Plant proteins are usually less digestible than animal proteins, but they may benefit gut health through effects on the microbiome. The long-term impact of diets rich in plant products on protein digestion and metabolism is still unknown, especially in older adults with higher protein needs. This study aims to compare the digestion and use of pea proteins in young and older adults, both vegetarians and omnivores. Volunteers are divided into four groups: young omnivores, young vegetarians, older omnivores, and older vegetarians. They take part in two clinical investigation days. On the first day, nitrogen retention and protein metabolism is measured after consumption of a pea-based meal. On the second day, amino acid digestibility of pea is evaluated. These results will provide valuable data on how plant proteins are digested and metabolized depending on age and diet. They will also help determine whether aging reduces the availability of plant proteins. This knowledge is important to support nutritional strategies for populations with specific protein needs, such as older adults.