Commercial lactose-free kefir (Altinkilic, Turkey) beverage, which is produced from cow milk with 3.1% fat and 7.5% protein content and stored in appropriate conditions, was used in this study. The kefir (intervention) group was provided with 250 mL of plain lactose-free kefir containing at least 107-1010 probiotic bacterial strains (Lactobacillus, Lactic acid, Streptococcus, Acetic acid bacteria sp) and yeasts (Kluyveromyces marxianus, Torulaspora delbrueckii, Saccharomyces cerevisiae, Candida). 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.

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.

Commercial lactose-free kefir (Altinkilic, Turkey) beverage, which is produced from cow milk with 3.1% fat and 7.5% protein content and stored in appropriate conditions, was used in this study. The kefir (intervention) group was provided with 250 mL of plain lactose-free kefir containing at least 107-1010 probiotic bacterial strains (Lactobacillus, Lactic acid, Streptococcus, Acetic acid bacteria sp) and yeasts (Kluyveromyces marxianus, Torulaspora delbrueckii, Saccharomyces cerevisiae, Candida). 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.

Gastrointestinal Symptom Rating Scale developed by Revicki et al and validated in Turkish by Turan et al. (2017) was used to evaluate gastrointestinal health of participants. GSRS is a disease specific instrument comprising of 15 items combined into 5 symptom clusters namely abdominal pain (items 1, 4 and 5), reflux (items 2 and 3), indigestion (items 6, 7, 8 and 9), diarrhea (items 11,12 and 14) and constipation (items 10, 13 and 15). In order to score the items, participants state how he/she felt about these problems in the past week. GSRS questions are rated on a 7-point Likert scale ranging from "no discomfort" to "very severe discomfort". A score of 1 is given for the absence of symptoms, and 7 for frequent and severe symptoms. High scores indicate severity of symptoms and diseased state.

Body composition analysis, of the participants, was performed with TANITA MC-780 body composition analyser. By means of bioelectrical impedance analysis, basal metabolic rate and body composition (fat mass, muscle mass, fluid mass, lean mass, bone mineral mass, visceral fat, phase angle and edema index) were evaluated. Height (with SECA 206), body weight (with TANITA MC-780), neck and waist circumferences (with SECA 201) of participants were recorded. Heights were measured while standing upright and without shoes, feet were placed side by side, head was placed on the Frankfort Horizontal Plane, occipital region toucing the stadiometer. The body mass index (BMI) of the participants was calculated by dividing the body weight (kg) by the square the height (metres) and the evaluation was made according to the classification of the World Health Organization.

The Warwick-Edinburgh Mental Well-Being Scale developed by Tennant et al. (2007) and validated in Turkish by Keldal (2015), was used to evaluate the mental health status of individuals. The WEMWBS total score is calculated by adding the scores assigned to each of the 14 items (1=never to 5=always). The minimum score that can be obtained is 14, whereas the maximum score obtained is 70. Higher scores are associated with higher mental well-being.

Participants were asked to maintain their usual physical activities. The physical activity levels of the participants were evaluated with the Short Physical Activity Assessment Tool. The tool consisted of two questions focusing on 20 minutes of vigorous physical activity (running, weightlifting, cycling, etc.) and 30 minutes of moderate physical activity (mowing grass, lifting light loads, etc.). A maximum score of 8 and a minimum score of 0 could be obtained by this tool. A total score of 0-3 was considered insufficiently active, and a score of ≥4 was considered sufficiently active.

Pittsburg Sleep Quality Index , developed by Buysse et al. (1989) and validated in Turkish by Ağargün et al. (1996), was used to assess sleep quality.. The tool comprises of 7 sub-components and 18 items, including subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbances, use of sleeping medication and daytime dysfunction. The total score is calculated by evaluating the sub-components of the scale over 0-3 points. A score of 0-4 indicates good sleep quality, and a total score of 5-21 points indicates poor sleep quality.