The study will use a parallel randomized design with two arms; participants will either be randomized to a non-nutritive sweetener (NNS) beverage condition (active) or a water condition (control). A total of 432 overweight and obese participants (Body Mass Index; BMI range: 27-35 kg/m2) will be enrolled in the study. A 3-month weight loss diet phase will initially be completed, with participants receiving nutritional advice at weekly group sessions whilst additionally incorporating the active or control products into their diet (weeks 0-12). This programme is based on a model successfully used in an American sample (Peters et al., 2014) which will be adapted for a UK sample. This will be followed by a 9-month weight maintenance phase (weeks 13-49) in which participants will be advised to follow a general healthy diet in accordance with dietary guidelines (continuing to incorporate the active or control beverages) but will only attend monthly group sessions. This will be followed by a one year post assisted weight maintenance period in which participants will continue to consume the active or control beverages every day and will be asked to complete questionnaire measures, cognitive tasks, biomarkers of health (blood samples, if willing) and body composition data (weight, sagittal diameter and hip and waist circumference measures; DXA (for Subset 2)) at the end of this period. A subset of participants (Subset 1) will additionally undergo appetite probe days (n = 116), attending the Study Centre at 15 points throughout the trial (3 sessions at baseline, 3 sessions at the start of the weight loss programme, 3 sessions at the end of the weight loss programme, 3 sessions at the end of assisted weight maintenance and 3 sessions at the end of the one year post assisted weight maintenance period). A separate subset (Subset 2) will complete biological assessment measures (n =50) involving body composition (DXA) scans at 4 points throughout the trial (at baseline, end of weight loss, end of assisted weight maintenance, and at a year post assisted weight maintenance visit) and all participants will be asked to provide blood samples at these four points but will not be excluded from the trial should they not wish to.