Immunosenescence is the age-related decline of the immune system, involving a state of chronic inflammation and a decrease in the diversity/adaptability of the lymphocyte repertoire. The consequences of immunosenescence are multiple, including increased susceptibility to infections and poorer vaccine responses.

In order to fight against immunosenescence, many therapeutics are envisaged, with the aim of restoring, renewing and/or reprogramming the immune system. Among these leads, therapeutic fasting, and in particular the "fasting mimicking diet" (FMD), seems to modify the reprogramming and renewal of the immune system with, on the one hand, the elimination of inefficient "overactivated" cells and, on the other hand, the activation of hematopoietic stem cells. FMD consists of a significant reduction in food intake, limited to 3-5 days/month. The FMD seems to be the best compromise between efficacy and tolerance (already evaluated in humans, including oncology).

Inclusion Criteria :

• Patient affiliated or entitled to a social security scheme
• Residing in a Long-Term Care Unit (LTCU) or a Facility for the Elderly Dependent (EHPAD)
• A person deemed a priori, by the geriatrician member of the monitoring committee, to be capable of following the dietary recommendations required for the study.
• A person deemed a priori, by the clinician in charge, to be able to follow the dietary recommendations required by the study.
• Patient who has received informed information about the study and has co-signed a consent to participate in the study with the investigator.

Exclusion Criteria :

• Inability and/or unwillingness to follow dietary recommendations and/or perform follow-up examinations required for the study
• Iso-resource group 1 or 2 according to the AGGIR grid
• BMI < 18.5 kg/m2 or recent weight loss > 5% in 1 month or recent weight loss > 10% in 6 months from previous weight
• Albuminemia < 30g/l
• All swallowing disorders requiring a specific diet.
• Influenza vaccination received prior to inclusion
• Episode of influenza or suspected influenza during the 2020-2021 season and prior to inclusion
• Any acute medical or surgical event less than 2 weeks prior to inclusion
• Presence of systemic inflammatory or autoimmune disease
• Presence of an innate or acquired immune deficiency (including taking immunosuppressive treatment)
• Presence of a chronic disease which, according to the physician, makes the follow-up of a "fasting diet" and/or the consumption of large amounts of liquids (cf. fasting diet modalities) risky (e.g. insulin-dependent or insulin-requiring diabetes, severe heart failure, severe kidney failure, active cancer, significant undernutrition...) or impossible (e.g. advanced neurodegenerative pathology).