http://www.ahrq.gov/downloads/pub/evidence/pdf/nutrition/nutrtp2.pdf

The workgroup concluded that the systematic review approach could be productively used to inform the development of reference values. Challenges identified in this exercise include prioritizing and defining research questions when the volume of literature is large, relying on intermediate (surrogate) outcomes when few or no studies directly linked nutrient intake with clinical outcomes are available, and determining reliable nutrient biomarkers. Ultimately, an objective, unbiased systematic review of a defined question could be useful; not only in helping to set nutrient reference values, but also for increasing the transparency of the decision making process.