The purpose of this study is to determine if taking iron supplement pills improves exercise performance in low-oxygen conditions.

Hypoxia (low oxygen) causes the blood vessels in the lungs to constrict (hypoxic pulmonary vasoconstriction). This increases the pressure (afterload) the right ventricle faces as it pumps blood to the lungs. Increased right ventricular afterload during hypoxia may compromise exercise capacity. Intravenous iron administration prior to hypoxic exposure has been shown to blunt the hypoxia-induced increase in right ventricular afterload. This may be through iron's action in the Hypoxia Inducible Factor (HIF) pathway. Iron is a cofactor for prolyl hydroxylases that degrade HIF subunits and thus may "turn off" HIF-related pathways of pulmonary artery vasoconstriction and remodeling. However, it is not known whether oral iron supplementation similarly reduces right ventricular afterload in hypoxia, or what impact iron has on right ventricular function and exercise capacity in hypoxia.

This is a human physiology study that will characterize the impact of oral iron supplementation on right ventricular function and exercise performance in hypoxia. It is a follow-up "sub-study" to a separate, "parent" study (NCT05272514) by the same investigators which evaluates resting and exertional right ventricular performance in normoxia and hypoxia in 10 healthy individuals. In this follow-up study, 5 individuals who completed the parent study will be eligible to enroll. As part of the parent study, participants will complete baseline echocardiography to assess right ventricular function and cardiopulmonary exercise testing to assess exercise performance in normoxia and hypoxia. After enrolling in this study, participants will take an oral iron supplement (ferrous sulfate 325 mg oral daily) for 30 days. They will then return for one visit. First, participants will complete submaximal exercise while breathing room air. Submaximal exercise will include 5 minutes each at 40% and 60% of baseline hypoxic (fraction of inspired oxygen [FiO2] 12%) maximal oxygen uptake (VO2max) achieved during parent study. After 10 minutes' rest, echocardiographic measurements will be obtained at upright rest with FiO2 21%, 17%, 15%, and 12% to characterize the impact of progressive hypoxia on resting right ventricular function. Participants will then repeat submaximal exercise tests at FiO2 12%, followed by a short period of recovery. Thereafter, participants will complete a symptom-limited cardiopulmonary exercise test at FiO2 12%. Measurements will include heart rate/rhythm, oxygen saturation, blood pressure, gas exchange parameters (oxygen uptake [VO2], carbon dioxide production [VCO2], and minute ventilation), rated perceived exertion and resting echocardiographic measurements.

Inclusion Criteria:

• Age 18 - 60
• For women, premenopausal status

Exclusion Criteria:

• Active cardiovascular or pulmonary disease (e.g. hypertension, coronary artery disease, cardiomyopathy, arrhythmia, valvular abnormalities, diabetes, peripheral vascular disease, tobacco use, chronic obstructive pulmonary disease, asthma, interstitial lung disease, restrictive lung disease, or pulmonary hypertension)
• Use of cardiac- or pulmonary-related medications
• Prior history of high altitude pulmonary edema or high altitude cerebral edema
• Body mass index < 18.5 or > 30
• Anemia
• Iron deficiency
• Iron supplementation (oral or intravenous) in the preceding 60 days
• Systemic anticoagulation or aspirin use that cannot be temporarily held for the study
• Pregnancy
• Non-cardiopulmonary disorders that adversely influence exercise ability (e.g. arthritis or peripheral vascular disease)
• Dedicated athletic training (defined here as spending >9 hours per week in vigorous physical activity [≥6 mets])
• Regular high-altitude exercise (defined here as engaging in vigorous physical activity [≥1 hour at ≥6 mets] at ≥8,000 ft for >2 days per week over the preceding 4 weeks)
• Residence at ≥8,000 ft for 3 or more consecutive nights in the preceding 30 days