None of us need FaceApp to know the broad strokes of what will happen when we grow older: a few more wrinkles, a few more grey hairs, a little less muscle strength — accompanied, of course, by a lot more wisdom.
A lesser-known change that occurs with age has to do with the gut microbiota — the community of microorganisms (including bacteria, viruses, and fungi) found in the human digestive tract. These gut microbes, which have been characterized as an “organ” in their own right, are linked with various aspects of metabolism and overall health. Our community of gut microbes tends to shape-shift in older age. But what scientists haven’t figured out yet is whether it’s aging per se, or the lifestyle factors that accompany aging, that ultimately drive gut microbiota composition later in life.
Complex genetic and environmental factors contribute to gut microbiota composition from cradle to grave — but nevertheless, scientists have found some clear patterns. The story of a human’s gut microbes starts at the moment of birth: when born vaginally, an infant is colonized by the mother’s gut and vaginal microbiota; when born by cesarean section, the infant’s colonizing microbes come from adults’ skin and the hospital environment (1). A healthy infant soon establishes a relatively low-diversity gut microbiota, dominated by bifidobacteria; the exact composition of the gut microbiota at this stage depends heavily on how the baby is fed (breast or bottle). Moreover, the exact species and strains in the gut vary greatly from infant to infant.
The introduction of solid foods is a transition point for the infant’s gut microbiota, with a spike in the diversity of microbes when fiber- and protein-rich foods are introduced (2). This begins the trend that will continue through childhood and into adulthood: the process of gradually diversifying the gut microbiota (3). And although it’s true that every child and adult has a unique gut microbial signature, variability decreases over time, as fecal samples from two adults often have more in common than fecal samples from two age-matched children.
A few transition stages occur along the way to adulthood, with distinct compositional features present in pre-adolescents (4) and in 20-year olds (5), but the gut microbiota of the average healthy adult is relatively diverse and stable (6). In most healthy adults it actually tends to be quite stubborn — except in the case of a major perturbation such as a course of antibiotics (7).
But things start to change again in older adulthood. The exact time when this starts to happen is unclear, but one study documented distinct changes in gut microbiota after the age of 60 (8). Another found that after age 65 or so, there’s a return to the greater inter-individual variability (which, as you will recall, was observed in infants) (9). Diversity generally begins to decrease, while enrichment of previously non-dominant taxa occurs (10). Scientists also observe more pathobionts (potentially harmful bacterial groups) and a loss of potentially important genes in the gut microbiota, like the ones that encode for the production of short-chain fatty acids (11). And these age-related changes in gut microbiota may not be trivial, because they just might be related to frailty and broad aspects of physical health (12).
So are the changes in the gut microbiota in older adulthood unavoidable? Maybe not. Because when researchers focus in on the gut microbiota of older adults in different living situations, they find patterns according to lifestyle rather than age alone.
The best example: when Irish researchers compared the gut microbiota of older adults living in different settings, they found it clustered by where they lived (9): community, day-hospital, rehabilitation, or long-term residential care. Gut microbiota groups ranged from the most diverse (people living in the community) to the least diverse (in long-term care). It should be noted that each of these settings brings with it a different lifestyle — from diet to pattern of social interactions — so the scientists couldn’t say with certainty which aspect of lifestyle drove the gut microbiota composition. (Diet was a strong contender, though.) And the evidence supported the importance of these differences in health: less diversity was correlated with increased frailty (9).
More support for chronological age being less important than lifestyle was provided by a study that examined super-healthy older adults in China (5). It turned out that despite their advanced chronological age, these individuals harbored gut microbial communities that closely resembled healthy people of a younger age. Thus, it could be the case that later in the lifespan, lifestyle and overall health status are important determinants of what the gut microbiota looks like.
Despite the fact that researchers have identified some particular microbes that occur in the guts of very healthy older people (compared to those who aren’t as healthy), these don’t constitute a quick anti-aging fix for the gut (13). More study is required to figure out whether these special microbes are actually driving changes in health. But all in all, the evidence supports some very good news: by controlling your lifestyle in older age, you may be able to maintain a young-looking gut microbial community that supports excellent health.
This is part 4 of a series covering “microbiota” provided by Paul Enck from the Tübingen University Hospital and science writer Kristina Campbell. Continuous updates on microbiota research can be found at www.gutmicrobiotaforhealth.com.