This study aims to understand why people with chronic kidney disease (CKD) find it hard to exercise and why their blood pressure rises during exercise. CKD means the kidneys can't filter blood well. Exercise capacity is how well someone can exercise. The study will test if regular exercise on a stationary bicycle or taking 6R-BH4 pills (a drug for a genetic disorder, not approved for CKD) can help. It will also see if histidine and beta-alanine supplements (common in sports) improve exercise measures.

Researchers will first measure how CKD patients' veins react to adrenaline (a hormone that increases heart rate) and check muscle health during exercise. Then, participants will either do aerobic exercise or stretching, with or without 6R-BH4 or supplements, for 3 times a week over 6 to 14 weeks.

• The study includes CKD patients and non-exercisers who can follow the study plan.
• Participants with severe CKD, certain health issues, or pregnancy cannot join.
• The study could help improve exercise capacity and blood pressure in CKD patients.