"Sudden adult onset Calorie Restriction shortens the lifespans of mice. Though the lifespan enhancing effects of calorie restriction have been known since the 1930s, it was also known that if adult mice were suddenly put on a calorie restricted diet, their lifespans were actually shortened. Dr. Walford found in the 1980s, however, that if mice were slowly transitioned from an ad lib to a calorie restricted diet, then their lifespans increased. The time for this transition recommended by Dr. Walford is a minimum of 6 to 9 months, but preferably 1 to 2 years. [p. 78, BY120YD]"

from the CR society