Inadequate exposure to microbial organisms, which are essential for healthy human development, may help to explain the reduced life expectancies experienced by people of 'low socio-economic status' (SES), according to a new review by a UCL researcher.
Around the world, households and individuals of low SES, which are defined by very low income/wealth and low educational level, are significantly more likely to experience multiple health issues, and in the most extreme situations, life expectancy can be 20-30 years lower than more wealthy and privileged individuals.
Published in the journal mSystems, the study assesses the numerous lifestyle factors and life choices (deliberate or otherwise) of people of low SES, that could lead to deficient or inappropriate microbial exposures, resulting in abnormal and ineffective microbiomes.
In humans, the microbiome refers to the complete set of genes contained in the entire collection of microorganisms that live in the body. It is essential to healthy development of all organs and systems.
Study author, Emeritus Professor of Medical Microbiology, Graham Rook (UCL Division of Infection & Immunity), said: "Low SES is associated with illness and reduced life expectancy and also with abnormal microbiomes.
"This paper summarises evidence that these two observations are linked, and that lifestyle factors that accompany low SES can reduce and distort microbial exposures and cause them to diverge from the physiologically necessary exposures with which humans co-evolved. This can explain much of the SES-associated health deficit."
In a review of published research, Professor Rook says the strongest evidence that the lifestyle factors or choices of people of low SES, can cause abnormal microbiomes are found in the following:
- Air pollution
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- Research shows exposure to unsafe levels of PM2·5 (air containing dangerous particles, dust, soot, dirt or smoke), nitrogen dioxide (NO2) and ozone (O3), is higher among low SES. This is both outside the home and inside.
- Studies show air pollutants may act directly on gut epithelial cells to drive intestinal inflammation and changes to the microbiota. Pollutants might also act indirectly by changing the microbiota of the natural environment.
- Lack of access to green space
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- Individuals of low SES, especially in urban settings, often have little access to green space (the natural environment).
- Studies of traditional farming communities and of the proximity of homes to green space reveal a crucial role for microorganisms from the natural environment and their spores in setting up the microbiome, and the regulation of the immune system. Lack of access to green space is linked to multiple chronic inflammatory and psychiatric disorders.
- Stress (caused by poverty, violence, noise, heat and sleep disorders)
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- Research shows stress in childhood is more common and extreme in those from low SES, and this adversity when young leads to high mortality later in life.
- The negative effect of stress on microbiota is proven in both animals and humans. Prolonged microbiome changes have been evidenced in stressed humans, mediated by stress-induced changes in gut mobility and function and redirection of blood away from the gut triggered by signals from the vagus nerve and enteric nervous system.
- Changes to mother/family dynamic
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- Research shows that mothers and siblings are major sources of the microorganisms that establish the infant's microbiome, and drive development of the infant's immune and metabolic systems.
- However, major lifestyle factors that reduce this transfer and correlate with increased immunoregulatory disorders are caesarean deliveries (high in individuals of low SES in some high-income countries), lack of breast feeding, and lack of mother-baby intimacy. This review shows those factors are seen more in some people of low SES.
- Poorer diet
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- Globally poorer diets are associated with low SES, particularly in urban settings. Research shows numerous dietary factors are associated with changes to the microbiome.
- Artificial sweeteners change the composition of the mouse gut microbiome in vitro, and this modified microbiota causes glucose intolerance. In humans, too much fructose (sugar found in fruit juice) has been shown to distort the microbiome of the gut. Low vitamin D levels are associated with autoimmune disorders and, importantly, vitamin D has effects on the immune system, including promotion of Tregs (lymphocytes that help to suppress autoimmunity), and modifies the gut microbiome, so at least some of the effects of vitamin D deficiency may be mediated via the microbiome.
- Smoking
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- Smoking is increasingly associated with low SES, and leads to numerous illnesses, including cardiovascular disease, periodontitis, chronic obstructive pulmonary diseases, Crohn's disease, various cancers, shrinking brain volume, MS and rheumatoid arthritis. The association with autoimmune disorders suggests compromised immunoregulation.
- Research shows that some of these effects are mediated via changes in the microbiota. Smoking causes clear changes in the oral, nasopharyngeal, airway and gut microbiomes. Some of these smoking-induced changes could be direct effects on microorganisms of chemicals in smoke, but smoking also modulates both the innate and adaptive immune systems.
- Vaccine hesitency
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- Research shows vaccine hesitancy or refusal, are most prevalent among people of low SES, and deprive individuals of two powerful benefits. Vaccines protect against the targeted infection, and some of them also provide a non-specific boost to the innate immune system.
- Antibiotic misuse
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- Research shows, excessive and unnecessary use of antibiotics, particularly in childhood is most prevalent in families of low SES, particularly of low educational level. Antibiotics disturb the developing microbiome, and predispose to obesity, childhood-onset asthma and allergic disorders (rhinitis, atopic dermatitis), celiac disease, attention deficit hyperactivity disorder (ADHD) and autism.
- Antibiotics also disrupt the processing of sex hormones by the gut microbiome, with important consequences for early menarche (first menstrual cycle) and development of secondary sexual characteristics in low SES girls, and subsequent breast cancer risks.
Professor Rook said: "Awareness of the involvement of microbial exposures may enable better targeting of societal measures to improve the health of more deprived groups.
"These measures include reducing pollution, better housing, better education about the value of breast feeding, natural birth and vaccines, encouraging lifestyle adjustments that maximise exposure to nature, dietary guidance, and abandoning broad spectrum antibiotics in favour of antibiotics that target an identified pathogen.
"We also need more attention to urban planning and we need construction materials and methods that increase exposure to the microorganisms of the natural world."
Explaining the importance of the microbiome, he added: "Since the dawn of human evolution millions of years ago, we have evolved to live with a diverse range of microorganisms, including bacteria and viruses.
"For the modern human, optimal development and function of all our essential physiological systems and organs including the brain, are dependent on a robust microbiome: this requires exposure to the microbiota of our mothers and the natural environment, especially in early life. Further we also know that some infections, if we survive them, modulate and strengthen the immune system, though this function can now be replaced safely by vaccines."
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- 'Colony of bacteria close-up 3D rendering illustration', Credit libre de droit on iStock
