• Correlation with autonomous nervous system activity [ Time Frame: 70-80 minutes and 190-200 minutes during euglycemic clamp ]
  Correlation of the change in peripheral insulin sensitivity by central insulin action with the simultaneous change of the autonomous nervous system (measured by heart rate variability).
• Differential effects dependent on female sexual hormones [ Time Frame: 60-90 minutes and 150-210 minutes during euglycemic clamp ]
  Correlation of the change in peripheral insulin sensitivity by central insulin action dependent on the time point in menstrual cycle
• Effect of menstrual cycle on insulin sensitivity before spray application [ Time Frame: 60-90minutes ]
  Will be measured by hyperinsulinemic euglycemic clamp
• Effect of menstrual cycle on brain insulin sensitivity [ Time Frame: 60-90 minutes ]
  Brain insulin sensitivity will be assessed by functional magnetic resonance imaging combined with intranasal insulin administration during the follicular and the luteal phase of the menstrual cycle
• Effect of menstrual cycle on processing of food cues in the brain [ Time Frame: 20 minutes ]
  Processing of food cues in the brain will be assessed by functional magnetic resonance imaging during the follicular and the luteal phase of the menstrual cycle

• Correlation with autonomous nervous system activity [ Time Frame: 70-80 minutes and 190-200 minutes during euglycemic clamp ]
  Correlation of the change in peripheral insulin sensitivity by central insulin action with the simultaneous change of the autonomous nervous system (measured by heart rate variability).
• Differential effects dependent on female sexual hormones [ Time Frame: 60-90 minutes and 150-210 minutes during euglycemic clamp ]
  Correlation of the change in peripheral insulin sensitivity by central insulin action dependent on the time point in menstrual cycle
• Effect of menstrual cycle on insulin sensitivity before spray application [ Time Frame: 60-90minutes ]
  Will be measured by hyperinsulinemic euglycemic clamp

The human brain is an insulin sensitive organ. Brain insulin action modulates peripheral insulin sensitivity in young lean men. As a underlying mechanism, the investigators previously detected suppression of endogenous glucose production and stimulation of glucose disappearance to peripheral tissue in response to brain insulin delivery by nasal spray. Whether this holds true in young woman is unknown, since differences in brain insulin response between sexes have been reported. The investigators will address this question by combining the delivery of insulin to the brain as nasal spray with hyperinsulinemic euglycemic clamp experiments in natural cycling women.

In the planned randomized, placebo controlled cross-over study, female participants will undergo four hyperinsulinemic euglycemic experiments with tracer dilution, two in the first phase and two in the second phase of their menstrual cycle. On one of the study days per menstrual phase, subjects will receive intranasal insulin administration, on the other placebo spray. The protocol has been successfully applied previously in men. Based on the results of this trial, the investigators calculated a required sample size of N=10 for the planned study in women.

These experiments will help to better understand the role of brain insulin action in a broader sense. The results can be the basis for larger clinical trials that address the sex-specific impact of brain insulin resistance for glucose metabolism and diabetes risk.

"Effect of central insulin administration on whole-body insulin sensitivity in women" The human brain is an insulin sensitive organ. Brain insulin action modulates peripheral insulin sensitivity in young lean men. As a underlying mechanism, the investigators previously detected suppression of endogenous glucose production and stimulation of glucose disappearance to peripheral tissue in response to brain insulin delivery by nasal spray. Whether this holds true in young woman is unknown, since differences in brain insulin response between sexes have been reported. The investigators will address this question by combining the delivery of insulin to the brain as nasal spray with hyperinsulinemic euglycemic clamp experiments in natural cycling women.

In the planned randomized, placebo controlled cross-over study, female participants will undergo four hyperinsulinemic euglycemic experiments with tracer dilution, two in the first phase and two in the second phase of their menstrual cycle. On one of the study days per menstrual phase, subjects will receive intranasal insulin administration, on the other placebo spray. The protocol has been successfully applied previously in men. Based on the results of this trial, the investigators calculated a required sample size of N=10 for the planned study in women.

These experiments will help to better understand the role of brain insulin action in a broader sense. The results can be the basis for larger clinical trials that address the sex-specific impact of brain insulin resistance for glucose metabolism and diabetes risk.

• Drug: intranasal insulin
  application of 160 units human insulin
• Drug: Placebo spray
  nasal spray containing placebo solution

• Active Comparator: Insulin nasal spray
  160 Units of human insulin as nasal spray
  Intervention: Drug: intranasal insulin
• Placebo Comparator: Placebo nasal spray
  Nasal spray containing placebo solution
  Intervention: Drug: Placebo spray

Inclusion Criteria:

• female volunteer adults
• HbA1c <6.0%
• Age between 18 and 30 years
• Standard routine laboratory
• No underlying diseases
• No medication
• No hormonal contraception
• Understanding of the explanations of the study and the instructions

Exclusion Criteria:

• Persons with limited temperature perception and / or increased temperature Sensitivity to warming of the body
• Cardiovascular disease, such as manifest coronary Heart disease, heart failure greater than NYHA 2, recent myocardial infarction
• People with a hearing disorder or increased sensitivity to loud Sounds
• persons with claustrophobia
• Minors or non-consenting subjects are also excluded
• Pregnancy or breastfeeding women
• Surgery less than 3 months ago
• Simultaneous participation in other interventional studies
• Acute illness or infection within the last 4 weeks
• Neurological and psychiatric disorders
• Subjects with hemoglobin Hb <12g / dl (at screening)
• Allergic diseases
• Individuals with a history of heparin-induced thrombocytopenia (HIT)