Cardiorespiratory assessment of 24-hour crash-diet effects on altitude, +Gz, and fatigue tolerances.

Eleven male surrogates of general aviation pilots, 21-35 years old, were tested for altitude, +Gz, and fatigue tolerances with and without previous fasting for 24 h. Testing included 2 min of lower body negative pressure (LBNP) at --40 torr (equivalent to +2Gz) after 118 mn at 3,810 m chamber altitude and, after returning to ground level pressure, ergometry of 50 watts (W) for 6 min. The fast had no statistically significant effect on altitude and fatigue tolerances. One subject, who tolerated 2 min of LBNP in the nonfasting condition, lost useful consciousness during this test in the fasting condition. Although the remaining 10 subjects tolerated 2 min of LBNP in both fasting and nonfasting conditions without statistically significant differences in quantitated parameters, two of them during fasting manifested symptoms usually associated with impending syncope. Pilots should be informed that a 24-h fast may reduce the margin for safe tolerance of greater than or equal to +Gz flight maneuvers.