The goal of this observational study is to understand how diet may influence the disease characteristics of inclusion body myositis (IBM). Research findings will help determine whether dietary factors could play a role in managing IBM.

The study aims to answer the question: Does diet affect the muscle health and functional ability of people living with IBM?

Researchers will compare adults with IBM to healthy volunteers aged 40 years and older. This comparison will help to identify which findings are related to normal ageing and which are specific to IBM.

Participants will:

Attend an initial screening visit at the Manchester Metropolitan University Institute of Sport to confirm eligibility and explain study procedures.

Complete four weeks of home-based monitoring, including dietary records, physical activity monitoring, and questionnaires about lifestyle and symptoms.

Attend a second university visit for assessments of body composition, metabolism, and muscle function.

This study will employ an observational, case-control design.

A sample size of 32 participants with inclusion body myositis (IBM) has been determined based on a power calculation designed to detect a strong association (r = ±0.5) between dietary protein practices and various outcome measures related to muscle mass and function. Using a two-tailed test with an alpha level of 0.05 and power of 80% (β = 0.2), the analysis indicated a required sample size of 29 participants. The sample size equation used was appropriate for Pearson's Correlation analysis. To account for an estimated 10% dropout rate, the final target recruitment number is 32. In addition, 15 age- and activity-matched control participants will be recruited. Power calculations for comparisons between IBM and control groups returned lower sample size requirements, reflecting the lower variability typically observed in healthy populations.

Identification of participants with IBM will be carried out at the Tertiary Neuromuscular Service, Salford Royal Hospital (Manchester, UK), and the Neuromuscular Centre (Winsford, Cheshire, UK). Those identified will be approached in clinic or by email with a Participant Information Sheet, the study poster and a Consent to Contact Form. The completed Consent to Contact Form will allow the Manchester Metropolitan (ManMet) research team to follow up with an invitation to a screening visit at the ManMet Institute of Sport (IoS)- a phone call or email. Control participants will be recruited from the local community. If agreed, a phone call will take place to arrange a screening and enrolment visit.

The screening visit to the IoS will last up to 1.5 hours but may end prematurely if the participant is found to be ineligible. During the visit, participants will meet with a researcher who will provide a detailed explanation of the study, including time commitment, risks, and participant rights. Informed consent will then be obtained. Participants will then complete four questionnaires to highlight any contraindications to testing and ensure appropriate matching between the disease and control group. These include: a Demographic and General Health Questionnaire, the sIBM Physical Functioning Assessment, an MRI Safety Checklist and the Physical Activity Scale for Individuals with Physical Disabilities. At the end of this visit, participants will be fitted with two activity monitors secured using medical-grade adhesive (one on the wrist and one on the thigh). They will also be given a paper-based food record or shown the Libro App that will serve as a digital alternative, and a set of questionnaires.

Participants will then complete a four-week home-based monitoring period. During this time, they will wear the activity monitors continuously for seven days to record physical activity, sedentary behaviour, and sleep patterns. They will also complete two 3-day food diaries, spaced three weeks apart, to capture dietary intake (food and drink, excluding water). Additionally, participants will complete a Food Frequency Questionnaire, which asks how often different food items have been consumed over the past year. Throughout this period, participants will also complete the series of validated questionnaires at their own pace. These include a Quality of Life Questionnaire (Short Form-36v2) for overall health and wellbeing, the Nottingham Extended Activities of Daily Living (NEADL) Scale for independence in daily activities, the Modified Fatigue Impact Scale (MFIS), the Visual Analogue Scale (VAS) for pain, the Pittsburgh Sleep Quality Index (PSQI), and the SWAL-QOL questionnaire, which assesses how swallowing impacts quality of life.

After the home-based data collection is complete, participants will return to the IoS for a single laboratory based testing session lasting approximately 3-4 hours. In preparation for this visit, participants will be asked to fast for at least three hours, avoid caffeine for 12 hours, and alcohol for 24 hours. They will also be advised to wear comfortable sports clothing with no or minimal metal components. At the start of the visit, height and weight will be recorded to calculate body mass index. Limb dominance will be ascertained. A small finger-prick blood sample will be taken to measure serum 25(OH)D. Resting metabolism will then be measured using indirect calorimetry. Following this, a whole-body DXA scan will be used to assess body composition, including total and regional fat mass, lean mass, and bone mineral density. This scan will only be undertaken once the participant completes the X-ray Imaging Checklist. Muscle strength and function will be assessed next. Leg strength will be evaluated using an isokinetic dynamometer. Handgrip strength will also be measured using a handheld dynamometer. Finally, participants will undergo an MRI scan lasting approximately 45 minutes. Images will be taken of the thigh and forearm muscles, as well as the head and neck, to assess muscle size, fat infiltration, inflammation, and swallowing function. Standard MRI safety procedures will be followed, including a repeat of the MRI safety checklist prior to scanning.

With respect to the study's primary outcome (relationships between habitual dietary protein intake and total lean mass, as well as muscle strength and physical function measures) correlation analyses will be used to explore the relationships between continuous variables. Depending on data distribution, Pearson's correlation coefficients (parametric) or Spearman's rho (non-parametric) will be used to examine associations between protein intake (g/day, g/kg/day) and a) total lean mass/ fat-free mass (DXA), b) muscle strength (grip strength, knee extensor and flexor torque) and c) Functional capacity (sIFA and NEADL scores).

