Prolactin is a multifunctional hormone which acts in different cell types rather than just modulating reproduction and lactation. One additional point is its action on glucose metabolism which is described especially in rodents. Prolactin affects proliferation, survival and insulin production of pancreatic beta cells in animal models. But also in human studies prolactin is linked to insulin action. Low systemic levels of prolactin are associated with increased prevalence of diabetes and while higher levels are associated with higher insulin sensitivity.

Acute metabolic effects of prolactin in young, healthy and non-lactating women should be accessed. Therefore, the induction of a transient increase of circulating prolactin levels is necessary to study the acute effects of this hormone on metabolic processes. This can most likely be achieved in non-pregnant and non-lactating women without pharmacological intervention.

As the main stimulus of prolactin release from the pituitary gland is the mechanical stimulation of the breast / nipple, like a suckling baby, stimulation by a breast pump might trigger prolactin release.

Due to other studies in this field, only a minority of women respond to breast stimulation with an increase in prolactin levels. Therefore, this pilot study will assess if an electric milk pump can stimulate prolactin release in non-lactating women. The second aim of the study is to identify factors which determine response and non-response to breast stimulation.

Prolactin will be measured in blood serum. The primary objective of this study is to identify women in whom a 30-minute breast stimulation causes a marked prolactin release. Response is defined as an increase at least twice the baseline value.

Second outcomes are the quantification and duration of prolactin increase as well as to detect differences in responders and non-responders. Therefore, demographic data as well as anthropometric data will also be assessed. A 10-minute electrocardiography and non-invasive measurement of resting energy expenditure by indirect calorimetry will also be performed. History of menstrual cycle, intake of contraceptive, parity and lactation experience will be recorded. Physical activity level and feeling of hunger will be assessed by questionnaires.

Feeling of hunger, resting energy expenditure, blood pressure, heart rate and body temperature will be assessed repeatedly to register differences before and after stimulation. Blood and urine samples will be assessed for safety.

Inclusion Criteria:

• Women, aged between 18 and 45 years
• premenopausal (women, who had menses at any time in the preceding 24 consecutive months)
• BMI < 25 kg/m²
• Ability to understand and voluntarily sign an informed consent document prior to any study related procedures

Exclusion Criteria:

• Type 1 or Type 2 diabetes
• Intake of medication which may affect prolactin levels (e.g. Metoclopramide, Methyldopa, opiates, cimetidine), except birth control pill/hormonal contraception known hyperprolactinemia or hyperthyroidism
• any other serious illness that might impact the results as judged by the investigator (e.g. malignant disease, coronary heart disease)
• Pregnant / breastfeeding women (weaning at least 6 month ago)
• plastic breast surgery (with detachment of milk ducts)
• allergy to BPA-free plastic

There is no plan to make individual participant data available to other researchers.