This prospective, open-label pilot/feasibility study of 10 youth with T1D is to evaluate glycemic and metabolic changes taking place with a very low carbohydrate diet.

First, to evaluate the feasibility of following each type of VLCD (high protein or high fat), participants will follow a standard carbohydrate diet for 2 weeks, followed by a 1 week transition to a VLCD, and then follow the a high protein VLCD for 2 weeks followed by a high fat VLCD for 2 weeks. The study will consist of 4 in person or virtual visits which will take place over ~7 weeks. All participants will be initially studied for 2 weeks while ingesting a standard diet recommended by the American Diabetes Association, followed by a 1-week transition to a VLCD, and followed by 2 weeks on each type of VLCD (high protein, high fat). During the first (baseline) period fasting β-hydroxybutyrate levels will be measured 2-3 times per week. During the VLCD study periods, fasting β-hydroxybutyrate levels will be measured daily in the morning using a blood ketone meter; insulin doses will be collected using insulin pump downloads and continuous glucose monitoring (CGM) profiles will be used to assess glycemic excursions on each diet, as well as the time in hypo-, eu- and hyperglycemic ranges. The purpose of this study is to examine changes in sensor glucose levels, basal and bolus insulin doses, and metabolic factors following implementation of a very low carbohydrate diet (VLCD). Sensor-derived time glucose ranges will serve as a surrogate marker of hemoglobin A1c given the short duration of the study. Safety measures of the diet will also be assessed, including daily fasting ketone levels.

The first hypothesis of the study is that use of a VLCD with strict monitoring of ketosis will reduce glycemic variability and increase time in target range, defined as 70-180 mg/dL by reducing the time in the hyperglycemic (>180 mg/dL) and hypoglycemic ranges (<70 mg/dL). Decreased carbohydrate intake will lead to less postprandial hyperglycemia. Lower insulin doses for meals and snacks as a result of decreased carbohydrate intake will lessen postprandial hypoglycemia as well.

Inclusion Criteria:

• BMI 19-30 m2 for individuals at least 18 years old or BMI < 95 percentile for individuals less than 18 years old
• Participants 18 years of age must be able to read and provide written consent
• Participants under 18 years of age must be able to read and provide written assent
• Participants are managed using an insulin pump or injections
• Participant has or is willing to wear a CGM for the duration of the study
• Participant is willing to complete diet logging procedures stated above

Exclusion Criteria:

• A1c < 6.5% or > 10%
• Recent history of more than 1 of diabetic ketoacidosis (DKA) in the past 6 months
• Treatment with glucose-lowering drugs other than insulin
• Unstable psychiatric disorders, including eating disorders (DSM-V criteria)
• Weight loss medications within the last 6 months
• Females who are pregnant, lactating or planning to become pregnant in the next 6 months
• Another medical condition that precludes participation in the study

Up to 7 weeks total