http://www.who.int/bulletin/archives/80(2)106.pdf

A substantial body of literature indicates that nutritional status affects labor outcomes, particularly productivity. While the exact mechanisms underlying these relationships are not entirely clear, this literature is distinguished by the co-existence of carefully conducts experimental studies and observational studies that have documented sizeable effects of nutrition on productivity indicators. Relative to the links between nutrition and labor outcomes, these links have not been as well established in the scientific literature. In part, this is a reflection of limited data on non-nutrient physical health indicators, particularly in observational studies. In part, it is a reflection of the difficulties associated with using self-reported health status. Creative design of experimental studies that might shed new light on the meaning of different health indicators is long overdue