The gut microbiota ferments indigestible food components, thereby yielding metabolites that impact host metabolism. Although not completely understood, saccharolytic fermentation is regarded as beneficial for the host, whilst proteolytic fermentation is considered detrimental. Currently, most measurements of microbial fermentation occur in faeces long after actual fermentation has taken place. Real-time in vivo measurements are necessary to increase our understanding regarding the gut microbiota-host metabolism axis in humans. A whole room calorimeter system will be equipped with sensor technology to measure real-time hydrogen (H₂), methane (CH₄) and hydrogen sulphide (H₂S) excretion in humans. A double-blind, randomized, placebo controlled, crossover pilot study including 12 normoglycemic lean individuals and 12 prediabetic/insulin resistant overweight/obese individuals will performed to investigate whether we can precisely measure the switch from proteolytic (as reflected by H₂S) towards saccharolytic fermentation (as reflected by expired H₂ and CH₄) after the consumption of a complex fibre mixture over a 36 hours period. Additionally, changes in intestinal gas production will be related to human energy and substrate metabolism. Participants will receive supplements of a complex fibre mixture consisting of long-chain inulin and resistant starch or cellulose (placebo) in a randomized order starting dinner before entering the whole room calorimeter. During clinical investigation days, intestinal gas excretion patterns, indirect calorimetry and plasma metabolites and hormones will be assessed throughout the 36 hours spent in the whole room calorimeter. Additionally, faecal, urine and room air samples will be analysed to asses volatile organic compound (VOC) profiles. Development and validation of this new measurement method provides us with a unique opportunity to better understand the link between the gut microbiota, its products and human metabolism. This provides a basis for novel personalised nutritional and lifestyle strategies and/or the development of functional foods and nutraceuticals to improve gut and metabolic health.