Vitamins could be dissolved in the PFC, along with elements such as Calcium in the form of organometallic complexes for easy absorbtion in the lungs. Perhaps this could stop bone loss? I would think that some spiking would be necessary for long term living, else the PFC would leach out necessary nutrients. Speaking of nutrients, perhaps the stomach, (and other messier organs) would become obsolete, and all food intake and waste elimination could occur in the lungs. Sugars, proteins, and amino acids could dissolve at very low concentrations in PFC, but with the constant high volume contact with the lungs, perhaps this is adequate. Contact with the blood of the lungs likely exceeds that of the intestines. Food and water consumption and elimination have always been problems in space, but I don't think that they are any more complex for the PFC immersion case.