The shell of several species of freshwater turtle is a very special and potent Yin Essence tonic that also builds blood. It is said to increase fluids, build hormones and to replenish substances lost due to stress and excessive behavior. Turtle shell is thus included in "restorative" formulas. These are formulas designed to rejuvenate some one who has aged themselves prematurely by living excessively or because they have experienced severe stress. The same formulas can also be used as an occasional replenisher. The Yin essence is constantly drained by life itself, and since restorative formulas are quick and effective, they can be used from time to time to slow down aging.