Protein in Urine
Kidneys act as a filter for the blood.
Like a colander used to strain pasta, the protein
(pasta) is kept out of the urine
while the bad stuff (the boiling water) is passed through.
With kidney injury, the colander becomes less fine, with bigger openings. This, as you would expect, allows not just water but pasta (protein) to be lost. Causes for this are many. Consult a nephrologist.