Iron is an essential micronutrient responsible for a whealth of biological processes of the human body. Iron plays a fundamental role especially in oxygen transport, by binding to hemoglobin in the blood circulation. Iron intake from the diet needs to be in balance the unavoidable losses that occur daily via different pathways. The recommended daily requirement of iron is based on the balance between absorption and losses and is calculated to maintain a balance so that the absorption equals losses. At higher altitudes, the absorption of iron may be higher due to adaptation mechanisms in response to low oxygen concentration, and to maintain a larger erytroid compartement. However, the long-term effects of altitude on iron balance are unknown. Filling this knowledge gap is important to better understand iron deficiency in populations living at high and moderate altitudes. Therefore, the investigators plan to study people that live at two different altitudes and measure iron absorption and losses over the long term. This will be useful to formulate specific recommendations for this population groups, to expand the knowledge base to better prevent iron deficiency in Switzerland but also worldwide. Participants will be asked to consume a dose of stable iron isotopes. After one year from isotope administration, the isotopes will be equally distributed in all body compartments and any change in the isotope abundance with the normal occurring body iron can be detected. From this point onwards, the investigators can observe the iron turnover and calculate iron absorption and losses per unit of time. At 4 different visits blood samples will be taken from each participant.

This study compares iron absorption in two population groups residing at different altitudes in Switzerland-one below 1000 meters above sea level and the other above 1500 meters. Additionally, the study aims to determine whether daily iron intake recommendations should be adjusted for high-altitude populations and to establish appropriate thresholds for diagnosing iron deficiency and iron deficiency anemia in these conditions. In each group 60 participants (30 men and 30 women) will be recruited. Each participant will be administered with an oral dose of iron isotopes in the form of ferrous sulphate 57FeSO4 dissolved in water. Ascorbic acid (vitamin C) will be additionally given with the isotope dose to increase the absorption and the enrichment in the body compartments and sirup will be added to the solution to confer a sweet flavour. There will be no other intervention throughout the course of the study.

The investigators will collect blood samples during the second year from study start at a frequency of 4 months for a total of 4 visits, at months 12, 16, 20 and 24. Venous blood samples for the measurement of the isotopic signal will be collected, and at two additional study visits the investigators will also measure hemoglobin using the so-called carbon monoxide (CO) rebreathing method that includes a CO rebreathing into a closed-circuit system in combination with pure oxygen. Before and after the 6 min re-breathing, capillary blood will be collected by finger prick in triplicates. At the study visits with blood withdrawal, the investigators will ask participants to fill questionnaires about dietary intake and physical activity.

Inclusion Criteria:

• Females and males aged 18-45 y
• BMI: 18 to 25 kg/m2
• Body weight <70 kg for females, <85 kg for males
• Plasma ferritin >25 µg/L
• Signed informed consent

Exclusion Criteria:

• Hb <12 g/dL (females), Hb <13 g/dL (males), corrected for altitude using the 2024 WHO guidance values for hemoglobin adjustments
• Inflammation (CRP > 5 mg/L)1
• Plasma ferritin >150 µg/L for females and >200 µg/L for males
• Chronic disease known to affect iron metabolism
• Chronic medications (except for oral contraceptives)
• Pregnancy (tested at screening) or intention to become pregnant during the course of the study
• Lactation
• High physical activity (MET-min/week > 3000 calculated according to the IPAQ, short version, administered at the screening)
• Difficulty with blood withdrawal
• Earlier participation in a study using stable iron isotopes or participation in any clinical study within the last 30 days
• Smoking
• Blood donation, blood transfusion or significant blood loss (more than 400 mL) over the past 6 months
• Intention to regularly donate blood during the 2 years duration of the study
• No residence in the respective location/altitude for at least the last 2 years
• No intention to reside in the respective location/altitude at least for the next 2 years
• Participants who are planning to spend an extended period (>12 weeks) at a different altitude from their residence location
• Spending more than 2 nights per week at a different location from residence location
• Inability to understand the information sheet and the informed consent form due to cognitive or language reasons