Protein in the urine (proteinuria) reflects how well the kidneys are filtering and retaining essential proteins. Under normal conditions, little to no protein should pass into urine. Even small, persistent elevations—especially albumin—can signal early changes in kidney function or vascular health, often before other labs become abnormal. In otherwise healthy individuals, transient increases can occur after exercise, dehydration, or illness, but sustained proteinuria is more meaningful and may reflect subtle endothelial or metabolic stress.