Cod liver oil, the ratio of vitamins A and D, frequent respiratory tract infections, and vitamin D deficiency in young children in the United States.

We would like to take this opportunity to respond to the Commentary by Cannell and his distinguished colleagues' (herein referred to as the "Commentary"). We agree that extensive basic research and adult clinical research performed in the past few decades have demonstrated that the levels of vitamin D required for optimal health are much higher than those needed to prevent rickets in children or osteomalacia in adults; it is also clear that vitamin D deficiency and insufficiency, newly defined for optimal health, are widespread.2 However, as discussed below, we take a different view on several issues criticized by Cannell et al' in their Commentary.