suggested to reduce whole-body protein catabolism even in hypocaloric settings (Carraro et al., 1990).
Excessive levels of protein in operational rations may have undesirable effects. In a study of combat engineers engaged in a 30-d field-bridging exercise, men subsisting on the MRE consumed only 2,500 kcal/d during the study. However, even with wastage of at least one-third of the rations provided, they consumed components that provided 103 g protein per day, achieving 100 percent of the military Recommended Daily Allowance (Thomas et al., 1995). As further evidence of the high protein intake, 24-h urinary nitrogen excretion doubled by 30 days on the MRE diet. This level of intake was equivalent to that of energy-restricted Ranger students, except that for the engineers this was a voluntary restriction. High protein intake and positive balance may even produce premature satiety in field feeding. Tyrosine augments the anorectic action of sympathomimetic drugs in rats (Hull and Maher, 1990); the right mix in a high-protein food might similarly augment the sympathetic activation of soldiers working hard in the field and produce an anorectic effect. Although highly speculative, it is conceivable that this explains the inadequate energy intake observed in field studies with the MRE, where the average intake is 3000 kcal/d, and the average estimated energy requirement is 4000 kcal/d. High protein content of the rations also potentially increases calcium excretion, although studies feeding protein up to 2 g/kg BW indicate no calciuretic effects (Spencer and Kramer, 1986). Protein intake in excess of requirements increases water loss with increasing urea excretion, a factor that could be decisive in isolated desert environments. Conceivably, a low protein ration could be developed for limited use in dry environments to reduce the logistical burden of water transport requirements.
The potential brain effects of some of the amino acids that serve as neurotransmitter precursors, such as tyrosine and tryptophan, are intriguing for actions beyond appetite control. Manipulation of serotonin levels with tryptophan (Spring, 1984) might prove useful in the prevention of stress casualties, estimated to be as high as one in every four medical casualties in future conflicts. Tyrosine, acting as a precursor of catecholamines, may be useful in sustaining soldiers' performance in high-stress environments. Several human and animal studies suggest that 85-179 mg/kg of tyrosine can improve mental performance and reduce anxiety in stressful conditions and with inadequate rest (Lieberman, 1994; Neri et al., 1995). John Thomas and his colleagues at the Naval Medical Research Institute have reported preliminary data from studies with Marine sharpshooters on maneuvers in Alaska that indicated a trend toward restoring marksmanship performance degraded by cold and fatigue (Shurtleff et al., 1994). Improved field tests for assessment of military performance still need to be developed to better evaluate such ration benefits, as recommended following the first major workshop of the Committee on Military Nutrition Research (CMNR) (National Research Council, 1986).