Table 1 Overview of PICOS eligibility criteria

Participants

Humans with a diagnosis of type 1 diabetes, type 2 diabetes, prediabetes, gestational diabetes, impaired fasting glucose, or impaired glucose tolerance (according to WHO guidelines [21, 22]), or humans with overweight/obesity (BMI ≥ 25 kg^.m²) where the relevant outcomes were collected and reported.

Animal studies using a diabetes-related disease model (see human criteria above), or overweight/obese animals where the relevant outcomes were collected and reported.

There will be no restriction on age or comorbidities and no restrictions on the methods used to induce disease in animal studies.

Intervention

Supplementation with carnosine or β-alanine. We will exclude studies that use a multi-ingredient supplement intervention.

Human studies will include oral administration only, whereas in animal studies we will also consider administration by other means (e.g. intraperitoneal or intravenous injection).

Comparator

Comparisons for human studies will be between placebo and the experimental intervention.

Comparisons for animal studies will be between placebo or control (no intervention) and the experimental intervention.

Studies without a control or placebo group will be excluded.

Outcomes

Outcomes relating to glycaemic control and insulin resistance. Primary outcomes include changes in (i) fasting glucose, (ii) glycated haemoglobin, and (iii) 2-h glucose following a glucose tolerance test. Additional outcomes include changes in fasting insulin, glucose tolerance test parameters, and homeostatic model assessment parameters (see supporting information for a full list).

Study designs

Studies will be limited to non-randomised and RCTs, including cluster RCTs. We will exclude cohort studies, cross-sectional studies, case series, case reports, commentary, and review articles.