The aim of this study was to determine body mass reduction and microbiota composition change after the weight loss diet (50 % fat, 25 % proteins and 25 % carbohydrates).

For high fat group a total of 19 overweight/obese participants (14 females, 5 males, aged 25 to 43) with BMI of 28.9-44.4 kg/m2 were assigned to low-calorie low-carbohydrate high-fat weight loss diet for 4 weeks.

As a control group, 58 subjects (39 females, 20 males, aged 24 to 55 years) were recruited and divided into 3 groups based on BMI: 18-25, 25-30, 30-39 kg/m2 (N=33, N=16, N=9, respectively).

Inclusion criteria for participants included no previous history of gastrointestinal (GI) disease, no reported antibiotic use in preceding 3 months, or taking any medication known to alter bowel motility, no history of food allergies and not taking medications.

Before the study all participants were informed about the study and participants were signed a written consent.

At day 0 baseline samples and data were collected (faecal sample, Bristol score and bowel habits, food diary about the period of at least three days before faecal sampling, body weight). After collection of baseline samples subjects in the high fat group started diet plan for 4 weeks while control group contiunued their habitual diet. After the 4 week intervention period samples and data were collected again (faecal sample, Bristol score and bowel habits, food diary about the period of at least three days before faecal sampling, body weight).

Inclusion Criteria for obese group:

• BMI > 30 g/m2

Exclusion Criteria:

• antibiotic treatment within last three months;
• food restrictions due to allergies, any kind of food intolerance, veganism/vegetarianism or other extremes in food consumption practices;
• chronic illnesses;
• current pregnancy or breastfeeding
• travel to subtropical or tropical regions within one month before the study.