citric acid is the easiest for your body to expell (that is, from a fruit/plant, not the preservative). It is released through your lungs, you exhale it with CO2. Stronger acids are released through your urine.
It is debatable whether or not a citric fruit fast can cause the body to be acidic. In fact, afterwards, I had a urinary tract infection for 3 weeks. It wasn't from the citric acid . It was a cleansing reaction from the fast due to harsh acids being released from my cells at a rapid rate, through my bladder, and causing a bacterial infection because they thrive off acid. Well, that's my theory anyway :). Prove me wrong if you wish...