The folly of an RDA for vitamin D 9. January 2011 William Davis (26) Tom is a 50-year old, 198-lb white male. At the start, his 25-hydroxy vitamin D level was 28.8 ng/ml in July. Tom supplements vitamin D, 2000 units per day, in gelcap form. Six months later in January (winter), Tom's 25-hydroxy vitamin D level: 67.4 ng/ml. Jerry is another 50-year old white male with similar build and weight. Jerry's starting summer 25-hydroxy vitamin D level: 26.4 ng/ml. Jerry takes 12,000 units vitamin D per day, also in gelcap form. In winter, six months later, Jerry's 25-hydroxy vitamin D level: 63.2 ng/ml. Two men, similar builds, similar body weight, both Caucasian, similar starting levels of 25-hydroxy vitamin D. Yet they have markedly different needs for vitamin D dose to achieve a similar level of 25-hydroxy vitamin D. Why?It's unlikely to be due to variation in vitamin D supplement preparations, since I monitor vitamin D levels at least every 6 months and, even with changes in preparations, dose needs remain fairly constant. The differences in this situation are likely genetically-determined. To my knowledge, however, the precise means by which genetic variation accounts for it has not been worked out. This highlights the folly of specifying a one-size-fits-all Recommended Daily Allowance (RDA) for vitamin D. The variation in need can be incredible. While needs are partly determined by body size and proportion body fat (the bigger you are, the more you need), I've also seen 105 lb women require 14,000 units and 320-lb men require 1000 units to achieve the same level of 25-hydroxy vitamin D. An RDA for everyone? Ridiculous. Vitamin D is an individual issue that must be addressed on a person-by-person basis.