Humans have evolved for millions of years in a habitat that is very different from the environment most of the world’s population now live in. By studying the health of ancestral populations, traditional people unaffected by western lifestyle, and contemporary hunter-gatherer societies, we learn that “non-communicable” chronic diseases such as cancer, cardiovascular disease and acne vulgaris are not a natural part of human life, but a result of the modern, western lifestyle (1). Homo sapiens isn’t adapted for big mac’s, indoor living, inactivity and other factors associated with life in the industrialized world, and we therefore face several evolutionary mismatches that drive the so-called diseases of civilization. A lot of researchers have focused on the western diet and lifestyle in relation to our human genome, but we’re now learning that the impact on the second genome in our body – the human microbiome – could be of even more importance.
Humans face a gene-environment mismatch
The DNA we receive from our mother and father code for thousands of different enzymes, hormones, and other proteins. Although we inherit our DNA from mom and dad, the way we express the genes in our genome is largely determined by our diet and lifestyle (2,3). The western dietary pattern, characterized by high intakes of sugar, flours, and refined fats, seems to increase expression of genes involved in chronic disease (4,5). We also know that our human genome changes fairly slowly, and several researchers have therefore focused on the connection between our human genes and the western diet and lifestyle.
Although it’s interesting to study expression and adaptation in our human genome, we’re now learning that the changes to the human microbiome – the aggregate of all the microorganisms (and their DNA) that live in and on the human body – could be even more important in terms of the modern diseases of civilization. The human microbiome is primarily composed of bacteria, viruses and fungi that collectively harbor a genetic repertoire that is more than 100-times greater than that of the human host (6). The human body is essentially a microbial world, with critters living in places such as the gastrointestinal tract, lungs, vagina, and skin. Some recent reports even suggest that bacteria are found in the brain (7).
If these microbes were just passive bystanders on our journey through life, we wouldn’t really care that much about the composition and functions of the microbiome. However, as my regular readers are well aware of; the microbiome provides essential functions that we can’t live without, and the structure and diversity of the microbial rainforest inhabiting our body could be the key to understanding chronic disease.
The western lifestyle is a master manipulator of the microbiome
I recently read an excellent scientific paper where the authors suggest that the increased prevalence of autoimmune, inflammatory and allergic diseases in the industralized world results from a loss of species from the ecosystem of the human body. This is in line with a lot of the things I talk about on the blog, but instead of focusing on dysbiosis like so many other researchers, these guys have steered their attention towards biome depletion…
The primary factor associated with allergic and autoimmune disease is apparently loss of species diversity from the ecosystem of the human body, the human biome. Species depleted or even eliminated from the human biome include a wide range of pathogens, commensals and mutualists whose reproductive cycle is greatly diminished or even eliminated by modern sanitation, water treatment and medical practices [16–18]. Importantly, the human biome, as with other biomes, not only includes species that are permanent residents of the ecosystem but also species that interact transiently with the ecosystem . The absence of species from the human biome leaves the immune system in a hypersensitive state that, when combined with environmental triggers and genetic predisposition, leads to allergic and autoimmune disease.
The western lifestyle is a master manipulator of the human microbiome because antibiotics, western diets, caesarean sections, formula feeding, modern hygiene and urban living have a significant impact on the ecosystems in and on our body (8,9,10). We’re not only changing the balance between beneficial bacteria and inflammatory bugs, but we’re quickly depleting the diversity of genes and microbes that make up the human superorganism.
Just like some species of animals and plants are now extinct from the planet as a result of human activities (e.g., global warming, forest fires), some old friends that used to make up the human microbiome are probably gone forever. In the same way that species diversity in a rainforest ensures ecosystem resilience, a species-rich human superorganism is also considered beneficial.
Although humans are very similar in terms of our human genome, the composition of the microbiome can vary significantly from one person to another. We also know that the microbial communities in and on our bodies are altered fairly rapidly as a result of dietary changes, drug use, hygiene, etc. So, while we can’t change the DNA we got from our mother and father, we actually have the ability to alter most of our total genetic reportoire by manipulating the microbial communities in our body.
Diseases of civilization are a result of the western diet and lifestyle
While modern medicine has been very succesful in combatting infectious disease (what are the consequences of overriding nature’s own systems of natural selection?), the perspective on chronic disorders such as cancer, diabetes and heart disease seems to lack the basic fundamental principles (1). However, if we acknowledge that diseases of civilization are a result of the western diet and lifestyle, it quickly becomes apparent that we have to study non-westernized societies and hunter-gatherer populations that are free from diseases of civilization. While we don’t have to emulate the lifeway of our paleolithic ancestors to be healthy, learning more about the diet and lifestyle of Homo sapiens in its “natural state” does provide an essential framework for understanding health and disease.