The purpose of this study is to determine the effects of interventions with high-protein diet alone or in combination with resistance training on muscle mechanical function (muscle power and strength), frailty status, functional performance, muscle mass and quality of life.

Study design: two-phase randomized controlled trial with three arms.

Phase 1 stabilization phase lasting one month. Participants with low protein intake (<1.0 g/kg/day) are provided with national guidelines for daily protein intake and publicly available information material and recommended to increase their protein intake to meet the guidelines. Compliance to the recommendation/ guidelines (eating >1.0 g/kg/day) will be evaluated by 4-day food records at the end of the stabilization phase. Only compliant participants are eligible for phase 2. Participants that are not compliant will be followed as "natural observational group" but will not be considered for the primary statistical analysis. Data collection in these participants will only include assessment of selected demographic, functional and nutritional parameters and semi-structured interviews to identify potential barriers for not improving protein intake. Semi-descriptive data will be presented if possible.

Phase 2: Three-arm randomized controlled trial lasting four months:

i) Protein-only (PROT): participants will receive milk-based protein-rich products with the aim of targeting a daily protein intake of 1.5 g/kg; ii) Exercise and Protein (EXEPROT): participants will receive the protein intervention as in the PROT-group combined with progressive explosive type heavy-resistance strength training two times per week iii) Recommendations (REC) participants receive the national nutritional recommendations for older adults and are recommended to follow these recommendations over the course of the study.

Nutritional data will be collected pre and post the stabilization phase. Collection of the primary and secondary outcomes will occur at baseline (pre phase 2) and at the 4-month follow-up (post phase 2). Nutritional assessment will also be evaluated halfway, at the 2-month follow-up.

Compliance Adherence to the exercise protocol and protein supplementation is recorded during the study. For the exercise protocol adherence will be considered as achieving minimum to 75% of valid training sessions, considered as minimum 70% of the exercises planned for each session. For the protein supplementation, adherence will be evaluated as daily protein intake of ≥ 1.35 g/kg (estimated from 24 hours recall and four-day food records) at the 2-month follow-up and at the 4-months follow-up. Compliance will also be estimated from during each delivery of products with a set of questions (e.g. how much of the supplement is consumed? Any changes to the habitual food intake?) and regular phone follow-ups. If participants are unable to reach the protein target additional face to face or phone interview will be planned to support adherence. For the REC-group adherence is defined as following the recommended intake of protein (1.0-1.3 g/kg/day).

Sample size and statistics Lower leg muscle power is the primary outcome of this study. Due to lack of studies similar to this study design (e.g. age and frailty status of the participants, type of exercise and level of protein supplementation) the investigators have calculated sample size using a combination of studies and methods. Based on findings from a 2017 study the effect of 12-weeks of protein supplementation (two daily supplements of 20 g milk protein) in combination with resistance training in +80-year-old healthy adults increased muscle power by 15% (SEM ± 5%) in comparison with -7% (SEM ± 6%) in the control group (receiving protein supplementation only). A result from a 2018 study indicate that 12 weeks of nutritional supplementation (0.8 g/kg/day, 1.2 g/kg/day or 1.5 g/kg/day) to pre-frail or frail older adults >70 years resulted in an increase in muscle mass (estimated by Dual energy X-ray Absorptiometry, DXA) of approximately 4% in the group receiving 1.5 g/kg/day. Unpublished data from the investigators research group show that change in muscle mass (estimated by DXA) accounted for 1,95% of the change in muscle power in older adults following 12 weeks of resistance training. Hence, the estimated effect of an increase in muscle mass of 4% on muscle power is 7,8%. Adding this to the results from the 2017 study gives an estimated change on 0,8% in the PROT-group. Assuming, that the change in muscle mass are comparable in the three groups the investigator therefore expect a change in muscle power of 15%, 0,8% and -7% with a standard deviation (SD) of 30 in the EXEPROT-, PROT- and REC-group, respectively.

Setting a power of 0.8 sample size with 37 participants in each arm should be enough to detect a significant difference in muscle power (significance level at 0.05). Adding 25% to account for dropouts a total of 150 participants is needed.

Intention-to-treat analysis will be performed. In addition, per-protocol analysis of participants that are compliant to the protocol will be performed.

Inclusion Criteria:

• Community-dwelling adults ≥ 80 years
• Pre-frail or frail (evaluated by the physical frailty questionnaire, SHARE-FI75+)
• Intact cognitive function (Mini Mental State Evaluation ≥4)
• Medically stable evaluated by medical screening including blood sample
• Able to participate in group-based exercise without personal assistance
• Able to understand, read and write Danish

Exclusion Criteria:

• Allergic/ intolerant to the interventions (kidney diseases, lactose intolerance etc.)
• On a weight losing diet

Outcome assessors will not be informed about allocation of the participants prior to the assessment of outcomes. The participants will be informed not to give any information to outcome assessors about the result of their randomization