Nutrient intake as a time signal for circadian rhythm.

In considering nutrition and circadian rhythms, time-of-eating behavior is an inherited, genetically controlled pattern that can be phase-shifted by conditioning or training. In addition, there are metabolic responses to meal eating, such as entrainment of a number of enzyme levels, hormone concentrations and other metabolic and physiologic activities. To separate truly inherent rhythms from entrained responses, it is necessary to determine which oscillations continue their circadian rhythms under completely free-running conditions of no zeitgeber, such as darkness vs. light or meal eating vs. fasting. Where meal eating provides a time signal to a biological clock experiment, healthy animals-not starving but constantly receiving nutrition to avoid an eating time signal-should be used. The ability to determine which daily rhythms are not genetically inherent, but rather are responses to a particular time signal, requires the elimination of all but this time signal. By using free-running conditions that are totally without time-giving signals, entrainment by (response to) meal feeding is readily separated from inherited endogenous circadian rhythm. Both types of daily rhythm exist and both involve the biological clock mechanisms . A method by which to identify each is proposed.