Effects of Dietary and Fluid Restrictions upon Physical Performance, Cognition and Vigilance
Introduction Prolonged fasting almost inevitably leads to a reduction of both physical and psychomotor performance. A progressive decrease of blood glucose and glycogen reserves limits the possibility of undertaking the anaerobic component of nearmaximal exercise and hampers isometric muscular contraction; it also impairs many components of cerebral function, since glucose is the primary metabolite of the brain. In a longer-term perspective, the depletion of lean tissue mass weakens the muscles, reducing their maximal contractile force. These changes are exacerbated by a reduction of motivation (which is an important determinant of most laboratory tests of physical performance), and in athletes there is also likely to be a relaxation of training schedules. On the positive side, the decrease in body mass as fat and lean tissue are metabolized reduces the energy cost of activities that involve the displacement of body mass. However, a substantial dehydration reduces blood volume and maximal cardiac output, thus impairing peak aerobic performance, and an alteration of sodium ion concentrations at the muscle membrane impairs muscle contraction. Although many of the impediments to performance have a physiological cause, the psychological element is also important, and for this reason, effects are usually greater in human studies than in animal research.