In Metal Gear Solid 3 and 4, there are bars underneath your health bar. What happens when they are empty? I have never encountered that as I've always eaten/cured that bar.
It has equal effects in both MGS3 and MGS4, but the depleting of the bar differs:
This is a secondary gauge that has various effects on Naked Snake, the most prominent being that the fuller it is, the quicker Snake's Life gauge recovers after his injuries are treated.
Like the Stamina gauge, its status affects regeneration of the Life gauge. The gauge decreases during battle, as the stress of combat affects Old Snake's ability to keep FOXDIE at bay. It increases if Snake gets out of a stressful situation, "uses" a magazine, or hides in the Cardboard Box or Drum Can. During certain cutscenes the gauge increases or decreases during conversations with other characters, either due to Snake suffering from the nanomachines suppressing the FOXDIE or as comic relief.