Effects of Kefir Consumption on Health Outcomes: Gastrointestinal System, Immunity, Biochemical Parameters, Body Composition, Sleep Quality and Mental Well Being in Healthy Adults

The goal of this clinical trial is to evaluate the effects of the administration of kefir on gastrointestinal system, mental health, biochemical parameters, immunity and inflammation processes as well as the sleep quality, elucidating the possible health effects of kefir consumption in healthy adults. The main questions it aims to answer are:

1. Will be better the gastrointestinal system symptoms severity of volunteers in the intervention group will be better compared to the control group?
2. Will be better the mental health status of volunteers in the intervention group will be better compared to the control group?
3. Will be better the biochemical parameters of volunteers in the intervention group will be better compared to the control group?
4. Will be better the immunity and inflammation processes of volunteers in the intervention group will be better compared to the control group?

The participants were asked to consume 250 mL of plain lactose-free kefir daily for 6 weeks. Any particular time was not specified for kefir consumption. Kefir consumption routine of participants was followed up by regular phone calls every week and evaluated by examining their consumption records. Those who consumed less than 85.0% (skipping kefir consumption maximum once a week) of the planned kefir consumption during the study period were excluded from the study.

In order to evaluate the general characteristics, history of diseases and nutritional behaviors, a face-to-face questionnaire was conducted on all participants by the researchers. The questionnaire form contained information about the demographic information (age, gender, educational status, marital status, etc.), history of diseases (diagnosed diseases, drugs used etc.), basic nutritional habits (number of major meals/snacks consumed in a day, water consumption, etc.) and anthropometric measurements (body weight, height, body mass index, waist circumference, neck circumference). In the intial (pre-intervention) and final (post-intervention) stages of the study, anthropometric measurements of the participants were taken, and body composition analysis was done. In addition, biochemical findings and 3-day food consumption records were taken in the intial (pre-intervention) and final (post-intervention) stages of the study. Effects of kefir consumption on gastrointestinal system, biochemical findings, mental health, immunity and inflammation, sleep quality, was evaluated by assessment of the relevant status of participants before and after kefir intervention and comparison with the control group. Short Physical Activity Assessment Tool was used for evaluating physical activity levels of the participants, mental health status by The Warwick-Edinburgh Mental Well-Being Scale (WEMWBS). Gastrointestinal system symptoms were evaluated by using the Gastrointestinal Symptom Rating Scale (GSRS), sleep quality by the Pittsburg Sleep Quality Index (PSQI) and diet quality by the Healthy Eating Index-2015 (HEI-2015). Measurements were recorded prior to kefir intervention (initial measurement) and post kefir intervention (final measurement) for kefir and control groups.

Eligibility Criteria:
Inclusion Criteria:

* Volunteer to consume 250 mL of plain lactose-free kefir once a day,
* Being healthy
* Being 18-35 years of age.

Exclusion Criteria:

* Follow a special diet for health reasons,
* Have an allergy to any food, have hepatic, renal and biliary diseases, have immunodeficiency, autoimmune and chronic gastrointestinal diseases, have a history of cancer, diabetes or cardiovascular diseases,
* Being pregnant/breastfeeding,
* Using vitamin-mineral and probiotic supplements,
* Using antibiotics in last one month and throughout the study period,
* Consuming probiotic foods or kefir on a regular basis,
* Smoking