and Europe. BMJ, 340, b5463.
- Holick, M.F., et al. (2012). Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology and Metabolism, 96(7), 1911-1930.
- Pfeifer, M., et al. (2000). Effects of a short-term vitamin D and calcium supplementation on body sway and secondary hyperparathyroidism in elderly women. Journal of Bone and Mineral Research, 15(6), 1113-1118.