Failure of dietary restriction to influence natural killer activity in old rats

Natural killer activity of Sprague-Dawley rats maintained on an ad libitum versus restricted diet was compared using an 18 hour 51Cr-release assay, against the K562 erythroleukemic line, Yac-1 lymphoma cells and SV40-3T3 cells. The results indicated that no enhancement of natural killer function was induced by dietary restriction of 10.5-month-old rats from weaning. Prolongation of the restricted diet into late life (24 months) similarly did not enhance basal natural killer activity over levels observed in the ad libitum controls. This suggests that the improved resistance to some tumours seen after prolonged dietary restriction depends on another defensive mechanism, reduced metabolic activity and/or a reduction of available nutrients at cancerous foci.