If you eat a lot of salty food, you pee less. And (drum roll please), if you eat a lot of watermelon, you pee more!
Anyway, this explains why doctors take the piss from their patients. That is, they want urine samples because they show what people eat and drink. But pee also shows how well your body functions and gives clues to problems like “infections, inherited metabolic disorders, kidney disease, bladder cancers, diabetes, substance abuse, exposure to toxins, inadequate or excessive fluid intake.”
Conclusion: They call urine #1 for a reason!