Food 4 Health - He Oranga Kai: Assessing the efficacy, acceptability and economic implications of Lactobacillus rhamnosus HN001 and β-glucan to improve glycated haemoglobin, metabolic health, and general well-being in adults with pre-diabetes: study protocol for a 2 × 2 factorial design, parallel group, placebo-controlled randomized controlled trial, with embedded qualitative study and economic analysis

BACKGROUND: The rates of pre-diabetes and type 2 diabetes mellitus are increasing worldwide, producing significant burdens for individuals, families, and healthcare systems. In New Zealand, type 2 diabetes mellitus and pre-diabetes disproportionally affect Māori, Pacific, and South Asian peoples. This research evaluates the efficacy, acceptability, and economic impact of a probiotic capsule and a prebiotic cereal intervention in adults with pre-diabetes on metabolic and mental health and well-being outcomes. METHODS: Eligible adults (n = 152) aged 18–80 years with pre-diabetes (glycated haemoglobin 41–49 mmol/mol) will be enrolled in a 2 × 2 factorial design, randomised, parallel-group, placebo-controlled trial. Computer-generated block randomization will be performed independently. Interventions are capsulated Lactobacillus rhamnosus HN001 (6 × 10(9) colony-forming units/day) (A) and cereal containing 4 g β-glucan (B), placebo capsules (O(1)), and calorie-matched control cereal (O(2)). Eligible participants will receive 6 months intervention in the following groups: AB, AO(1), BO(2), and O(1)O(2). The primary outcome is glycated haemoglobin after 6 months. Follow-up at 9 months will assess the durability of response. Secondary outcomes are glycated haemoglobin after 3 and 9 months, fasting glucose, insulin resistance, blood pressure, body weight, body mass index, and blood lipid levels. General well-being and quality of life will be measured by the Short-Form Health Survey 36 and Depression Anxiety Stress Scale 21 at 6 and 9 months. Outcome assessors will be blind to capsule allocation. An accompanying qualitative study will include 24 face-to-face semistructured interviews with an ethnically balanced sample from the β-glucan arms at 2 months, participant focus groups at 6 months, and three health professional focus groups. These will explore how interventions are adopted, their acceptability, and elicit factors that may support the uptake of interventions. A simulation model of the pre-diabetic New Zealand population will be used to estimate the likely impact in quality-adjusted life years and health system costs of the interventions if rolled out in New Zealand. DISCUSSION: This study will examine the efficacy of interventions in a population with pre-diabetes. Qualitative components provide rich description of views on the interventions. When combined with the economic analysis, the study will provide insights into how to translate the interventions into practice. TRIAL REGISTRATION: Australian New Zealand Clinical Trials Registry, ACTRN12617000990325. Prospectively registered on 10 July 2017. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s13063-019-3553-7) contains supplementary material, which is available to authorized users.