Plant-based Diet in Older People: What Consequences for Muscle Health?

This exploratory, open-label, randomized, controlled, two-arm clinical study aims to evaluate how increasing the proportion of plant-based foods in the diet affects skeletal muscle health in older adults. The primary objective is to assess the impact of a plant-enriched diet on physical performance, specifically maximal quadriceps strength after a 3-month nutritional intervention.

Secondary objectives are to determine the effects of a plant-enriched diet on: (1) whole-body and muscle protein metabolism, (2) modifications in the skeletal muscle proteome and metabolome, (3) functional capacities, (4) body composition, (5) gut microbiota composition, and (6) sex-related differences in metabolic adaptability. The study will compare diets containing different proportions of plant-based foods in healthy older men and women.

Participants will undergo medical evaluation, nutritional follow-up, body composition measurements, muscle function testing, indirect calorimetry, and biological sampling including blood tests, and microbiota profiling. BBlood samples will be collected for proteomic and metabolomic analyses to identify molecular pathways and potential muscle biomarkers associated with adaptations to plant-enriched diets. In parallel, circulating amino acid concentrations will be measured to assess protein bioavailability, and blood samples will be incubated with skeletal muscle cells to evaluate the anabolic potential of the ingested proteins. Digestive tolerance, micronutrient status, and metabolic health markers will also be monitored.

The procedures are associated with minimal anticipated risks. Blood sampling may cause bruising and indirect calorimetry may result in mild discomfort. Plant-enriched diets may induce transient digestive symptoms or weight loss, which will be closely monitored. Participants experiencing clinically significant weight loss will be withdrawn from the intervention and followed until stabilization.

The findings of this study may strengthen public health recommendations promoting sustainable dietary transitions in older adults and help develop nutritional strategies aimed at maintaining optimal muscle health and supporting healthy aging.

• Plant-enriched dietary intervention (Other): Participants follow a diet enriched in plant-based foods with approximately 70% of protein from plant sources and 30% from animal sources for 3 months, with tailored dietary guidance, meal planning support, and compliance monitored through food diaries and periodic follow-up.
• Control diet (Other): Participants follow a standard diet with approximately 70% of protein from animal sources and 30% from plant sources for 3 months, with general nutritional guidance and compliance monitored through food diaries and periodic consultations.

Primary outcomes

• Maximal Quadriceps Strength [Baseline and Month 3]: Maximal voluntary contraction of the quadriceps muscle will be measured using a standardized dynamometer. This outcome evaluates the primary effect of the dietary intervention on skeletal muscle strength in older adults.

Secondary outcomes

• Protein metabolism [Baseline and Month 3]: Assessment of plasma amino acid concentrations, ex vivo anabolic response, proteomic and metabolic profiling, and related biomarkers (glucose, insulin, cytokines).
• Short Physical Performance Battery (SPPB) total score [Baseline and Month 3]: Physical performance will be assessed using the Short Physical Performance Battery (SPPB), which combines balance, gait speed, and chair stand performance into a composite score ranging from 0 to 12 points. Higher scores indicate better physical function.
• Handgrip strength [Baseline and Month 3]: Handgrip strength will be measured using a calibrated hand dynamometer. The maximum grip strength will be recorded and reported in kilograms (kg). Higher values indicate greater muscle strength.
• Distance covered during the 6-minute walk test [Baseline and Month 3]: Functional exercise capacity will be assessed using the 6-minute walk test. The total distance walked during 6 minutes will be recorded and reported in meters (m). Higher values indicate better functional capacity.
• Appendicular lean mass [Baseline and Month 3]: Appendicular lean mass will be assessed by dual-energy X-ray absorptiometry (DXA) and reported in kilograms (kg). Higher values indicate greater skeletal muscle mass.
• Relative abundance of dominant gut bacterial taxa [Baseline and Month 3]: Description: Stool samples will be analyzed using microbiome sequencing methods. Relative abundance of predefined dominant bacterial taxa will be quantified and reported as percentages of total bacterial abundance.
• Sex-specific changes in maximal quadriceps strength [Baseline and Month 3]: Quadriceps maximal voluntary contraction force will be measured using a standardized dynamometer and reported in Newtons (N). Changes from baseline will be compared between male and female participants to evaluate sex-specific responses to the dietary intervention.
• Sex-specific changes in serum metabolomic and proteomic profiles [Baseline and Month 3]: Serum metabolomic and proteomic profiles will be analyzed using validated metabolomics and proteomics platforms. Changes in predefined metabolites and proteins will be compared between male and female participants to evaluate sex-specific responses to the dietary intervention.
• Sex-specific changes in appendicular lean mass [Baseline and Month 3]: Appendicular lean mass will be assessed by dual-energy X-ray absorptiometry (DXA) and reported in kilograms (kg). Changes from baseline will be compared between male and female participants to evaluate sex-specific responses to the dietary intervention.

Sex: All, ages: 65 Years to any

Inclusion Criteria:

* Men and women aged 65 years and older
* Body Mass Index (BMI) between 18.5 and 30 kg/m² (exclusive)
* Physical activity < 3000 MET-min/week, assessed using the ONAPS-PAQ questionnaire
* Laboratory tests compatible with participation in the study
* Adequate venous status, as assessed by study nurses, allowing catheter placement
* Willingness to participate in study procedures, including indirect calorimetry, functional tests, and assigned dietary intervention (plant-enriched or standard diet)
* Affiliation with the French Social Security system

Exclusion Criteria:

* Men and women aged 65 years and older
* Body Mass Index (BMI) between 18.5 and 30 kg/m² (exclusive)
* Physical activity < 3000 MET-min/week, assessed using the ONAPS-PAQ questionnaire
* Laboratory tests compatible with participation in the study
* Adequate venous status, as assessed by study nurses, allowing catheter placement
* Willingness to participate in study procedures, including indirect calorimetry, functional tests, and assigned dietary intervention (plant-enriched or standard diet)
* Affiliation with the French Social Security system
* Exclusion period from a previous study or total compensation exceeding €6,000 in the 12 months prior to study initiation (verified through the National Research Volunteer Database)
* Patients under legal guardianship, curatorship, deprived of liberty, or under judicial protection
* Claustrophobia

• Yves BOIRIE, Principal investigator, University Hospital, Clermont-Ferrand