Manchester Intermittent and Daily Diet Type 1 Diabetes App Study (MIDDAS-Type 1)

Inclusion Criteria: * T1D for 12 months or longer * HbA1c 53-108 mmol/mol * BMI ≥ 30 kg/m2 and \<50kg/ m2 or ≥27.5 kg/ m2 and \<50kg/ m2 in high-risk minority ethnic groups i.e. South Asian, Black African and African Caribbean * Multiple daily injections (MDI) or continuous subcutaneous insulin infusion (CSII) * Completed Dose Adjustment For Normal Eating (DAFNE) education * Access to a Freestyle Libre handset and sensors to monitor blood glucose * Willing to use the Freestyle Libre® flash glucose monitoring system to monitor blood glucose (flash and capillary) and blood ketones and to record carbohydrate and insulin. * Access to and ability to use a telephone. If no access to a smartphone running iOS or Android (to view the LibreLink app®) then access to a computer (to upload results to the LibreView website). * Willing to undertake Optifast® LEDs and have previously sampled Optifast®. * Negative urine pregnancy test at screening and agreement to maintain contraception or abstinence for the trial (where appropriate) * Ability to read, understand and communicate in English. Exclusion Criteria: * Evidence of severe hypoglycaemia in the last 12 months (more than one episode requiring third party assistance) or hypoglycaemia unawareness. * Patients with non-stable retinopathy, or grade R2 or later, or had no retinopathy screen within 12 months. * Patients who lack capacity or are unable to read or understand written or verbal instructions in English or those diagnosed with learning difficulties. * Confirmed pregnant via a pregnancy test at screening, planning pregnancy in the next 3 months, or currently breast feeding. * Participants who are currently on treatment with Orlistat or other pharmacological treatments for weight loss e.g. Glucagon-like-peptide-1 (GLP-1). * Participants who are currently taking a Sodium-Glucose Co-Transporter-2 (SGLT2) inhibitor. * Diagnosed Gastroparesis. * Participants who have previously had bariatric surgery for weight loss including gastric bypass and sleeve gastrectomy. * Patients who are on chronic use of steroids (more than 20mg daily of prednisolone or its equivalent). * Patients with known hypersensitivity to any of the ingredients of Optifast® e.g. fish, milk, soy. * Taking prohibited medications (see Appendix 3) including warfarin or novel anticoagulants (NOAC), low molecular weight heparin (LMWH) or equivalent anti-coagulants and anti-psychotic medication or other psychotropic medications that may cause excessive weight gain. * Substance abuse or harmful alcohol use as indicated by a score of 16 or above on the Alcohol Use Disorders Identification Test (AUDIT).\[36\] * Participants with a diagnosed eating disorder, or patients with severe binge eating assessed by a score of 27 or more on the Binge Eating Scale (BES).\[37\] * Participants with severe depression assessed by a score of 15 or more on the Patient Health Questionnaire-9 (PHQ-9) questionnaire.\[29\] * Participants with severe anxiety assessed by a score of 15 or more on the General Anxiety Disorder (GAD-7) questionnaire.\[28\] * Participants with very low self-efficacy assessed by a score of 35 or less on the Weight Efficacy Lifestyle Questionnaire (WEL-SF).\[30\] * Participants with severe loss of renal function (eGFR \< 30mL/min/1.73m2). * Participants with psychiatric or physical comorbidity or scheduled for major surgery, which in the opinion of the treating medical physician, Chief Investigator (CI) or Multidisciplinary Team (MDT) would compromise their safety or adherence to the study. * Unsatisfactory use of the Freestyle Libre® flash glucose monitoring system or unsafe use of DAFNE/insulin adjustment principles during the 14-day "run-in" period that in the opinion of the medical team may undermine the participant's safety on the trial. This includes flash and capillary monitoring of blood glucose and ketone testing. * Patients who are currently participating in a diabetes drug trial.