Cut Down on Carbohydrate in the Dietary Therapy of Type 2 Diabetes - The Meal Box Study

Methods: This is a 12-month investigator-initiated, randomized, controlled, open-label, superiority trial with two parallel groups. The study examines the effect of a CRHP diet (which is reduced in carbohydrate and increased in protein and fat) compared with a CD diet (which follows the currently recommended macronutrient intake for patients with T2D). The study will include 100 T2D patients with overweight or obesity.

The CD diet and CRHP diet comprise 50 percentage of energy (E%) and 30 E% from carbohydrate, 17 E% and 30 E% from protein, and 33 E% and 40 E% from fat, respectively. The CD diet and the CRHP diet do not principally differ in the quality of carbohydrate, protein, and fat; they both comprise of nutritious, organic, and sustainable food items.

Participants will be randomized in a 1:1 ratio to either the CD diet or the CRHP diet for 12 months. About 2/3 of the total calculated energy requirements on both diets will be delivered to the participants free of charge, as meal box solutions containing breakfast meals, snack meals, and dinner meals to optimize compliance and adherence to the assigned diet.

After 12 months of intervention with meal boxes, a 12-month follow-up period will be initiated for participants who wish to continue to follow the diet they were allocated to at baseline. During the 12-month follow-up period there will be no provision of meal boxes. Assistance is provided to help the participants adhere to the diet by regular counselling with RCD's and support via the Liva app.

The daily energy requirements for body weight maintenance will be calculated by multiplying resting energy expenditure using the Mifflin-St Jeor equation, with an estimated physical activity level (estimated by using a physical activity questionnaire at the beginning of the study). Based on the calculated daily energy requirements, participants will be divided into one of three energy level groups; in all groups, the amount of recommended total daily energy intake will exceed the amount required for weight maintenance and subjects will be instructed to consume the diets ad libitum until satiety is achieved.

The dietary interventions are implemented under the guidance of registered clinical dietitians (RCDs) in a free-living setting without any instruction or requirement for weight loss or increased physical activity level. Participants will be instructed to eat until satiety is achieved and allowed to consume alcoholic beverages within the recommendations from the Danish Health Authorities. Dietary advice and counselling regarding food choices and preparation of food concerning the allocated diet, especially the self-prepared lunch meals and how to navigate at special occasions, will be given under the guidance of RCDs.

Medication will be kept unchanged during the study, if possible. Rescue medication will be commenced if a HbA1c target of 58 mmol/mol is not reached after six months. If study participants obtain a HbA1c below 48 mmol/mol antidiabetic medication will still be kept constant.

Diurnal urine samples, fasting blood samples, dietary records and questionnaire responses will be collected every third month of the study. In addition, at baseline and 12 months, participants will undergo a standardized test battery including magnetic resonance imaging (MRI) and spectroscopy (MRS) for measurement of abdominal subcutaneous and visceral adipose tissue, and ectopic fat in the liver and pancreas, dual X-ray absorptiometry (DXA) scans for body composition, handgrip strength and 30-second chair-stand for muscle strength, oral glucose tolerance test (OGTT), continuous glucose monitoring (CGM), Holter-recording, diurnal blood pressure measurement, and dietary records.

Statistical analysis plan:

Intention-to-treat (ITT) analyses including all available data from all randomized participants will be conducted to estimate the effect of prescribing/encouraging a CRHP eating pattern but may differ from the actual effect of the eating pattern in presence of imperfect adherence to the diet by the participant. The treatment effect (CRHP vs CD) will be reported as an estimated marginal mean with corresponding two-sided 95% confidence interval (CI) and p-value and considered statistically significant if the 95% CI does not include zero.

Primary outcome A constrained linear mixed model (cLMM) will be used to model the mean HbA1c (including data in original form) over time within each diet group while adjusting for important covariates including sex, age, BMI, T2D duration, insulin resistance by the HOMA2IR index, and glucose-lowering medications (metformin, DPP-4 inhibitors, SGLT-2 inhibitors, and GLP-1 receptor agonists. The cLMM will include 9 mean parameters (a single at baseline and one for each group and follow-up timepoint) and an unstructured pattern stratified by treatment group to model the residual variance within individuals. Missing data is handled by the cLMM using a full information approach. The estimated marginal mean difference in changes from baseline to 12 months of follow-up between the two groups will be tested using a Wald test on the appropriate parameter of the cLMM (interaction group and 12 month).

Secondary outcomes and exploratory outcomes Similar cLMMs as for the primary outcome will be used to model mean change in body weight and intrahepatic fat content over time while adjusting for covariates..

Additional exploratory outcomes will be analysed using the cLMM, similarly as for the primary outcome, when continuous or using Fischer's Exact test when categorical where the existence of an association between the outcome and the group variable will be assessed.

Superiority and non-inferiority testing Our statistical analyses test for superiority, but if they fail to reject the null hypothesis, non-inferiority tests will be conducted secondarily for the primary and secondary outcomes, using the non-inferiority margin of 3 mmol/mol for HbA1c change, 5 kg for weight loss, and 25% relative change in intrahepatic fat. No adjustment for multiple testing is needed when switching between superiority and non-inferiority testing.

Ethics and dissemination: The National Committee on Health Research Ethics of the Capitol Region of Denmark has approved the trial (H-21057605). The study will be conducted in accordance with the Declaration of Helsinki II.

Results will be submitted for publication in international peer-reviewed scientific journals, regardless of being positive, negative or inconclusive.