Optimal vitamin D level different for various individuals

One of thee frustrating things is that RDI's and my book about optimal vitamin intake all focus on population statistics or averages. That is what is the level that is best for most people, not what is best for you. Technology is moving in this direction to allow people to optimize their own vitamin levels. May this happen with haste....

A just out paper looked at supplementing 800IU of vitamin D(3) in the Southern USA. Initially over 85% had sub optimal levels of vitamin D (hence why people need to supplement). However after 6 months of supplementing every day with 800IU, only half of the people had optimal levels of vitamin D.
This dramatically shows how some people need more vitamin's than others.

To put these numbers into perspective I recommend in my book that people obtain 400IU of vitamin D a day. So they are supplementing at twice the level I recommend, yet half of the population was still not obtaining optimal status with 800IU a day........

Reference: Marjorie L McCullough, ScD, RD, Roberd M Bostick, MD, MPH, Carrie R Daniel, PhD, MPH, W. Dana Flanders, MD, MPH, Aasma Shaukat, MD, MPH, Jill Davison, MPH, Udaya Rangaswamy, BS and Bruce W Hollis, MD Vitamin D Status and Impact of Vitamin D3 and/or Calcium Supplementation in a Randomized Pilot Study in the Southeastern United States. Journal of the American College of Nutrition, Vol. 28, No. 6, 678-686 (2009)