To meet dietary requirements from food alone, you would have to eat fifteen eggs or six pounds (almost three kilos) of Swiss cheese every day, or, more plausibly if not more palatably, swallow half a tablespoon of cod liver oil. In America, milk is helpfully supplemented with vitamin D, but that still provides only a third of daily adult requirements. In consequence, some 50 per cent of people globally are estimated to be vitamin D deficient for at least part of the year.16 In northern climes, it may be as much as 90 per cent.
