in his book "how and why we age", hayflick says that caloric restriction may not be effective in humans, citing data from the baltimore longitudinal study of aging which shows that being thin does not favour longevity. similarly, it is sometimes claimed that moderate "obesity" in later life may improve survival, but newer research has identified confounding factors such as weight loss due to terminal disease. once these factors are accounted for, the optimal body weight above age 65 corresponds to a leaner body mass index of 23 to 27.