Picture every blood test as a dot on a large map. Plot thousands of those dots and they won't scatter at random: they line up along two main roads. One road separates men from women, the other stretches from youth to old age. Follow these roads and you can see how a person grows up, lives and grows old.
Researchers drew on data from NHANES, a large American health and nutrition survey. For each age and sex, they averaged 54 blood markers, from hormone levels to cell counts. Then they applied a method that compresses 54 dimensions into two main axes, as if we were looking at a three-dimensional object from its most revealing angle. The first axis turned out to be sex: in teenagers the male and female trajectories diverge, by ages 20-40 the difference peaks, and in old age they converge again. The second axis is age: it shows how blood composition changes over the years.
To measure how fast a person moves across this map, the researchers introduced a simple metric: the distance covered in a year. It turned out that women's speed rises sharply during menopause, and after 75 everyone speeds up. But the most interesting part is the comparison between healthy people and the general population. Those without chronic diseases lag behind their peers: by age 79 their bodies look 16 years younger. Obesity, smoking and depression also push the trajectory forward, but each factor works its own way: obesity starts to matter as early as adolescence, smoking in adulthood, and depression hits middle-aged women hardest.
A life-course map built from a drop of blood helps us see aging not as a smooth decline but as a series of stages moving at different speeds. Researchers used to reduce the body's age to a single number, but now it's clear that at least two coordinates matter: sex and age. This approach can be applied to any health data to compare groups of people and assess how much a lifestyle or a disease accelerates the journey along life's road.
By age 79, healthy people lag behind the general population along the physiological trajectory by 16 years, meaning their body looks as if they were 63.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.