As there are fluctuations in the hormonal balance during the menstrual cycle, it is assumed that due to the interaction between the hormonal system and the immune system, some diseases might be worsened in the course of a cycle. This interaction is particularly important during the phases of the cycle when the corpus luteum is formed and secretes estrogens and progesterone (secretion phase, luteal phase), as well as during menstruation itself. To date, there is no comprehensive review regarding the molecular signaling pathways that are active during and influence the menstrual cycle.

The menstrual cycle influences blood glucose levels. Women are subject to greater fluctuations in blood glucose levels than men according to the phases of the menstrual cycle (menstrual phase, proliferative phase, ovulation, luteal phase). Since there are contradictory results regarding the influence of the menstrual cycle on glucose metabolism, the glucose and insulin response, as well as the glucose-dependent intestinal hormone glucagon-like peptide 1(GLP-1) after food intake are determined in the study presented here using an oral glucose tolerance test.

Within the menstrual cycle there are strong hormonal fluctuations. Therefore, it is obvious that the interaction between the endocrine system and the immune system play an important role in the progression of different diseases during the course of a cycle, especially during the luteal phase and menstruation. The molecular signaling pathways that are active during the menstrual cycle and have an influence on it have not been extensively elucidated to date.

Furthermore, glucose homeostasis, on which the female cycle also can have an influence, is part of the research. Study results show that when an oral glucose tolerance test (oGTT) is performed, women are subject to greater fluctuations in blood glucose levels than men. This seems to be dependent on the phases of the female menstrual cycle. It was shown that glucose metabolism was impaired in the luteal phase. Contradictory to this, in another study no significant influence on glucose metabolism by the menstrual cycle could be detected. Based on these equivocal results, in this study the postprandial glucose and insulin response, as well of the glucose-dependent gut hormone GLP-1 in the different cycle phases will be examined.

For this purpose, an intervention study is set up in which blood samples are taken at five different time points and the subjects participate in an oGTT at four of these different time points in their individual menstrual cycle. Furthermore, anthropometry measures and body composition will be determined with the help of Bodpod and bioelectrical impedance analysis (BIA) measurements. A diary will also be kept during the entire study duration in order to record the habitual diet. In addition, different subgroups are studied: continuous measurement of body temperature or glucose levels using sensors, continuous collection of stool samples and documentation of daily nutrition using an app.

To account for regular daily variations in immunological measurements that are not related to the menstrual cycle, a cohort of healthy, male individuals will be examined.

Inclusion Criteria:

• Women or men
• Age between 20 and 30 years
• BMI: 18,5 - 25 kg/m2
• Written informed consent to participate in the study
• if applicable: Regular menstruation in the last 6 months
• if applicable: Cycle length between 21 and 35 days for each cycle in the last 6 months (self-reported).

Exclusion Criteria:

• Smoking
• Mental illness
• Chronic illnesses
• Antibiotic therapy 3 weeks before the start of the study
• Alcohol, medication and/or drug abuse (BTMG-liable or other psychotropic substances)
• Participation in another clinical study at the same time or within the last 30 days
• Hospitalization in the last 3 months
• Inflammatory condition in the last 30 days or during the course of the study
• Other exclusion criteria at the discretion of the physician/investigator
• History of infection with SARS-CoV2 before or during the study period
• Regular physical activity with very high intensity (athletes)
• Coagulation disorders
• Planning a restrictive diet for weight loss or medical reasons in the next 3 months
• Gastrointestinal disease associated with malabsorption (e.g., Crohn's disease)
• Whole blood donation within the last 3 months (women) or last 2 months (men)
• Known infertility
• Infection with Chlamydia in the past
• if applicable: Pregnancy, lactation
• if applicable: Use of any contraceptive (hormonal or chemical contraception) within and during the 6 months prior to study entry. An exception are mechanical contraceptives such as condoms
• if applicable: planning pregnancy in the next 3 months
• if applicable: attempted conception in the past 6 months
• if applicable: smear test with abnormal findings in the past 6 months (without subsequent abnormal findings)
• if applicable: untreated gynecological infection or of the urogenital tract in the last 6 months
• if applicable: gynecological surgery in the last 12 months
• if applicable: current or past other gynecologic complaints

The study participants are healthy young women with a regular menstrual cycle or heathy young men aged 20-30 years. The subjects are predominantly female students at the University of Bonn.