Effect of fasting on circadian rhythmicity in deoxyribonucleic acid synthesis of several murine tissues.

These studies were done with CD2F1 adult mice that had been standardized to 12 hours of light alternating with 12 hours of darkness to determine what effect a 24-hour fast had on circadian rhythms in DNA synthesis of 10 different regions of the gut, as well as in the pancreas, liver, thymus, spleen, bone marrow, lung and testis. The mitotic index of the corneal epithelium also was studied. The overall responses varied rather dramatically. For example, in the different regions of the gut the response ranged from no statistically significant change in the colon to a strong statistically significant decrease in DNA synthesis in the cecum. In short, one cannot generalize about the effect of short-term fasting on the entire gut, but when there was any statistically significant effect, it always was a decrease. The spleen was the only tissue that showed no statistically significant response in DNA synthesis. In the bone marrow, however, a statistically significant increase in DNA synthesis was recorded at 8 and 24 hours after fasting began. In the lung there was a rather dramatic increase in DNA synthesis at 8 hours, but this was followed by decreases of 37 and 55% at 16 and 24 hours, respectively. There was a statistically significant increase of 83% in the mitotic index 24 hours after the fasting began. The data clearly demonstrate the necessity of considering circadian variation when evaluating the effects of fasting on cell proliferation.