Vitamin D Insufficiency
Vitamin D insufficiency is a serum 25 hydroxyvitamin D (25(OH)D) concentration that is below the level needed for optimal bone and muscle health but above the level that causes overt deficiency. It is very common worldwide, especially at higher latitudes, in winter, and in people with dark skin, covered skin, obesity, malabsorption or little outdoor time.
- Categories used in practice: deficiency below 25 nmol/L (10 ng/mL); insufficiency 25 to 50 nmol/L (10 to 20 ng/mL); sufficiency 50 nmol/L (20 ng/mL) or more; levels above 125 nmol/L (50 ng/mL) are unnecessary and above 250 nmol/L (100 ng/mL) risk toxicity. Some clinicians use 75 nmol/L (30 ng/mL) as a target in osteoporosis, malabsorption and chronic kidney disease.
- Sources: sunlight (skin synthesis by ultraviolet B, from about April to September at latitudes above 35 degrees), oily fish, eggs, fortified foods and supplements. Skin synthesis falls with sunscreen, covering, melanin and age.
- Causes: low sun exposure, dark skin, age over 65, obesity (sequestration in fat), malabsorption (coeliac disease, inflammatory bowel disease, pancreatic insufficiency, bariatric surgery), chronic kidney and liver disease, nephrotic syndrome, and drugs that increase vitamin D metabolism (phenytoin, carbamazepine, phenobarbital, rifampicin, glucocorticoids, antiretrovirals).
- Consequences: secondary hyperparathyroidism, bone loss, osteomalacia in deficiency, rickets in children, falls and muscle weakness; evidence of benefit from supplementation is strongest for those with deficiency, bone disease or at high risk.
