This is a randomized controlled single-blinded intervention study in 111 overweight and obese subjects with risk factors of developing type 2 diabetes, with the aim to investigate effects of isoenergetic high cereal fiber versus high protein diets over 6 and 18 weeks. We also investigate effects of a combined high-cereal-fiber/high-protein (mix) diet, and effects in a control group. All diets are based on foods from plants and whey products commonly assumed to be healthy. This trial is designed as a proof of principle study focusing on participants that are likely to adhere to the respective isoenergetic diets, in order to show whether high protein versus high cereal fiber diets indeed affect insulin sensitivity, as indicated by epidemiological observations. Study participants will receive intensive and regular nutritional advice in order to achieve the respective dietary targets. Dietary adherence of the participants will be supported by providing tailored dietary supplements in all four groups, twice daily over 18 weeks. Supplements are provided for twice daily consumption for all participants in all four dietary intervention groups, throughout the 18-wk intervention (each of the participants will be instructed to consume a total 252 portions of tailored supplements during the intervention, which will handed to the participants in sealed single portions at weeks 0, 3, 6 and 12, totalling 63 portions at each occasion). The packaging of the sealed portions for the 4 intervention arms do not indicate the type of dietary intervention for the participant, but are coded for identification by the supplier. Energy contents of the supplements are considered when calculating individual energy intake. Supplements have been specifically designed for this intervention study, in order to facilitate achieving dietary targets. A 6 week strictly controlled isoenergetic study period (regular group and individual sessions, daily food diaries/FFQs over the first 6 weeks with adaptation of food intake, if necessary, as indicated by direct analysis of FFQs and food protocols, and by weighing the participants; 3-day food protocols at weeks 0, 6, 12, and 18; various biomarkers of dietary adherence) will be followed by a further 12 week ad libitum period, with ongoing intake of the respective diet including the supplements, but no further dietary advice. The primary outcome measure is the detection of diet-induced changes in insulin sensitivity during weight maintaining isoenergetic conditions. Secondary outcomes are changes in factors that may explain potential diet-induced alterations of insulin sensitivity. During the 12-weeks ad libitum period we will further investigate potential effects of the respective diet on changes in body weight, body composition and biomarkers related to energy intake. Participants will be re-invited for anthropometric measurements and measurement of biomarkers in blood (18 weeks after completion of the study), as well as for taking a second muscle tissue biopsy (subset of participants that agreed to this procedure).