Chronic dietary restriction influences tumor metastasis in the rat: parametric considerations

Chronic dietary restriction is a well-documented means of inhibiting tumor growth. This study examines the effects of chronic dietary restriction on tumor metastasis in the rat. We investigate the effect of 1) the degree of food restriction, 2) the effect of preexposure to food restriction, and 3) the duration of food restriction after tumor inoculation on tumor metastasis. We also compare two methods of dietary restriction: 1) the time that food is available and 2) the amount of food available. Our findings demonstrate that rats restricted to 50% of ad libitum diet for one week before inoculation with MADB106 tumor cells and for three weeks after inoculation exhibit a significant (p < 0.001) reduction in lung colonization compared with animals fed ad libitum. Animals restricted to access to food for 4 hrs/day (60% of ad libitum) for the same period of time exhibit significantly (p < 0.005) greater lung tumor colonization than animals fed ad libitum. Preadaptation to the feeding regimen for one week before tumor inoculation proved to be critical in inhibiting tumor metastasis. The tumor-inhibitory effect was not significantly influenced by the duration of restriction after inoculation or by the manner in which food restriction was implemented. Finally, we demonstrate that inhibition of tumor colonization may be mediated by enhanced natural killer cell activity in the early postinoculation period.