The aim of the study is to compare the use of amino acids in whole-body protein metabolism in elderly consumers at risk of metabolic syndrome, after ingestion of 2 foods differing only in the nature of their proteins (optimized blend of vegetable proteins versus milk proteins).

The protocol consists in studying the postprandial evolution of protein metabolism. Two products will be tested: one based on dairy proteins, the other on an optimized blend of vegetable proteins).

Written consent will be obtained from subjects after they have been informed of the aims, nature and possible risks of the study.

Prior to inclusion, subjects will have a medical check-up carried out in the Nutritional Exploration Unit (UEN), including an interview about personal and family history and medication taken, as well as a standard medical examination and a blood test for a biological work-up (9.5 ml). Compliance with inclusion/exclusion criteria will be verified during this examination.

Subjects included in the study will come to the UEN to undergo the pre-study dietary visit and to complete the two study days of 9.5 hours each, at least two weeks apart.

For each study day, volunteers will arrive in the morning with an empty stomach. Two vascular catheters will be implanted. One catheter will provide continuous intravenous infusion of a labelled amino acid (13C Leucine) for a total duration of 9.5 hours. The second catheter will be used for blood sampling (8 ml), performed on an empty stomach and then, kinetically after ingestion of one of the two study products, in the form of food prepared on site. The IV-infused tracers will be used to monitor the kinetics of leucine appearance in plasma, and to quantify amino acid utilization flows at whole-body level (protein synthesis, degradation). Blood samples will also be taken to determine aminoacidemia, glycemia, insulinemia and plasma concentrations of hormones involved in regulating food intake (grelin, cholecystokinin, Glucagon-like-peptide-1 (GLP1), peptide YY (PYY).

Exhaled gases will be collected during the study days. Indirect calorimetry measurements lasting 20 minutes will be taken in the post-absorptive and post-prandial periods. Seven indirect calorimetry measurements will be taken on each study day. Finally, subjects will be asked to rate their satiety on a visual analog scale, at different times during the study days.

Subjects will also be asked to complete a tolerance questionnaire on each study day.

At the end of each study day, volunteers will have a snack at the study center before their departure. Any intercurrent events will be recorded in the observation notebook.

Allocation of the product sequence to subjects will be carried out by a random draw (generation of a random list) before the start of the study.

The volume of blood drawn per volunteer is 128 ml/study day, to which must be added 9.5 ml for the biological screening test, i.e. 265.5 ml for the entire study.

During the 3 days preceding each study day, the subjects' protein intake will be monitored. Standard menus were proposed, providing 1.2 g protein/kg/d. The meal on the evening before each study day will be standardized and identical for each day. To check compliance with the dietary instructions, volunteers will complete a dietary record for the 3 days preceding each study day.

Inclusion Criteria:

• Waist circumference > 94cm
• Triglycerides > 1.5 g/L or HDL cholesterol < 0.4 g/L
• Fasting blood glucose < 1.26g/L
• Biological profile considered compatible with study participation,
• Venous status judged satisfactory by the nurses, allowing catheter placement for the study,
• Individuals able to sign the informed consent form,
• Persons subject to a social security scheme.

Exclusion Criteria:

• Subjects on special diets (vegetarian, vegan, etc.),
• Subject showing a weight loss ≥ 5% in 1 month or ≥ 10% in 6 months compared to usual weight.
• Subject refusing to stop taking dietary supplements for the duration of the study,
• Subject refusing to consume any of the proposed products,
• Subject with food allergy(ies) to one or more components of the products under study, or with a contraindication to the consumption of these products,
• Subjects with a pathology or treatment (antidiabetics, hypolipidemics, corticoids, neuroleptics) judged by the investigator to be incompatible with the study,
• Subjects with dysthyroidism that has not been stabilized for at least 3 months,
• Subject with unstabilized hypertension for at least 3 months,
• Subject with a medical and/or surgical history deemed by the investigator to be incompatible with the trial,
• Subject having undergone surgery that could interfere with the study objectives (as judged by the investigating physician) in the 3 months preceding the study,
• Smoking > 5 cigarettes/day,
• Alcohol > 3 glasses/day,
• Claustrophobic subject,
• Person refusing to be registered with the French Ministry of Health's Fichier National des Volontaires Sains,
• Major under guardianship or with limited rights,
• Subject not affiliated to social security,
• Subject in a period of exclusion from a previous study or having received a total amount of compensation exceeding 6000 euros over the 12 months preceding the start of the trial (after verification in the Fichier des Volontaires pour la Recherche Biomédicale).

Participants will not know which protein ingredient is included in the test-meal.