Despite strong evidence that tight control of blood sugar reduces the risk of diabetes complications, most people with type 1 diabetes do not achieve recommended blood sugar targets. This randomized controlled trial will test whether a very-low- carbohydrate ketogenic diet can effectively and safely improve blood sugar control in adults with type 1 diabetes.

A very-low-carbohydrate ketogenic diet (≤50 g carbohydrate/day) could reduce glycemic variability, total daily insulin requirement, and HbA1c in people with type 1 diabetes (T1D). Indeed, several case series and observational studies of using a ketogenic diet (KD) in people with T1D have observed such benefits. However, no randomized controlled trials (RCTs) have evaluated the efficacy of KD for >7 days in people with T1D. In addition, there are serious concerns regarding the safety and tolerability of a KD in patients with T1D, including the potential for an increased risk of hypoglycemia, diabetic ketoacidosis, dyslipidemia, insulin resistance, decreased bone mineral density, and impaired quality of life. This study is a 12-week RCT to evaluate the clinical efficacy, metabolic function, safety, socio-behavioral impact, acceptability and potential for dissemination of an isocaloric KD compared with an American Diabetes Association-recommended control diet in adults with T1D.

Inclusion Criteria:

• Age ≥18 and ≤65 years
• T1D diagnosed >1 year prior to screening
• HbA1c 7.0%-9.0%
• Stable insulin delivery method for the past 30 days
• Ability to read all device instructions and insulin pump settings
• eGFR ≥60 mL/min/1.73 m2
• Use of an insulin pump or insulin delivery by multiple daily injections
• Use of personal CGM for at least 12 weeks and willing to change to Dexcom CGM for the duration of the study, if using a different sensor, to reduce variability in glucose values associated with different CGM products
• Use of cellular phone with data capability for wireless connectivity to the CGM system.

Exclusion Criteria:

• Body mass index <20.0 or >34.9 kg/m2
• Severe gastroparesis or history of bariatric surgery
• Diabetes-related hospitalization (including for diabetic ketoacidosis or severe hypoglycemia) within 12 months of screening
• Poorly controlled hypertension (SBP ≥160 mmHg or DBP ≥100 mmHg)
• Taking diabetes medications, other than insulin (particularly SGLT2 inhibitors, which are associated with an increased risk of euglycemic DKA)
• Structured exercise >210 minutes per week
• Pregnant, lactating, not using effective birth control if premenopausal, or planning to become pregnant within the 6-month study period
• Unstable weight (>4% change in the last 2 months)
• Significant organ system dysfunction (e.g., severe pulmonary, renal, hepatic, or cardiovascular disease)
• Anemia (Hgb <10 g/dL)
• Major psychiatric illness
• Active tobacco use (>8 cigarettes/day) or illegal drug use
• Regular alcohol consumption (>10 standard drinks per week)
• Use of medications known to affect the study outcome measures or increase the risk of study procedures that cannot be temporarily discontinued for this study
• Familial hypercholesterolemia
• Active eating disorder
• Dietary restrictions incompatible with a very-low-carbohydrate KD, vegan diet, vegetarian diet, severe lactose intolerance, severe aversion/sensitivity to eggs, fish, nuts, wheat, or soy, and any anaphylactic food allergy
• Already consuming a low-carbohydrate (<130 g/day) diet
• Persons who are not able to grant voluntary informed consent
• Persons who are unable or unwilling to follow the study protocol or who, for any reason, the research team considers an inappropriate candidate for the study.