Responses to Appetite and Taste in the Brain Circuits That Control Eating Behaviour

Inclusion Criteria: * Female and male adults aged 18-50 years. * Right-handed. * No MRI contra-indications. * Weight stable at time of recruitment, defined as less than 5% variation in body weight over the preceding 3 months. * Proficient in written and spoken English. * Able to comply with study protocol. * Willing and able to provide written informed consent. * Participants must be registered with a general practitioner (GP). * Willing for their treating consultant or their GP and the study team to be informed in the event of an incidental abnormal finding being detected by the investigations or assessments that form part of this study. * Able to lie flat for 1 hour. Group-specific inclusion criteria: * SG group: * BMI of 35-50 kg/m2 * Planned to undergo primary SG surgery and fulfilling eligibility criteria for bariatric surgery. * Lifestyle intervention group: * BMI of 35-50 kg/m2 * Willing to undergo a lifestyle intervention programme with a view to achieving approximately 10% weight loss. * Normal weight group: * BMI 18.5-24.9 kg/m2 Exclusion Criteria: * Body weight greater than 130kg (due to weight limit of MRI Scanner). * Contraindications specific to MRI scanning: metallic implants (cardiac pacemakers, aneurysm clips, permanent eye lining, cochlear implants or anyone who has been exposed to metallic flakes or splinters). * Type 1 and type 2 diabetes (in view of altered circulated gut hormone profiles and gustatory function). * Menopause (in view of findings that the menopause affects gustatory function). * Smoking (in view of the fact that smoking affects salivary gut hormones and gustatory function). * Vitamin B12 or zinc deficiency (in view of deficiencies affecting gustatory function). * Untreated severe depression. * Eating disorders including bulimia and self-induced vomiting. * Neurological or psychiatric conditions. * Acute illness or chronic conditions that may impact upon gustatory and olfactory function. * Acute illness or chronic conditions that may impact upon post-operative weight loss. * Known or suspected history of HIV, Hepatitis B or C or other blood-borne diseases (in view of safety regulations regarding exposure to blood products) * Pregnancy or lactation.