See Pusey, R. P. p. 151, note 3:  “Dr. Gaisford, on my applying to him, kindly answered me,—῾συναναλίσκεσθαι.  It appears to me that this word can only be explained by a periphrasis.  The writer appears to me to mean that the elements are not thrown off like ordinary food, but that they become blended or assimilated to the body, and waste away as the body wastes away.’  Mr. Field gives the same meaning.