The human body hosts a community of microorganisms, collectively known as the human microbiome. These microbes reside on and within us, particularly in the gut, where they outnumber human cells. The composition and variety of these communities play a profound role in health. Researchers increasingly agree that this internal ecosystem’s diversity is declining.
What is Microbiome Diversity?
Microbiome diversity refers to the variety of microbial species and their relative proportions within a community. It has two main components: richness and evenness. Richness is the number of distinct microbial species. Evenness describes how equally abundant each species is, with a more even distribution meaning no single species dominates.
A microbiome with high diversity is beneficial for health. Such a diverse community often exhibits greater resilience, allowing it to better withstand disturbances and return to stability. This range also contributes to functional redundancy, where multiple species perform similar roles, ensuring essential processes continue even if some microbes are diminished.
How Scientists Detect Diversity Loss
Evidence for decreasing microbiome diversity comes from several approaches. One method compares microbial communities of modern, industrialized populations with traditional societies. Studies show individuals in urban, industrialized environments have less diverse microbiomes than those in traditional, agrarian settings. These comparisons highlight differences in microbial richness and evenness.
Another approach analyzes historical samples, like ancient human feces (coprolites) and dental calculus. Extracting and sequencing DNA from these specimens reconstructs past microbial profiles. These studies reveal ancestral human microbiomes had greater diversity than commonly observed today. DNA sequencing technologies, including 16S rRNA gene sequencing and shotgun metagenomics, are fundamental for identifying and quantifying microbial species in these comparisons.
Key Factors Driving Diversity Down
Several interconnected factors linked to modern lifestyles contribute to declining human microbiome diversity. These reshape the microbial landscape.
Dietary Shifts
Modern industrialized diets are a primary contributor to reduced microbiome diversity. These diets are high in processed foods, unhealthy fats, and simple sugars, and low in dietary fiber. Processed foods often contain additives like emulsifiers and artificial sweeteners that disrupt the gut microbial community. This shift deprives beneficial gut microbes of their primary food source.
Dietary fiber acts as a fermentable substrate for many gut bacteria, promoting their growth and beneficial metabolites like short-chain fatty acids. Insufficient fiber intake causes these fiber-degrading microbes to decline, leading to a less diverse and resilient gut ecosystem. Studies show even short-term consumption of low-fiber, high-fat diets can rapidly reduce gut microbiota diversity.
Antibiotic Use and Medical Practices
Antibiotics, while treating bacterial infections, broadly impact microbial communities. They often indiscriminately eliminate both harmful pathogens and beneficial bacteria, significantly reducing overall microbial diversity. The effect can be rapid, with changes persisting for months or even years after a single course. This disruption can alter gut metabolic activity and increase susceptibility to other infections.
Certain medical practices also influence early life microbiome development, impacting diversity. Cesarean section (C-section) deliveries bypass the maternal birth canal, preventing newborns from acquiring the diverse microbial inoculum transferred during vaginal birth. C-section infants often have a different initial microbial colonization pattern, with reduced diversity compared to those born vaginally. Similarly, formula feeding, versus breastfeeding, can lead to different microbial colonization patterns in infants.
Modern Hygiene and Lifestyle
Modern hygiene practices and urbanized lifestyles contribute to reduced microbial exposure, impacting microbiome diversity. The “hygiene hypothesis” suggests decreased exposure to diverse microorganisms in early life may affect immune system development. Living in clean, indoor environments with less contact with natural elements like soil and animals limits opportunities for acquiring diverse microbes.
Widespread use of antimicrobial products, such as hand sanitizers, further reduces microbial exposure on the skin and potentially within the body. While effective at killing pathogens, frequent use can also reduce skin microbiome diversity. This collective reduction in environmental and direct microbial contact contributes to a less diverse human microbiome.
The Evolving Scientific View
The scientific community recognizes the ongoing trend of decreasing human microbiome diversity. Evidence from diverse research consistently points to a shift in microbial composition compared to past generations and less industrialized populations. While contributing factors have been identified, their full long-term implications remain an active investigation. Researchers continue to explore the complex interplay between human health and these intricate microbial ecosystems.