Longitudinal and cross-sectional observations of growth and body composition with age in laboratory populations of the male annual cyprinodont fish, Nothobranchius guentheri.

-Body weight and length were followed on individual fish throughout major portions of the lifespan in laboratory populations of the male annual fish, Nothobranchius guentheri. In addition, these same populations, hatched at different times, were killed on the same calendar day, and one half the fish of each population were analyzed for total body fat, water and protein. Since the proportion of body fat was always low, trends of body weight reflected that of the fat-free body (FFB = body weight minus body fat). If the data are only evaluated cross-sectionally, it appears that (1) growth ceases by 268 days; (2) fish lose FFB and shorten by 460 days; and (3) protein is lost from the FFB by 460 days. However, the availability of the longitudinal data for body length and weight allowed for more critical evaluation of the cross-sectional data; and indicated that the first two conclusions were not valid, and the third could have alternative interpretations. Tim LOSS of fat-flee body (FFB = body weight minus body fat) has been considered to be a manifestation of mammalian aging. The data supporting this view come from (i) the apparent loss of body weight in senescence in the human population (Build and Blood Pressure Study, 1969); (2) the premorbid loss of body weight in experimental rat colonies (Everitt, 1957); (3) the apparent loss of muscle mass in rats studied late in life (Lowry et aL, 1942; Neumaster and Ring, 1965; Rockstein and Brandt, 1962; Yiengst et al., 1959); and (4) the observed drop in the content of K 4°, a predominantly intracellular ion of assumed constant concentration in the FFB, with age (Forbes and Reina, 1970; Novak, 1972). In none of the above studies was fat-free body or body fat measured directly, and the conclusions were dependent on the validity of the assumptions within each experiment. In addition, virtually all of the above studies were carried out cross-sectionally, in that young and old subjects were sampled at the same time, in contrast to longitudinal studies, where the same subjects are followed throughout the lifespan. In a previously reported longitudinal study of FFB in the rat, it was observed that in healthy, long-lived animals, there was no loss of FFB with age, and that interpretation of cross-sectional data had to be made with caution (Lesser et al., 1970, 1973). In the present study, body weight and length were followed through major portions of the adult lifespan in six laboratory populations of the male, annual killifish, Nothobranchius guentheri. In addition, these same populations, hatched at different times, were killed on the same calendar day, and one half of the fish analyzed for total body water, fat and protein. The availability of longitudinal data of weight and length allowed for broader interpretation of the cross-sectional data of body composition and led to somewhat different conclusions. The similarities of the observations made for this fish with those in the rat and in man suggest that many aspects of the time course of aging may be similar among the vertebrates.