This study is a five-arm, parallel-group, assessor-blinded randomized controlled trial designed to evaluate the effects of resistance-dominant multicomponent training combined with nutritional supplementation on intrinsic capacity and disability risk among rural Chinese older adults. A total of 220 participants aged 60 years or older with mobility decline, nutritional risk, sarcopenia risk, or pre-frailty/frailty will be randomly assigned to one of five groups: control, nutrition supplementation alone, resistance training alone, resistance training plus nutrition supplementation, or resistance-dominant multicomponent training plus nutrition supplementation. The intervention will last 12 weeks. Exercise interventions will be conducted three times per week, and nutritional supplementation will include leucine-enriched whey protein and vitamin D. The primary outcomes are the Short Physical Performance Battery score, FRAIL score, and activities of daily living/instrumental activities of daily living scores. Secondary outcomes include gait speed, Timed Up and Go test, sit-to-stand performance, handgrip strength, nutritional status, protein intake, serum 25-hydroxyvitamin D, calf circumference, and appendicular skeletal muscle mass index.

This study aims to determine whether resistance-dominant multicomponent training combined with leucine-enriched whey protein and vitamin D supplementation can improve intrinsic capacity and reduce disability risk in rural Chinese older adults. The study focuses on the locomotion and vitality domains of intrinsic capacity, which are closely related to mobility function, nutritional status, muscle reserves, and the ability to maintain independent daily living.

Eligible participants will be rural community-dwelling adults aged 60 years or older who present at least one of the following risk characteristics: reduced physical performance, low gait speed, nutritional risk, sarcopenia risk, or pre-frailty/frailty. After baseline assessment, participants will be randomly allocated in a 1:1:1:1:1 ratio to one of five study groups: control group, nutrition supplementation group, resistance training group, resistance training plus nutrition supplementation group, and resistance-dominant multicomponent training plus nutrition supplementation group.

The traditional resistance training program will include progressive functional resistance exercises such as chair sit-to-stand, chair-assisted squat, calf raise, elastic-band knee extension, elastic-band hip abduction, elastic-band rowing, wall push-up, and shoulder press. The resistance-dominant multicomponent training program will use resistance training as the core component and will additionally include balance, gait, aerobic, flexibility, and breathing relaxation exercises. Exercise sessions will be conducted three times per week for 12 weeks.

The nutritional supplementation will consist of approximately 20 g/day whey protein, 2.5-3.0 g/day total leucine, and 800-1000 IU/day vitamin D3. The supplement will be administered twice daily. On training days, one dose will be taken within 30 minutes to 2 hours after exercise, and another dose after breakfast. On non-training days, the supplement will be taken after breakfast and dinner.

Outcome assessments will be performed at baseline and at week 12. The primary outcomes are the Short Physical Performance Battery score, FRAIL score, and activities of daily living/instrumental activities of daily living scores. Secondary outcomes include 4-meter gait speed, Timed Up and Go test, five-times sit-to-stand test, 30-second chair stand test, handgrip strength, Mini Nutritional Assessment Short Form score, daily protein intake, serum 25-hydroxyvitamin D, body mass index, calf circumference, appendicular skeletal muscle mass index, and quality of life. Safety outcomes include renal function, blood calcium, falls, musculoskeletal injury, gastrointestinal symptoms, and other adverse events.

Inclusion Criteria:

1. Aged 60 years or older.
2. Rural residents who have lived in the study area for at least 6 months and have no plan for long-term absence during the study period.
3. Able to walk independently or with a cane for a short distance.
4. Meeting at least one of the following risk characteristics: Short Physical Performance Battery score of 9 or lower, usual gait speed of 0.8 m/s or lower, Mini Nutritional Assessment Short Form score of 11 or lower, SARC-F score of 4 or higher, possible sarcopenia according to the Asian Working Group for Sarcopenia 2019 criteria, or FRAIL score of 1 to 3.
5. Able to understand the study requirements and provide written informed consent.

Exclusion Criteria:

1. Unstable cardiovascular or cerebrovascular events within the past 6 months.
2. Severe uncontrolled hypertension, defined as resting systolic blood pressure of 180 mmHg or higher or diastolic blood pressure of 110 mmHg or higher.
3. Severe cognitive impairment that prevents understanding of or cooperation with the study procedures.
4. Severe osteoarticular disease or neurological disease affecting exercise safety.
5. Severe renal dysfunction, such as estimated glomerular filtration rate lower than 30 mL/min/1.73 m², or considered unsuitable for protein supplementation by the study physician.
6. Hypercalcemia, active kidney stones, or contraindications to vitamin D supplementation.
7. Allergy to whey protein or any component of the nutritional supplement.
8. Regular use of protein powder, beta-hydroxy-beta-methylbutyrate, creatine, or other nutritional supplements that may affect muscle metabolism within the past 3 months.
9. Participation in another interventional study.

Individual participant data will not be shared because the study involves personal health information from rural older adults, and no formal data-sharing plan or participant consent for public data sharing has been established.

Due to the nature of the exercise intervention, participants and exercise instructors cannot be fully blinded to exercise allocation. Outcome assessors will be blinded to group assignment. For the nutritional supplementation component, the active supplement and the isocaloric placebo will be similar in appearance, taste, packaging, and administration frequency.