Data assessment

The initial examinations started in September 2018 and took place over a period of three weeks. In a personal consultation with a member of the study team, consisting of a medical doctor and a public health specialist, the participants were informed in detail about the content of the study and the implementation of the intervention. They received a brochure specifically developed for this purpose, with a detailed description of the fasting procedure and further evidence-based information on background and aim. They were asked to start TRE immediately after the initial examinations. If possible, the appointment for the follow-up examination was arranged exactly three months later.

The pre and post anthropometric measurements were carried out according to a standardized protocol. The participants’ height was measured to the nearest 0.1 cm (Stadiometer, Seca^®, Germany), and body weight to the nearest 0.1 kg using calibrated and balanced portable digital scales (Seca^®, Germany). Waist circumference (WC) was measured in the middle, between the iliac crest and the lower ribcage, using a flexible metal tape (Lufkin Industries Inc., Texas, USA). The Body Mass Index (BMI) was calculated (body weight/height in meters squared) and classified into normal weight (<25), overweight (≥25), and obese (≥30). The Waist-to-Height Ratio (WHtR) was calculated (WC/body height) and categorized as abdominal obesity from a threshold of 0.5. A blood sample was drawn to determine blood lipids (low-density lipoprotein (LDL), high-density lipoprotein (HDL), total cholesterol, triglycerides). Blood analyses were carried out at the Department of Clinical Chemistry at University Hospital Ulm, which is accredited according to DIN EN ISO 15189.

During the three-month intervention period, the participants documented the time of their first and their last meal as well as the duration of sleep in a pre-printed diary. The quality of sleep was assessed in the same diary with a visual analogue scale.

In December 2018, the follow-up examinations were carried out according to the same principle as the initial examinations, with questionnaires, anthropometry and blood sampling.

The participants completed a short questionnaire in order to collect information on their health behavior and lifestyle. Questions were partly based on questionnaires from previous studies and surveys [19], [20] or, with regard to specific variables, self-developed. At follow-up, the questionnaire was extended with questions on the feasibility, perceived health effects and side effects, and experience with and attitude towards TRE. Side effects as an important aspect of any intervention in humans were assessed in several questions. The first aimed at the occurrence of hunger outside the eating phase (never, less than once a week, once a week, several times a week, daily). The second asked about attacks of ravenous hunger, dizziness, nausea, and other side effects that could be reported in text form. A multi-selection was possible. The third question recorded the frequency of the side effects from the beginning of the implementation through less than once a week, once a week, several times a week to daily. The fourth question related to whether the side effects mentioned in questions one and two had improved in the course of the treatment. The possible answers were “yes”, “no”, and “I have not experienced any side effects”.

In addition to information on lifestyle and health behavior, the pre-questionnaire collected general data on occupational activity. The post-questionnaire then referred to possible changes in lifestyle and health in the past three months, perceived health effects and side effects, as well as to questions about the implementation of the intervention such as feasibility, and experience with and attitude towards TRE. HRQoL was assessed at baseline and follow-up by the Visual Analog Scale (VAS) taken from the EuroQol 5-Dimension (EQ-5D) questionnaire [21].