The goal of this clinical trial is to test the effect of different protein-enhancement strategies versus carbohydrate control on exercise-induced muscle damage in physically active older adults. The main question it aims to answer is:

What is the effect of three different protein-enhancement strategies (whey protein (WP), mixed plant-based protein (MPP), and use of protein-rich food products (PFP) on exercise-induced muscle damage in older adults compared to isocaloric carbohydrate control?

Participants will:

• Use a protein-enhancement strategy (WP, MPP or PFP) or carbohydrate control for a total of 5 weeks;
• Participate in a single walking bound (30/40/50 km) two weeks after the start of the study;
• Participate in a consecutive walking bound (4 days of 30/40/50 km) in the last week of the study period;
• Complete different questionnaires and give blood samples to test for muscle damage.

Inclusion Criteria:

• 60 years or older
• Registered for the Nijmegen Four Days Marches (i.e., able to walk 30-50 km/day)
• A habitual protein intake <1.0 g/kg/d based on the protein 55+ (PRO55+) screening tool
• Able to understand and perform the study procedures
• Able to use a mobile device

Exclusion Criteria:

• Allergic or sensitive for milk proteins, or lactose intolerant
• Diagnosed type I or type II diabetes mellitus
• Diagnosed intestinal diseases influencing the uptake of protein (i.e., active inflammatory bowel disease, Crohn's disease)
• Consumption of other freely available protein supplements during the study period.
• If the subject intends to perform additional exercise bouts that cause muscle damage in the 4 days before the single- and multiple exercise bouts.

Individual Participant Data (IPD) will be shared with researchers within the department of Medical Biosciences of the Radboud University Medical Center (Radboudumc). The department has a database of data from the past 15 years of Four Day Marches data. This research data will be added to the database. Data may be shared with other researchers upon request.

The data will become available after completion of the study and will be available for at least 15 years.

IPD will be shared with researchers within the department of Medical Biosciences of the Radboudumc. Data may be shared with other researchers upon request.

The study consists of 4 arms of which 3 arms that receive a supplement and 1 arm that will use an e-health application to enhance protein intake. The 3 supplement arms are double-blind. The e-health arm is not; since it is not possible to blind either the participants in this group neither the investigators.