What is Urea?

Urea production rises and falls with how much protein is being broken down. The kidney complicates that: it filters urea out and then reabsorbs part of it, and how much it reabsorbs depends on the body's fluid state. Blood urea therefore carries two signals at once, what a person has been eating and how well hydrated they are.

Creatinine arrives at a steadier pace, because it comes from muscle rather than food. Comparing the two shows whether a change in urea belongs to the kidney or to diet and hydration. eGFR is built on creatinine alone, so it carries none of that diet and hydration signal.

Protein in the diet drives production; the body's fluid state governs how much of the filtered urea comes back. And if the kidneys are filtering more slowly, urea accumulates whatever the diet.

Urea can climb within a day of a protein-heavy meal or a spell of dehydration, then fall back over the following days. Kidney filtration changes far more slowly than that. A year between measurements averages out the noise from meals and drinking, so what shows through is the kidneys' filtering rather than the last few days' habits. This is Do Health's monitoring schedule, not a general recommendation for everyone.