This is the first study to investigate the effect of the day-to-day variability of adrenal hormone levels on abdominal symptoms in healthy individuals. There is a possibility that a steeper diurnal cortisol slope during the day and lower cortisol levels in the evening is related to bouts of diarrhea. The results of this study are similar to those of two previous studies [7, 14]. Previous research suggests that compared with control patients, IBS patients show significantly higher levels of cortisol in the morning and lower levels in the evening . Therefore, the cortisol secretion patterns influencing vulnerability to abdominal symptoms may be similar in healthy individuals and individuals with IBS. Moreover, Sugaya et al.  reported that individuals with IBS had lower cortisol responses under acute stress. The result of this study is similar to that of Sugaya et al.  with respect to the relationship between the lower cortisol response and abdominal symptoms. The results of the present study are similar to those of Patachioli et al.  and Sugaya et al. ; however, the analysis of the day-to-day variability of cortisol provided a more definitive conclusion. The previous studies only indicated the characteristic of cortisol at a specific point in time. From these results, we indicate the continuity between healthy individuals and individuals with IBS with respect to the relationship between cortisol and abdominal symptoms.
We found no significant effects of day-to-day variability of DHEA-S on bowel symptoms. However, the 8-day average DHEA-S level at awakening was negatively related to all variables of abdominal symptoms. These results suggest that chronically reduced levels of DHEA-S at awakening or baseline levels are related to the degree and duration of abdominal pain, diarrhea, and infrequent bowel movements. In addition, the 8-day average of the DHEA-S slope was related to the duration of abdominal pain. Therefore, a flat circadian rhythm of DHEA-S can increase the duration or frequency of abdominal pain. Moreover, the 8-day average DHEA-S level before bedtime was negatively related to the frequency of bowel movements and stool property. In fact, we assert that low DHEA-S levels may have an affect on the frequency of bowel movements and stool property to a higher degree than the slope between the morning and the evening.
Previous research has suggested the possibility that higher cortisol and lower DHEA (or DHEA-S) levels may result in altered immune function, which in turn may cause abdominal pain [2, 5, 10]. However, our findings do not suggest that higher cortisol levels affect abdominal symptoms. Sugaya et al.  indicated that the secretion of a secretagogue, which induces the production of an adrenal hormone (e.g., adrenocorticotrophic hormone and corticotrophine-releasing hormone), may be normal in individuals with IBS because they showed a lower cortisol response than healthy controls and their DHEA profiles were similar to those of healthy controls. Therefore, considering the results of the present study and those of Sugaya et al. , there is a possibility that lower cortisol levels may aggravate abdominal symptoms due to a dysfunction of negative feedback regulation to corticoprophin-releasing hormone secretion. We can speculate that chronically low DHEA-S levels may cause the deterioration of abdominal pain and diarrhea with respect to a decreased effect of DHEA-S on immunological function that is opposite to cortisol.
This study provides some new insights and provides valuable information for future research; nevertheless, there were three limitations to the present work. First, we were unable to consider the relation between the participants dietary habits and their abdominal symptoms. Although inclusion of items about dietary habits may yield more accurate results, we feel that future studies should employ a less demanding saliva sampling process over the 8-day period. Second, the diurnal cortisol slope was calculated by only determining cortisol levels immediately after awakening and before bedtime; samples were not obtained for any other times. Hence, our results may not be useful for determining the cause of abdominal pain taking place early in the day. In the future, the relationship between the circadian rhythm of adrenal hormones and abdominal symptoms must be determined by taking multiple saliva samples throughout the day. Therefore, future studies should aim to maintain a good balance between consideration for the participants and collecting multiple samples. Third, the number of participants in this study was rather small.