Climate change is disrupting the human gut in a new path to illness
Climate-driven food shortage and undernourishment could affect the composition of the human gut microbiota, exacerbating the effects of climate change on human health, according to a new review article published in The Lancet Planetary Health.
The article comes on the heels of a growing number of studies that highlight the key role food and nutrition play in maintaining a healthy microbial population in the human gut, leading to better metabolic and intestinal health.
Diversity disrupted
According to the review, climate-induced changes in the yield and nutritional quality of plants, seafood, meat, and dairy could disrupt this microbial diversity, tipping the balance towards microbial strains associated with malnutrition and particular diseases.
The review also warns that these effects will be more pronounced in low and middle-income countries (LMICs) since these regions face the brunt of climate stressors, including higher temperature and atmospheric carbon dioxide, which affect their agricultural output and increase render deficiencies in these areas more common.
Indigenous communities that depend more than other demographic groups on local food sources and which have been shown to have greater gut microbial diversity may also be more susceptible to climate-related changes, the review reads.
Research has already found that high atmospheric carbon dioxide levels can diminish the quantity of plant micronutrients like phosphorus, potassium, zinc, and iron, along with protein concentrations in vital crops such as wheat, maize and rice. These effects add to the complexities that affect the gut microbiota.
While the effects of food and nutrition are direct, the review also examined the role of changes in water, soil, and other environmental microbiota as a result of climate change.
A fine balance
In another recent review, published in Dialogues in Health, researchers from the Indian Institute of Public Health, Gandhinagar, analysed the impact of heat on human and animal health in India. They found that reports of foodborne and waterborne infectious diseases and malnutrition increase with heat.
Although these findings mirror common knowledge about food and water-related illnesses in warmer weather, the resulting implications for gut dysbiosis — the imbalance in gut microbial populations — also need to be considered for future heat-related mitigation efforts, The Lancet review said.
'While we know and research various effects of climate change on human health, one aspect remains understudied — the effects of changing climate on the microbial communities in the human gut,' Elena Litchman, author of the review in The Lancet and the MSU Foundation professor of aquatic ecology at Michigan State University, said. 'This, in part, could be explained by the fact that researchers studying human microbiota do not necessarily think about it in a climate change context.'
The human gut is home to about 100 trillion bacteria, fungi, protozoa, and viruses. Bacteria are this group's predominant members. The microbes' overall diversity in the gut influences several aspects of human well-being, including immunity, maintaining glucose levels, and metabolism.
According to a 2018 analysis in The BMJ, lower bacterial diversity has been observed in atopic eczema, types I and II diabetes, and inflammatory bowel disease, among other conditions. Researchers are also exploring how gut dysbiosis changes the central nervous system and leads to neurological disorders.
More research attention
The gut microbiome — the collective genome of the microbes in the gut — has far more genes than the human genome, producing thousands of metabolites that affect the individual's health and development.
'Our understanding of the gut microbiota's role in human health is still evolving,' While climate change is a growing concern in this context, establishing cause and effect is difficult as there are many confounding factors,' Sachit Anand, a paediatric urologist and assistant professor at AIIMS, New Delhi, said. In his research, Anand examines the role of gut microbiota in congenital anomalies of the kidney and urinary tract.
He added that understanding the interactions between the microbiota, the host, and the environment is now gaining more research attention, especially when evaluating an individual's susceptibility to specific diseases. As climate change becomes a key influencing factor in this 'triad', its impact cannot be ignored moving forward, he said.
It may be tempting to examine these interdependencies in a linear manner: i.e. that climate-induced changes in crops affect the diet and thus the gut microbiota, or that climate-induced increases in temperatures make enteric infections more prevalent, ultimately disrupting the gut's microbial population. But both Litchman and Tarini Shankar Ghosh, assistant professor at Indraprastha Institute of Information Technology Delhi, warned that many of these stressors are often playing out simultaneously.
As a computational biologist, Ghosh is interested in patterns in data about the human gut microbiome.
'If you take the example of low-income groups residing in urban environments, you are looking at the impacts of temperature, pollution, lack of quality food, and water supply,' he explained. 'There are multiple factors that are disrupting the gut microbiota at the same time.'
A new science
Ghosh also said dysbiosis has been found to be a diagnostic signature in many disease states. According to him, this means it is not just the tipping of the balance towards unfavourable microbial populations that is concerning: dysbiosis also signals a loss of interdependence between 'normal' microbial strains, leading to a loss of several metabolic functions in the host.
'What we need right now is to generate more data to understand how the so-called good bacteria interact with each other and benefit the host. Data generation must go hand-in-hand with connecting this information to climate change, so we know what is happening,' Ghosh said.
Thus, Litchman said, a multidisciplinary approach with researchers from disparate fields coming together is vital to understand the effects of climate change on human gut microbiota. But along with a lack of awareness of climate change's effects, a paucity of funding programmes to enable such interdisciplinary and international research is a major impediment to future research of this nature, she added.
On the flip side, with advances in computational biology and metagenomics — analyses of the genetic makeup of microorganisms in a given environment — researchers are inching closer to unearthing some of the gut microbiota's secrets. For example, Indian Institute of Science Education and Research, Bhopal, professor Vineet Kumar Sharma has developed an open-access database named GutBugBD. It provides information about how the gut microbiome can interact with and alter specific nutraceuticals and drugs, paving the way for therapeutic approaches to modulate gut microbiota in response to various changes.
This is just the start, according to Sharma: 'At the moment, we are merely doing broader surveys of the gut microbiota to understand what is there and how they are functioning. Even if we introduce healthy microbiota through, say, probiotics, we cannot know if the response will be the same between two individuals. Each person's gut microbiota is unique, and understanding this uniqueness is important for the way forward.'
Sharmila Vaidyanathan is an independent writer from Bengaluru.
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