Gut bacteria may drive a person's life expectancy, research suggests.
The micro-organisms in the gastrointestinal tract do far more than just digest food, with studies implying they help control a person's weight, regulate mood and even communicate with the immune system.
With the microbiome's role in ageing being less clear, scientists from the Institute for Systems Biology in Seattle analysed 9,000 people, aged 18 to 101.
They observed a "microbial drift towards a unique compositional state" in healthy 80-year-olds, compared to their less-fit counterparts.
These healthy older adults also had a "depletion" of certain core bacterial species, defined as those which are common to all humans.
Those who retained these specific gut bugs or had a "low gut microbiome uniqueness measure" were less likely to survive over the next four years, the results show.
A human's gut bacteria is known to change with age. This is rapid in early life, up to three years old, "followed by a long period of relative stability, ending with gradual changes associated with advanced age", the Seattle scientists wrote in the journal Nature Metabolism.
Past research has linked the microbiome to measures of fitness and frailty in people over 65.
Feeling "still very little is known" about how gut bacteria may influence survival, the Seattle scientists analysed three studies with 9,000 participants between them. They particularly focused on 900 individuals aged 78 to 98.
The results show having a unique microbiome was "highly correlated" with the presence of the compound tryptophan-derived indole in the blood, which has been linked to longer lifespans in mice.
The presence of the bacterially-derived phenylacetylglutamine in the blood "showed the strongest association with [microbiome] uniqueness", with the same compound found to be "highly elevated" in centenarians.
"Interestingly, this uniqueness pattern appears to start in mid-life – 40 to 50 years old – and is associated with a clear blood metabolomic signature, suggesting these microbiome changes may not simply be diagnostic of healthy ageing, but they may also contribute directly to health as we age," said lead author Dr Tomasz Wilmanski.
"This uniqueness signature can predict patient survival in the latest decades of life."
Bacterially-derived compounds are known to reduce inflammation in the gut, which may drive age-related ill health.
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Overall, the results suggest a healthy adult's gut microbiome continues to develop into old age.
The findings further "identified microbiome pattern of healthy ageing is characterised by a depletion of core genera found across most humans", particularly species of the Bacteroides group.
"Retaining a high Bacteroides dominance into older age, or having a low gut microbiome uniqueness measure, predicts decreased survival in a four-year follow-up," wrote the scientists.
Co-author Dr Sean Gibbons added: "Prior results in microbiome-ageing research appear inconsistent, with some reports showing a decline in core gut genera in centenarian populations, while others show relative stability of the microbiome up until the onset of ageing-related declines in health.
"Our work, which is the first to incorporate a detailed analysis of health and survival, may resolve these inconsistencies.
"Specifically, we show two distinct ageing trajectories – one, a decline in core microbes and an accompanying rise in uniqueness in healthier individuals, consistent with prior results in community-dwelling centenarians; and two, the maintenance of core microbes in less healthy individuals."
The "exciting" research may have "major clinical implications for monitoring and modifying gut microbiome health throughout a person's life", added co-author Professor Nathan Price.
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