Picture an interpreter who gets distracted by every rustle in the room and mixes up the words. Urine clocks of ageing worked much the same way for a long time: the kidneys filter urine, and that "noise" drowned out the signal of age.
Scientists collected urine from healthy people and trained a model to predict age from protein fragments. Then they measured how the error depends on kidney function: how much blood the kidneys clear per minute and how much of the protein albumin ends up in the urine. The result was a correction that switches off the kidney noise when the kidneys are working poorly.
After the correction, the clock predicted age with an error of 4.9 years. Accelerated ageing raised the risk of death by 48% and predicted heart attack, heart failure and kidney disease. Now the interpreter hears the words clearly, and we can learn how fast the body is ageing from a simple urine test.
The strongest single peptide in the clock comes from the protein PECAM-1, which sits on the surface of vascular cells. Its contribution to the age estimate is 0.634 years per one standard deviation.
The idea of a 'urinary clock' echoes Philip K. Dick's novel 'Do Androids Dream of Electric Sheep?', where age and health were determined by biological markers. Here urine becomes a similar window into biological age, only without the fantastic assumptions.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.