Where does your body end?
At first the answer seems obvious: at your skin.
Biologically, however, the picture is much more interesting.
Your skin, mouth, digestive tract and other body surfaces support enormous communities of microorganisms. Their genes, metabolites and interactions form part of the biological environment in which your own cells live.
Long before the modern microbiome became a major field of research, Herbert M. Shelton included a chapter called Internal Symbiosis within his wider discussion of Natural Hygiene and Orthobionomics.
The old terminology deserves another look.
What is internal symbiosis?
Symbiosis literally concerns organisms living together.
Internal symbiosis draws attention to a simple fact that is easy to forget: an animal body provides habitats for other forms of life.
These relationships can take many forms. Some organisms coexist without obvious harm. Some participate in mutually useful relationships. Others may become harmful when circumstances change.
The key idea is not that every microbe is beneficial.
It is that the human organism cannot be understood as though microbes and their environment are irrelevant.
The human microbiome changed how we think about the body
Modern microbiology has revealed complex microbial communities associated with the gut, skin, mouth and other sites.
These communities differ between body locations and between individuals. They can also change with age, diet, medication, environment and other factors.
Researchers now study how microbial communities interact with digestion, metabolism, the intestinal barrier and signalling between different parts of the body.
This modern research should not be confused with Shelton’s historical theory. He did not possess today’s sequencing technology or current microbiological knowledge.
What is interesting is the broader question he was asking: can we understand human life without understanding the other life associated with us?
You are an ecosystem
It can be useful to think of the body as an ecosystem rather than a sterile machine.
An ecosystem contains organisms, resources, environmental conditions and continual interactions.
The digestive tract provides an obvious example.
Temperature, acidity, oxygen availability, water, digestive secretions, food residues and transit time all help create the environment in which different microbial populations live.
Change those conditions and the ecological balance can change too.
This connects directly with our article on Normosymbiosis, Shelton’s broader idea concerning normal biological relationships between living things.
The gut is not just a tube for food
When people think about digestion, they often imagine food simply being broken down and nutrients absorbed.
But the colon also contains a dense microbial ecosystem.
Microorganisms can ferment food components that escape digestion higher in the gastrointestinal tract. In doing so they produce compounds that become part of the chemical environment of the colon.
Diet therefore does more than provide material directly to human cells. It also helps determine what substrates are available to microbial communities.
This creates an important connection with Shelton’s concept of Orthotrophy: food changes the internal conditions in which biological processes occur.
Food creates an environment
This point is worth considering carefully.
Every meal changes the digestive environment.
Different foods contain different carbohydrates, fibres, proteins, fats, water and plant compounds. Different amounts and combinations alter what reaches different parts of the digestive tract.
Those differences can influence microbial activity.
From a Natural Hygiene perspective, this reinforces a larger principle: food should not be considered only as a list of nutrients. It also changes the conditions under which the body and its associated organisms function.
The skin has an ecology too
The digestive tract is not the only example.
Human skin contains varied microbial communities, and different areas of the body provide very different habitats.
An oily area of skin is not the same environment as a dry forearm or a moist armpit.
Again, the environment influences which organisms are able to thrive.
This ecological perspective is more sophisticated than thinking of the body as something that should simply be made “germ-free”.
Microbes are not automatically enemies
Some microorganisms unquestionably cause disease, and serious infections can require medical treatment.
But it is equally incorrect to imagine that every microorganism associated with the human body is dangerous.
Many microbes live with us without causing disease. Some participate in processes that influence the local environment and interact with human physiology.
The important word is relationship.
A microorganism’s effect depends not only on its name but also on where it is, how abundant it is, what other organisms are present and the condition of the surrounding environment.
Context changes everything
A species that causes no problem in one part of the body may contribute to disease if it reaches another location.
An organism normally present in small numbers may behave differently if environmental changes allow it to become disproportionately abundant.
This is why ecological thinking is useful.
Instead of viewing biology only as a catalogue of individual organisms, we also look at the relationships between organisms and conditions.
Internal symbiosis and Orthobionomics
Shelton placed Internal Symbiosis within his work on Orthobionomics.
That placement makes sense.
Orthobionomics asks whether the conditions surrounding life are appropriate to its biological requirements.
Internal symbiosis asks us to recognise that the “environment” also exists inside and upon the body.
The body itself becomes habitat.
What we eat, how we live and what enters that environment can influence the conditions experienced by the organisms associated with us.
Are we human plus microbial?
Popular descriptions of the microbiome sometimes go too far and imply that humans are mostly microbes or that microbial cells vastly outnumber human cells.
Modern estimates suggest the numbers are of roughly the same order rather than the dramatic ratios once commonly repeated.
But the precise ratio is less interesting than the underlying fact.
A human life includes continuous interactions with vast numbers of microorganisms.
We are individual organisms, but we are not biologically isolated ones.
What happens when the internal environment changes?
Living communities respond to changing conditions.
This is another place where the idea connects with physiological compensation.
The human organism adapts to changing circumstances, and microbial ecosystems can shift as their circumstances change too.
That does not mean every microbial change causes illness, nor that manipulating the microbiome is a simple route to health.
It means that biological systems are dynamic.
The Natural Hygiene question
Natural Hygiene repeatedly returns to conditions.
Rather than beginning only with:
What organism is present?
we can also ask:
What kind of environment is this organism living in?
That question does not replace microbiology. It adds ecological context to it.
The Natural Body Intelligence perspective
At Natural Body Intelligence, we view the body as an interconnected living whole.
Internal symbiosis expands that picture further.
The digestive tract, skin and other surfaces are not empty spaces. They are biological environments populated by living communities whose relationships can change as conditions change.
This gives us another reason to move away from overly simplistic ideas of health.
The body is not a battlefield containing isolated parts. It is an extraordinarily complex biological landscape in continual relationship with the world around it.
Explore the forgotten language of Natural Hygiene
- Normosymbiosis — normal cooperative relationships between living things.
- Bionomy — the laws, functions and adaptations of life.
- Orthobionomics — matching conditions of life to biological needs.
- Orthotrophy — correct feeding within Natural Hygiene.
- Physiological Compensation — how the organism adapts when conditions are imperfect.
Historical context
Internal Symbiosis appears among the subjects Shelton discussed in his work on Orthobionomics, alongside NormSymbiosis, the basic needs of living, the laws of life and physiological compensation.
His terminology belongs to the historical Natural Hygiene tradition and should not be treated as a substitute for modern microbiology. What makes it worth rediscovering is the underlying insight: human biology has always existed in relationship with other life.