About 900 of these people were seniors who underwent regular checkups at medical clinics to assess their health. Dr. Gibbons and his colleagues found that in midlife, starting at around age 40, people started to show distinct changes in their microbiomes. The strains that were most dominant in their guts tended to decline, while other, less common strains became more prevalent, causing their microbiomes to diverge and look more and more different from others in the population.
“What we found is that over the different decades of life, individuals drift apart — their microbiomes become more and more unique from one another,” said Dr. Gibbons.
People who had the most changes in their microbial compositions tended to have better health and longer life spans. They had higher vitamin D levels and lower levels of LDL cholesterol and triglycerides, a type of fat in the blood. They needed fewer medications, and they had better physical health, with faster walking speeds and greater mobility.
The researchers found that these “unique” individuals also had higher levels of several metabolites in their blood that are produced by gut microbes, including indoles, which have been shown to reduce inflammation and maintain the integrity of the barrier that lines and protects the gut. In some studies, scientists have found that giving indoles to mice and other animals helps them stay youthful, allowing them to be more physically active, mobile and resistant to sickness, injuries and other stresses in old age. Another one of the metabolites identified in the new study was phenylacetylglutamine. It is not clear exactly what this compound does. But some experts believe it promotes longevity because research has shown that centenarians in northern Italy tend to have very high levels of it.
Dr. Wilmanski found that people whose gut microbiomes did not undergo much change as they got older were in poorer health. They had higher cholesterol and triglycerides and lower levels of vitamin D. They were less active and could not walk as fast. They used more medications, and they were nearly twice as likely to die during the study period.
The researchers speculated that some gut bugs that might be innocuous or perhaps even beneficial in early adulthood could turn harmful in old age. The study found, for example, that in healthy people who saw the most dramatic shifts in their microbiome compositions there was a steep decline in the prevalence of bacteria called Bacteroides, which are more common in developed countries where people eat a lot of processed foods full of fat, sugar and salt, and less prevalent in developing countries where people tend to eat a higher-fiber diet. When fiber is not available, Dr. Gibbons said, Bacteroides like to “munch on mucus,” including the protective mucus layer that lines the gut.
“Maybe that’s good when you’re 20 or 30 and producing a lot of mucus in your gut,” he said. “But as we get older, our mucus layer thins, and maybe we may need to suppress these bugs.”