The majority of adults with type 1 diabetes (T1DM) have either overweight or obesity. As such, dietary management has been recommended as an adjunct to insulin treatment to improve glycemic control and facilitate weight loss in patients with T1DM. Daily calorie restriction (CR) is the main diet prescribed to patients with T1DM for weight loss. However, many patients find it difficult to adhere to CR because calorie intake must be vigilantly monitored every day. In light of these problems with CR, another approach that limits timing of food intake, instead of number of calories consumed, has been developed. This diet is called "time restricted eating" (TRE) and involves confining the period of food intake to 6-8 h per day. TRE allows individuals to self-select foods and eat ad libitum during a large part of the day, which greatly increases compliance to these protocols. The simplicity of TRE, its accommodation of dietary preferences, and associated weight loss may translate to improved glycemic measures in patients with T1DM. The present study will be the first randomized controlled trial to compare the effect of TRE versus CR for weight management and improved glycemic control in adults with obesity and T1DM.

Inclusion Criteria:

• Previously diagnosed with T1DM and currently using insulin
• HbA1c: 6.5-9.5% (regardless of medication use)
• Age between 18 to 75 years old
• BMI between 25 and 50 kg/m2
• Independently living (i.e., subjects who do not live in a nursing home or institution)
• On either multiple daily doses of insulin injections (MDII) or using an insulin pump (with or without closed loop feature)
• Active prescription for daily use of a continous glucose monitor (CGM) or willing to be provided with a CGM during the study (at no cost)
• Active prescription for glucagon

Exclusion Criteria:

• History of severe hypoglycemia defined as requiring help from others, needed to use emergency glucagon administration in the past 6 months.
• Medical history of heart failure, unstable coronary artery disease, chronic obstructive pulmonary disease, requiring oxygen, cirrhosis, active cancer, history of stroke, cognitive impairment, end stage renal disease (eGFR ≤30 ml/min/m2)
• Previously diagnosed with T2DM
• Have a history of eating disorders (anorexia, bulimia, or binge eating disorder)
• Are not weight stable for 3 months prior to the beginning of study (weight gain or loss > 4%)
• Are not able to keep a food diary for 7 consecutive days during screening
• Are taking certain drugs that have significant weight loss outcomes (weight loss medications, specifically GLP1 agonist, GLP-1/GIP agonists)
• Are eating less than a 10-hour window at baseline
• Are perimenopausal (menses does not appear every 27-32d)
• Are pregnant, or trying to become pregnant
• Are night shift workers