A translator who has finally turned off the noise in the hall hears every word for the first time. In much the same way, urinary aging clocks long confused the noise of the kidneys with the signal of age: the kidneys filter urine, and their work changes the mix of protein fragments by which we judge the wear and tear on the body.
Researchers collected urine from 1811 people with healthy kidneys and trained a model to predict age from 12 019 peptides (small protein fragments). Then, in a large group of 7798 people with varying kidney function, they built a calibration surface: it adjusts the prediction depending on how quickly the kidneys clear the blood and how much of the protein albumin leaks into the urine. The correction kicks in only when the filter is impaired, and leaves healthy people untouched.
After the correction, the k-UPBioAge clock explained 87.6% of the variation in age with an error of 4.91 years. An acceleration of age by one standard deviation raised the risk of death by 48% after accounting for all factors. Acceleration also predicted heart attack, heart failure, and progression of kidney disease. In people with diabetes and kidney disease, age acceleration reached 13.1 years compared with healthy individuals.
Now the kidney has become an axis of calibration rather than an obstacle. The urinary clock reads as a systemic signal of aging, not as a reflection of the filter. This approach can be transferred to other body fluids produced by organs. And if diabetes or blood pressure drugs push the clock backward, we will have a cheap way to assess the effectiveness of anti-aging therapy. The translator is no longer bothered by noise, and he hears how the body ages.
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.