Inclusion Criteria:
* Men and women aged 18 to 65 years.
* History of at least two upper respiratory tract infections during the previous winter season.
* Body mass index between 18 and 30 kg/m².
* Generally healthy, as determined by the investigator.
* Women of childbearing potential must have a negative pregnancy test and agree to use an effective method of contraception during the study.
* Covered by French health insurance.
* Able and willing to provide written informed consent.
Exclusion Criteria:
* Current or recent upper respiratory tract infection or COVID-19 infection within 2 weeks before enrollment.
* Acute gastrointestinal infection, including gastroenteritis, within 2 weeks before enrollment.
* Use of non-steroidal anti-inflammatory drugs (NSAIDs) within 2 weeks before enrollment.
* Use of systemic antibiotics within 3 months before enrollment.
* Receipt of any vaccination within 6 weeks before enrollment.
* Surgery requiring general anesthesia within 1 month before enrollment.
* History of chronic respiratory disease, including asthma or chronic obstructive pulmonary disease.
* Food allergy or chronic/recurrent allergic upper airway conditions requiring regular medical treatment, including moderate to severe allergic rhinitis, hay fever, or chronic rhinosinusitis.
* Acute lower respiratory tract infection within 6 months before enrollment or ongoing long COVID.
* Immunodeficiency (including HIV infection) or autoimmune disease, except stable hypothyroidism.
* Severe or uncontrolled chronic disease, including cancer, significant liver, pancreatic, biliary, or renal disease, inflammatory bowel disease, or cardiovascular disease requiring intensive medical management.
* Untreated or inadequately controlled diabetes.
* History or current diagnosis of periodontitis.
* Current chemotherapy, immunosuppressive therapy, or chronic treatment with corticosteroids or opioids.
* Specific diets or major dietary changes within 3 months before enrollment, including high-protein, vegan, low-calorie, or low-FODMAP diets, or participation in a weight-loss program.
* Current use of dietary supplements that may affect immune or gut microbiota parameters, or discontinuation of such supplements less than 6 weeks before enrollment.
* Current smoking or e-cigarette use, or smoking cessation within 6 months before enrollment.
* Current drug or alcohol abuse, or a history of abuse within the previous 12 months.
* Regular consumption of more than 2 alcoholic drinks per day and unwillingness to maintain stable drinking habits during the study.
* Unwillingness to refrain from using over-the-counter cold remedies, nasal washes or sprays, antihistamines, corticosteroids, or NSAIDs during the study.
* More than 8 hours per week of vigorous physical activity or planned major changes in physical activity during the study.
* Any condition or circumstance that, in the investigator's judgment, may place the participant at unacceptable risk or interfere with study participation or interpretation of study results.
* Current participation in another clinical trial or being in an exclusion period from a previous clinical trial.
* Pregnant or breastfeeding.
* Persons deprived of liberty, under legal protection, or unable to provide informed consent.
* Individuals who have reached the regulatory compensation limit for participation in clinical research.
