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.

Plant proteins generally have lower digestibility than animal proteins but also provide positive effects on the digestive system, particularly on the gut microbiota. However, the long-term effects of consuming a diet rich in plant-based products on protein digestion are still unknown. In the context of the nutritional transition toward more plant-based diets, it is important to comprehensively study the effect of such diets on the bioavailability of plant proteins. This is especially relevant for populations with specific protein needs, such as older adults.

The aim of the study is to compare the digestive and metabolic bioavailability of pea proteins in young and older adults, vegetarians and omnivores.

Peas are labeled with stable isotopes (15N and 2H), which are completely safe for health. Four groups of 8 healthy volunteers (+ 4 possible dropouts) are recruited according to age and dietary habits: young omnivores (18-23 years), older omnivores (65-75 years), young vegetarians, and older vegetarians. The volunteers' diets are characterized using an online food frequency questionnaire.

Each volunteer participates in two clinical investigation days, separated by 4 weeks to 2 months. On the first investigation day, a postprandial test is conducted after ingestion of a pea-based meal (100 g dry weight) labeled with 15N, consumed in normal conditions (as a bolus). Blood samples are taken regularly over 8 hours (156 mL total), and urine is collected every 2 hours. Postprandial kinetics of plasma amino acids concentrations, incorporation of dietary nitrogen in plasma protein and free amino acids as well as plasma gut hormones and protein metabolism markers are determined. Desamination of dietary protein is evaluated through determination of dietary nitrogen in plasma and urinary urea.

On the second investigation day, participants undergo the dual isotope tracer method to assess the digestibility of amino acids from pea proteins. They consume a pea-based meal (100 g dry weight) labeled with 2H, distributed in portions every 30 minutes for 7.5 hours. Free amino acids labeled with 13C are added to each portion. Blood samples are collected regularly over 8 hours (130 mL total). Respiratory exchanges are measured by indirect calorimetry in 15-minute periods every hour, and breath samples are collected regularly. 2H and 13C enrichment in individual amino acids are determined in plasma from 5 to 8h during a plateau. Oxydation of 13C amino acids is evaluated through measurement of 13CO2 in expired air.

The results will provide data on the digestive and metabolic bioavailability of plant proteins and amino acids in young and older individuals, whether vegetarian or omnivore. This study will therefore contribute to advancing knowledge about the digestibility of plant proteins and how it changes with aging, a question for which little data is currently available.

Inclusion Criteria:

• Normal weight or overweight (18 < BMI < 30 kg·m-²)
• Male or female
• Aged 18-23 years (young groups) or 65-75 years (older groups)
• Following an omnivorous diet (regular consumption of meat products, with meat intake > 3 times/week and > 70% of protein intake from animal sources) or vegetarian diet (exclusion of meat and fish/seafood, with > 70% of protein intake from plant sources)
• In good general health (WHO = 0)
• Affiliated with a social security system
• Free and informed consent, provided in writing after receiving the information required by the Public Health Code

Exclusion Criteria:

• Adults under legal protection or unable to give informed consent (e.g., under guardianship, trusteeship, or legal protection)
• Any known food allergy
• Positive serology for HBsAg, anti-HBc, HCV, or HIV
• Anemia: hemoglobin level < 13 g/dL in men and < 12 g/dL in women
• Pregnant women or those who may be pregnant (based on a positive urine pregnancy test at inclusion)
• Excessive alcohol consumption (> 2 drinks/day). Harmful alcohol use will be assessed by the investigator at inclusion.
• Hypertension, diabetes, gastrointestinal, liver or kidney diseases, or severe heart disease. These conditions will be assessed by the investigator based on standard clinical evaluation and participants' self-reports at inclusion.
• Hypertension: significant arterial hypertension according to the investigator, or systolic blood pressure ≥ 140 mmHg and/or diastolic blood pressure ≥ 90 mmHg at inclusion.
• Diabetes: type 1 or type 2 diabetes, or fasting blood glucose > 1.25 g/L.
• Gastrointestinal disease: clinically significant gastrointestinal disorders (bleeding, vomiting, constipation/diarrhea grade > 1) as judged by the investigator, any inflammatory bowel disease, or acute gastroenteritis in the month prior to the intervention.
• Liver disease: any significant hepatic disorder according to the investigator, or ASAT/ALAT > 2.5 times the upper normal limit.
• High-level athletes (> 8 hours of training per week)
• Blood donation within 8 weeks prior to study start
• Absence of free, informed, written consent after receiving the information required by the Public Health Code
• Not affiliated with a social security system