Giving blood donors something to drink before donation can prevent fainting symptoms: is there a physiological or psychological reason?

The vasovagal reaction has been widely studied but its anatomic and physiological nature remains uncertain. The mechanisms underlying vasovagal reaction related to blood donation are not completely understood either. Does its occurrence depend on the blood donors' physical characteristics and health variables or psychological factors? On the basis that a psychological approach considerably prevents donor reactions, the effect of fruit juice ingestion was studied in a group of 1849 first-time high-school students as a simple strategy to avoid systemic reactions at blood donation. The reasons for the psychological effect of this hydration protocol are stressed also in light of previous physiological studies on the hemodynamic effects of water or carbohydrate drinks.