The objective of this study is to assess to what extent goat's milk is better tolerated than cow's milk among individuals who report being intolerant to cow's milk, and, if so, to understand the underlying factors. To achieve this, digestive discomfort sensations and postprandial kinetics of nutrients after ingestion of cow's or goat's milk will be compared in two populations: individuals intolerant to cow's milk but not to goat's milk, and individuals who show no symptoms of intolerance to either cow's or goat's milk. For this purpose, approaches based on stable isotope labeling of milk will be employed.

Objective:

Although milk has many nutritional benefits, such as being rich in high-quality proteins and calcium, some people limit or exclude its consumption due to poor digestive tolerance. Lactose intolerance is a well-known cause, but other factors may occur, remaining poorly understood. Goat milk is perceived by consumers as causing fewer intestinal issues than cow milk, particularly for sensitive individuals. However, there is little scientific evidence to support this perception and to confirm the supposed health benefits of goat's milk over cow's milk. The objective of this study is to assess to what extent goat's milk is better tolerated than cow's milk among individuals who report being intolerant to cow's milk, and, if so, to understand the underlying factors.

Design:

This study is crossover study, randomized for the order of testing (goat milk or cow milk), conducted in a single-blind design (participants are not informed about the test meal, which is masked by aroma), involving two groups of at least ten volunteers: individuals intolerant to cow's milk but not to goat's milk, and individuals who show no symptoms of intolerance to either cow's or goat's milk. Each subject will have consumed both milks at the end of the two sessions, with a minimum one-month interval.

Postprandial Test Procedure:

Two days before the test, subjects must exclude consumption of the main dairy products (yogurt, cheese, milk, and custard creams). Volunteers arrive in a fasted state on the investigation day and are hospitalized for one day per session. On the first visit, volunteers bring a stool sample. Upon arrival, body composition is measured via bioelectrical impedance and a venous catheter is inserted. Blood samples are taken 30 minutes pre-meal, every 30 minutes for the first 4 hours, and hourly thereafter, totaling 291 mL. Urine is collected before the meal and every 2 hours post-meal. Respiratory is measured hourly using an indirect calorimeter. Digestive tolerance and hunger are assessed hourly using questionnaires. Volunteers remain semi-reclined for about 9 hours and fast until the experiment ends, receiving hourly water.

The test meal consists of aromatized 500 ml of cow's or goat's milk, labeled with stable isotopes: nitrogen-15 and deuterium. The incorporation rate is minimal and safe for human health, commonly used in clinical studies. The 15N enrichment is measured in collected biological samples (blood, urine) to track the digestive and metabolic fate of dietary amino acids, enabling precise evaluation of nutrient availability kinetics in the body. After the investigation day, volunteers can go home post-meal. A follow-up call the next day checks for digestive symptoms, with the doctor assessing the need for symptomatic treatment.

The protocol has been approved by the Ethical Committee and authorized by the French Agency of Drugs and Health. The personal data management will be in accordance with the regulation on personal data protection ("regulation n° 2018-493 du 20 juin 2018").

Inclusion Criteria:

• Individuals who report being intolerant to cow milk but not to goat milk (one group) or tolerant to both cow milk and goat milk (another group)
• Good general health (WHO grade = 0)
• Affiliated with a social security scheme
• Free, informed, and express consent, in accordance with the public health code: 'No research mentioned in 2° of Article L. 1121-1 can be conducted on a person without their free, informed, and express consent.

Exclusion Criteria:

• Individuals under guardianship or curatorship
• Individuals under legal protection
• Any known food allergies
• Anemia: hemoglobin levels below 13 g/dL for men and 12 g/dL for women
• Pregnant women or those likely to be (based on a positive urine pregnancy test at the time of inclusion)
• Excessive alcohol consumption (>2 drinks/day). Harmful alcohol consumption is assessed by the investigator at the time of inclusion.
• Hypertension, diabetes, digestive tract diseases (except for irritable bowel syndrome, IBS), liver or kidney diseases, severe heart disease. The presence of these conditions will be assessed by the investigator based on usual clinical criteria and the volunteers' declarations during inclusion. Hypertension: Significant arterial hypertension as determined by the investigator or systolic blood pressure greater than or equal to 140 mmHg and/or diastolic blood pressure greater than or equal to 90 mmHg on the day of inclusion. Diabetes: Type 1 or Type 2 diabetes or any fasting blood glucose level > 1.25 g/l. Digestive tract disease: Gastrointestinal disorders deemed by the investigator as clinically significant (bleeding, vomiting, constipation/diarrhea grade >1), any inflammatory bowel disease, acute gastroenteritis in the month preceding the intervention. Liver disease: Any significant liver disorder as determined by the investigator or any AST/ALT > 2.5 times the upper normal limit.
• Elite athletes (> 8 hours per week)
• Blood donation in the 3 months preceding the start of the study
• Participation in a clinical study in the 3 months preceding the study
• Lack of free, informed, and express consent