Sarcopenia and probable sarcopenia are associated with loss of muscle strength, functional impairment, frailty, and an increased risk of adverse outcomes in older adults. In short-term interventions, structural or functional changes may be modest, whereas certain serum biomarkers of metabolic-nutritional response may be more sensitive. In this protocol version, serum transthyretin (TTR) has been selected as the primary outcome, without assuming that it represents, by itself, a direct measure of muscle mass or overall clinical efficacy.

Sarcopenia is a progressive skeletal muscle disorder associated with aging and an increased risk of disability, hospitalization, dependency, and mortality. Contemporary consensus definitions identify low muscle strength as the primary characteristic of the condition, while muscle quantity or quality and physical performance are used to support the diagnosis and determine its severity.

In older adults, anabolic resistance reduces the muscle's responsiveness to habitual dietary protein intake. Leucine has been proposed as an amino acid capable of stimulating key anabolic pathways and therefore represents a plausible candidate for short-term nutritional interventions. In turn, Durvillaea incurvata flour provides bioactive compounds and dietary fiber with potential antioxidant and anti-inflammatory properties, although clinical evidence supporting its use in sarcopenia remains limited.

From a methodological perspective, a 28-day follow-up period may be sufficient to detect early metabolic and nutritional responses, but it may be insufficient to demonstrate robust clinical changes in muscle strength, muscle mass, or physical performance. For this reason, serum TTR has been selected as the primary short-term response outcome in the present protocol.

The selection of serum TTR as the primary outcome requires careful interpretation. Serum TTR levels may be influenced by protein-energy intake, liver function, inflammatory status, and intercurrent clinical conditions. Accordingly, in this protocol, serum TTR will not be interpreted as a direct surrogate of muscle mass or as a stand-alone measure of clinical efficacy. Rather, it will be considered a serum marker of early metabolic and nutritional response, whose interpretation will be complemented by functional, body composition, inflammatory, and safety-related outcomes.

This study is therefore designed as an exploratory, randomized proof-of-concept trial to determine whether the combination of leucine and seaweed flour improves serum TTR and whether this response is accompanied by consistent secondary changes in muscle strength, mobility, body composition, and the metabolic-inflammatory profile.

Inclusion Criteria

• Age between 60 and 80 years at the time of enrollment.
• Ability to understand the study procedures and provide written informed consent.
• SARC-F score ≥4 during screening.
• Reduced muscle strength defined as handgrip strength <27 kg in men or <16 kg in women; alternatively, a time >15 seconds on the five-repetition chair stand test when handgrip dynamometry cannot be validly performed or is contraindicated.
• Availability to attend study visits and comply with daily consumption of the study product for 28 days.
• Willingness to provide fasting blood samples at the Day 0 and Day 28 visits. Exclusion Criteria
• Cognitive impairment according to the abbreviated Mini-Mental State Examination (MMSE), using the operational cutoff approved for the study (≤13 points).
• Clinically significant chronic liver disease, clinically significant chronic kidney disease, nephrotic syndrome, active cancer or recent oncological treatment, or any condition that may substantially affect serum TTR levels or increase participant risk.
• Acute infection, acute inflammatory condition, or clinical decompensation at baseline that may interfere with the interpretation of serum TTR measurements.
• Uncontrolled diabetes mellitus or any metabolic condition that, in the investigator's judgment, may increase the risks associated with the intervention.
• Food allergy or intolerance to any component of the study product, including gluten, egg, milk, or other specific cake ingredients.
• Use of leucine, HMB, or protein supplements within the previous 3 months.
• Chronic use of systemic corticosteroids, immunosuppressive agents, or other therapies that may significantly alter the metabolic response under study.
• Untreated thyroid disease or any other unstable endocrine disorder.
• Clinically significant edema, severe dehydration, or any condition that may compromise the validity of bioelectrical impedance measurements.
• Any medical, functional, social, or cognitive condition that may limit safe participation or adherence to the study protocol.

Participants, outcome assessors, clinical personnel, investigators responsible for data collection, and the statistician will remain blinded to treatment allocation throughout the trial.

Study products will be packaged identically and presented with the same external appearance and coded labeling. The individual responsible for packaging and maintaining the code-treatment correspondence will not participate in clinical assessments or statistical analyses.

Unblinding will only be permitted in the event of a medical emergency or when knowledge of treatment allocation is essential for participant management. Any unblinding event will be documented, including the date, time, reason, requesting individual, and authorizing individual.