Multiple linear regression models will be used to assess the predictive value of dietary variables (e.g., protein intake, total energy intake) on muscle-related outcomes (e.g., lean mass, strength, MRI-derived muscle volume). Models will adjust for relevant covariates, including age, sex, body mass index, and physical activity. These models will help establish whether dietary intake independently predicts muscle health beyond demographic and lifestyle factors.

Between-group comparisons for continuous secondary outcomes will use independent samples t-tests or Mann Whitney U tests, as appropriate. ANCOVA will be applied for adjusted group comparisons. For MRI-derived outcomes, two-way mixed ANOVA will examine group × region effects, with post-hoc Bonferroni correction. Friedman tests will be used for repeated non-parametric comparisons where needed. Associations between dietary, metabolic, and functional outcomes will also be explored using correlation and regression techniques, as have been described.

Subgroup analyses will be performed within the IBM group to assess how disease duration, physical activity level, or swallowing severity influence dietary intake, body composition, and function. Stratification may include disease duration (<5 vs. ≥5 years), biological sex (male vs. female) and dysphagia severity (Swal-QOL scores) differences between subgroups will be tested using ANOVA or Kruskal-Wallis tests. Interaction effects may be examined via factorial ANOVA.

This research is sponsored by ManMet University and funded by both the ManMet Faculty of Science and Engineering and Myositis UK.

Inclusion Criteria (Adults with IBM):

• Diagnosis of Inclusion Body Myositis: As defined by the revised European Neuromuscular Centre 2013 diagnostic criteria (Definite or Probable IBM)
• Age: Adults aged ≥40 years at the time of consent.
• Ability to Provide Informed Consent: Participants must be cognitively capable of providing written, informed consent.
• Disease Progression: Participants must be able to walk short distances, with or without the use of an assistive aid (e.g., a cane or frame) for balance, so that they can undertake study procedures without safety concerns. Must be able to consume solid or modified-texture food, with or without dysphagia adaptations (i.e., not exclusively tube-fed).
• Willingness to Participate: Willing to attend study visits and comply with all study procedures.

Inclusion Criteria (Control Participants):

• Disease-Free: Control participants must be generally healthy, with no known myopathies or metabolic diseases. They must not have any conditions that, in the judgement of a clinician, would compromise their safety, impair their ability to complete study procedures, or influence study outcomes.
• Age: Adults aged ≥40 years at the time of consent.
• Ability to Provide Written Informed Consent
• Willingness to Participate
• Matched to IBM participants according to their age and physical activity levels

Exclusion Criteria (Adults with IBM):

• Coexisting Neuromuscular or Metabolic Disorders: Presence of other neuromuscular or metabolic disorders that may confound the outcomes of interest.
• Severe Dysphagia: Participants requiring exclusive enteral or parenteral feeding, which would prevent participation in dietary intake assessments.
• Enrolment in an interventional trial within the past 3 months: Participation in an interventional study within the past 3 months involving interventions that may affect dietary habits, physical activity, or muscle metabolism.
• Contraindications to any Study Procedures: Any condition preventing safe completion of key assessments. For instance: severe osteoporosis contraindicating DXA scanning (T-score < -3.0), respiratory conditions, severe claustrophobia, severe cognitive impairments or communication barriers that may impair participant ability to comply with study procedures.
• Logistical Concerns: Any other logistical concern, as judged by the CI, that would compromise participant safety or the integrity of study data.
• English Comprehension: Participants will be excluded if they are unable to understand written or verbal English to a degree that would impair their ability to provide informed consent or fully understand the study procedures.

Exclusion Criteria (Control Participants):

• Diagnosed neuromuscular or metabolic Disorder
• Enrolment in an interventional trial within the past 3 months
• Contraindications to any study procedures such as those listed for IBM participants (e.g., DXA, MRI).
• Logistical or safety concerns, as judged by the research team.
• To minimise dietary overlap, individuals living in the same household as an IBM participant will not be eligible.
• Do not meet the physical activity criteria to be appropriately matched to an IBM participant
• Unable to understand written or verbal English

Adults aged 40 years and older will be recruited into two groups: individuals with a confirmed diagnosis of IBM and healthy volunteers without diagnosed neuromuscular or relevant chronic disease. Participants with IBM will be identified through specialist neuromuscular services in the UK, and control participants will be recruited from the local community. Control participants will be frequency-matched to the IBM group by age, sex, and habitual physical activity level.

De-identified individual participant data from all study assessments will be shared.

De-identified individual participant data and supporting materials will be made available following publication of the primary study results and will remain available indefinitely.

Access to de-identified individual participant data and supporting materials will be available to any member of the public, including researchers, clinicians, and other stakeholders. Materials will be hosted in an open-access repository (Open Science Framework) and can be downloaded directly from the project page. Data will be available in standard file formats (e.g., CSV, PDF, or R/Python code files) with a README describing variable definitions, units, and coding to ensure usability and reproducibility.