Imagine someone hauling two sacks up a hill. One is stuffed with sugar the body cannot process. The other is filled with the years the body has lived faster than the calendar. While the sacks are light, the climb is manageable. But if both are filled to the brim, the legs give way, and the person collapses not with one disease but with several at once.
That is exactly the picture researchers saw after following 8,767 residents of China for nine years in the CHARLS project. They tracked two things: how well the body handles sugar (shown by the TyG index, calculated from blood tests for glucose and triglycerides) and how quickly the body ages compared with passport age. The latter was assessed using blood biomarkers: creatinine, glucose, and C-reactive protein. These three substances are routinely measured in any clinic, but together they can say more about biological age than a birth date.
Researchers divided participants into four groups: those with both measures normal, those with only one elevated, and those with both high. Over nine years, 873 people (one in ten) developed cardiometabolic multimorbidity, meaning at least two cardiovascular diseases appeared, such as a heart attack and a stroke, or heart failure together with diabetes. In the group with two heavy sacks, the risk was 2.37 times higher than in those who traveled light.
Next, the researchers tried to merge the two measures into one by multiplying the TyG index and biological age. This produced the composite TyG-BA index. Each one-standard-deviation increase in this index raised the risk of multimorbidity by 62%. The relationship turned out to be nonlinear: up to a certain threshold, risk grew slowly, and beyond it, it accelerated sharply. When the researchers looked at four-year trajectories, people with a persistently high index had a 5.17 times higher risk than those whose index remained low.
But here is the important part: the combined index was not more accurate than the two separate measures. It predicted who would get sick well, but no better than if a doctor simply looked at TyG and biological age separately. So the authors honestly write: the composite index is a convenient tool for quickly screening people into a risk group, not a new biomarker that replaces everything else.
Returning to the hill metaphor: the two sacks can be carried separately, and the doctor sees each one. But if you tie them together with one rope, carrying becomes a bit more convenient, though the weight does not change. The same goes for the TyG-BA index: it adds no new information, but it helps you spot faster the person who needs to drop the load before it is too late.
Biological age by the Light-BA method uses just three biomarkers that are routinely measured in an ordinary clinic: creatinine, glucose, and C-reactive protein. This makes aging assessment accessible in virtually any laboratory.
The idea of measuring biological age and its link to disease echoes the film 'In Time' (2011), where people have built-in clocks counting down their lifespan. In reality, biological age can be estimated from a blood test, and it predicts the risk of multiple diseases.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.