The overall aim of this 12-week randomized controlled trial is to investigate if a dietary fiber supplement rich in arabinoxylans (AX) affects weight loss success differently according to baseline gut microbiota composition in subjects who have overweight or obesity.

105 participants will be randomized in a 2:1 ratio to receive 15 g/day of AX or placebo.

First, we hypothesize that participants who have a predominantly Prevotella enterotype (inferred by a high Prevotella/Bacteroides-ratio) will lose more body weight after AX supplementation compared to participants with a predominantly Bacteroides enterotype (inferred by a low Prevotella/Bacteroides-ratio). Specifically, we hypothesize that weight loss and P/B-ratio will be positively correlated in the AX supplementation-group whereas there will be no such correlation in the control-group. Consequently, we hypothesize that participants with the Bacteroides enterotype - and the lowest P/B-ratio - will have no benefit after AX supplementation while the weight loss effect of the AX supplementation will increase with increasing P/B-ratio.

Second, we hypothesize that the ability to digest starch in the upper gastrointestinal tract - evaluated by salivary alpha amylase gene (AMY1) copy number - will influence the interactions among AX intake, P/B ratio, and body weight change. Specifically, we hypothesize that there will be an association between body weight change and P/B ratio among subjects with a low AMY1 copy number, but not among the ones with a high AMY1 copy number, when consuming AX.

Inclusion Criteria:

• Apparently healthy men or women (self-reported nonpregnant, nonlactating, and not planning a pregnancy in the next 4 months)
• BMI: 25 to 40 kg/m2
• Non-smoker
• Want to maintain or lose weight
• Willing to consume wheat buns on a daily basis

Exclusion Criteria:

• Consumption of whole grain products with every meal
• Use of antibiotics 60 days prior to the start of the study. If a participant uses antibiotics prior to randomization, they will be invited to be re-screened 3 months after the last use of antibiotics, provided that it is realistic to complete the study no later than the scheduled LPLV
• Dietary supplements with pro- and/or prebiotics 6 weeks prior to study
• Self-reported eating disorders
• Being a bodybuilder (>4 strength training sessions per week)
• Alcohol intake above the recommendation from the Danish Health and Medicines Authority (>21 units of alcohol per week)
• Night- or shift work
• Chronic diseases e.g. cancer within the past 5 years (except adequately-treated localized basal cell skin cancer), asthma, thyroid disease, cardiovascular disease, type 1 or 2 diabetes, inflammatory diseases, and celiac disease
• Disorders: neurological, sleep, diagnosed psychiatric disorder and gastrointestinal and liver disorders
• Surgical treatment of obesity and abdominal surgery
• Inability, physically or mental, to comply with the procedures required by the study protocol as evaluated by the daily study manager, principal investigator or clinical responsible
• Subject's general condition contraindicates continuing the study as evaluated by the daily study manager, principal investigator or clinical responsible
• Simultaneous blood donation for another purpose than this study
• Simultaneous participation in other clinical intervention studies