Final answer:
Urobilinogen is most likely to be lost in dilute, alkaline urine due to its conversion to urobilin. Polyuria is indicative of conditions like diabetes insipidus or diabetes mellitus. Urine's color is mainly due to urobilin or urochrome and can indicate health issues when changed.
Step-by-step explanation:
In a dilute, alkaline urine, the constituent most likely to be lost (destroyed) is urobilinogen. Urobilinogen is susceptible to oxidation in alkaline conditions, which converts it into urobilin, making it less likely to be detected in urine tests. In the context of diabetes and urine analysis, the simplest way to distinguish between diabetes insipidus and diabetes mellitus is by the presence of polyuria, or the excretion of urine more than 2500 ml/day. The color of urine, primarily determined by urobilin or urochrome, can also be impacted by pathological conditions indicated by changes in color due to various causes.