Effect of fasting on the circadian rhythm of serum insulin levels.

The effects of 29 hrs of fast on gastric content, serum glucose and IRI levels were studied twice in normal mice. Both experiments showed a circadian rhythm of all parameters studied in control animals. Fasting in both experiments was unable to modify the circadian rhythm of gastric content and serum glucose levels, in spite of substantial modifications in their absolute values. Conversely, while the rhythm of serum IRI levels was not changed in one experiment, the other showed a variation with no level of significance. The latter coincides with the greatest decrease of both gastric content and serum glucose levels. Therefore, circadian IRI levels should be only within some range under the control of food intake. This suggests the possible existence of a threshold for absolute glucose and/or gastric content variation under which serum IRI level rhythm is unable to reach the level of significance.