Does it sound reasonable that if you eat one nanogram of bread during severe starvation, it would retard your death in precisely zero seconds?

No. You use energy at some finite rate (I'll assume 2000 kilocalories/day, dunno how much starvation affects this). A nanogram of bread contains a nonzero amount of energy (~2.5 microcalories). So it increases your life expectancy by a nonzero time (~100 nanoseconds). A similar analysis can be performed for anything down to and including a single molecule.