The billions of bacteria that call your gut home may help regulate everything from your ability to digest food to how your immune system functions.

Bacteria in your gut may reveal your true age

But scientists know very little of how that system, known as the microbiome, changes over time or even what a “normal” one looks like. Now, researchers studying the gut bacteria of thousands of people around the globe have come to one conclusion: The microbiome is a surprisingly accurate biological clock, able to predict the age of most people within years.

To discover how the microbiome changes over time, longevity researcher Alex Zhavoronkov and colleagues at InSilico Medicine, a Rockville, Maryland–based artificial intelligence startup, examined more than 3600 samples of gut bacteria from 1165 healthy individuals living across the globe. Of the samples, about a third were from people aged 20 to 39, another third were from people aged 40 to 59, and the final third were from people aged 60 to 90.

The scientists then used machine learning to analyze the data. First, they trained their computer program—a deep learning algorithm loosely modeled on how neurons work in the brain—on 95 different species of bacteria from 90% of the samples, along with the ages of the people they had come from.

Then, they asked the algorithm to predict the ages of the people who provided the remaining 10%. Their program was able to accurately predict someone’s age within 4 years, they report on the preprint server bioRxiv. Out of the 95 species of bacteria, 39 were found to be most important in predicting age.

Zhavoronkov and his colleagues found that some microbes became more abundant as people aged, like Eubacterium hallii, which is thought to be important to metabolism in the intestines. Others decreased, like Bacteroides vulgatus, which has been linked to ulcerative colitis, a type of inflammation in the digestive tract.

Changes in diet, sleep habits, and physical activity likely contribute to these shifts in bacterial species, says co-author Vadim Gladyshev, a Harvard University biologist who studies aging.