The enthusiastic editorial by Weiss and Litonjua (Thorax 2007;62:745–6) may have left some readers with the impression that a single cause of the rise in asthma, multiple sclerosis, inflammatory bowel disease and type 1 diabetes in the West has now been discovered, namely vitamin D deficiency, and that primary prevention of asthma and autoimmune disease is just around the corner. Indeed, their bold conclusion portrays vitamin D supplementation as a likely panacea for many ills worldwide.

Weiss and Litonjua argue that …