We humans are more than just our body
Good health needs biological cooperation
It is well understood now that humans and all other animals and plants on earth have evolved alongside important symbiotic microorganisms, referred to as their microbiota. In this relationship, we act as the host upon and within which our microbiota live and work with us cooperatively.
“…we are each a walking, talking multi-organism ecosystem.”
It is also understood that the symbiotic associations between host and microbiota are so complex and important that plants and animals cannot exist without their microbiota, and are, therefore, no longer described in biology as autonomous entities. Instead, every plant and animal with its associated microbiota (including humans) is an organized multi-species biological unit. These biological units are referred to by the new-ish term holobiont [1]. Viewed as such, we are each a walking, talking multi-organism ecosystem. The term holobiont, therefore, is used to describe the complex and beneficial symbiotic association between a host and its microbiome.
I have used two books edited by Bosch and colleagues [2,3] as source material for this post. Additional external references are cited and supporting blog posts are linked within text below.
Biological resilience
Our holobiont selves have been able to live healthily with each other no matter what life has thrown at us since the dawn of time. At the centre of this resilience has been our host metabolic flexibility, microbiome core functionality, and our ability to maintain balanced multi-species cooperation.
Access to real nutrition for our host and microbiota underpins the resilience within each of us. It is, however, important to understand that each of us is quite different and what works for me may not work for you and vice versa. There are, unfortunately, no universal one size fits all approaches to nutrition and a healthy lifestyle.
Mutual benefits
As host, we offer the benefit of range of different nooks and crannies upon and within which our microbiota can thrive. These include, to name just a few, skin, mouth, nose, lung, intestine, and vagina. Our intestine provides niches for our microbiota that do not exist anywhere else on earth and is critical for human health. It will be the focus of many future articles.
In exchange for unique places to live our microbiota provide us with a range of essential benefits, including protection against pathogens, nutrient production and recycling, beneficial fat storage, development and behaviour, and production of body heat. [4]
We are inseparable.
As we evolved, we developed a continuous, dynamic, and sophisticated symbiotic crosstalk between our human cells and our microbiota. This cross talk is essential for human health.
When our cells and microbiota are well nourished, our crosstalk can withstand environmental stressors and our holobiont is healthy. Conversely, if our body or our microbiota are disturbed by poor nutrition or damaging chemicals, our multi-organismal community crosstalk can be damaged and with it our health.
How is such close multi-species cooperation achieved?
Obvious questions that arise from considering the closeness of the symbiotic relationship between the holobiont host and microbiota are “What maintains the complex balance between the two, what prevents the host from killing its symbionts and the symbionts from overwhelming the tissues they reside in?” The answers appear again to be in the sophisticated communication between host and microbiota, irrespective of how many players are involved. Specifically, elements of our immune system are crucial in controlling host–microbiota interactions and maintaining the crosstalk that constitutes our health.
Fascinatingly, the immune system plays the same roles in living beings as diverse as plants, insects, corals, and all other mammals.
Unexpected consequences of not caring for the holobiont
There is mounting evidence that we have inadvertently damaged our health by ignoring the unique nature and needs of host and microbiota.
Our host metabolism is a marvel because it can use different sources of fuel flexibly when it is working properly. Unfortunately, it appears that over-consumption of things like processed carbohydrates and seed oils may be disrupting that natural flexibility. Consequently, we are witnessing increases in so-called chronic disease such as obesity, heart disease, type-2 diabetes, poly-cystic ovarian syndrome, hypertension, dementia, cancer, and non-alcoholic fatty liver disease.
“…by attempting to treat our human host bodies in isolation, we have upset the vital crosstalk with our microbiota and increased the incidence of diseases that were hitherto relatively rare.”
When we consider only our human bodies (the host) when trying to treat certain diseases we may be damaging our microbiota. For example, with the application of antibiotics and vaccines, we reduced morbidity and mortality from infectious diseases and increased overall life expectancy in the past 50 years. There is no doubt that this is a major achievement.
Unfortunately, there is also evidence of a dramatic rise over the same period of allergic (asthma, rhinitis, and atopic dermatitis) and autoimmune (multiple sclerosis, type-1 diabetes, and inflammatory bowel disease) diseases. It appears that by attempting to treat our human host bodies in isolation, we have upset the vital crosstalk with our microbiota and increased the incidence of diseases that were hitherto relatively rare.
Managing our multi-organismal selves
The evolutionary advantages conferred in arranging a living system as a holobiont with its sophisticated crosstalk may lie in its resilience. The fact that the holobiont model has persisted in such diversity of life on earth for such a long time is surely evidence for such.
Unfortunately, there is mounting evidence that much of our contemporary approach to nutrition and healthcare is at odds with and may even be damaging our holobiont biology and its inherent resilience. We can see this in the unexpected rise of so-called chronic diseases.
“If we are interested in health, we should adopt a lifestyle and approach to nutrition that takes care of our whole walking ecosystem, not just the individual moving parts.”
George Sugihara addressed this in contemplating conditions which may shift a system from stability into another, unhealthy state - “We like to see the world as consisting of separate parts that can be studied in an isolated, linear way, one piece at a time. These pieces then can be summed independently to make the whole. The trouble and real danger is that we persist with these linear tools and models even when systems that interest us are complex and nonlinear.”[5]
We are a multi-organism network of self-coordinating parts. If we are interested in health, we should adopt a lifestyle and approach to nutrition that takes care of our whole walking ecosystem, not just the individual moving parts. I’ll be addressing that in future posts.
References
Bordenstein, S. R., Theis, K.R. (2015) Host Biology in Light of the Microbiome: Ten Principles of Holobionts and Hologenomes. PLoS Biol, 13(8), e1002226. doi:10.1371/journal.pbio.1002226
Bosch, T.C.G and Miller, D.J., editors (2016) The Holobiont Imperative: Perspectives from Early Emerging Animals. New York: Springer
Bosch, T.C.G and Hadfield, M.G., editors (2021) Cellular Dialogues in the Holobiont. Boca Raton; CRC Press
Rosenberg and Zilber-Rosenberg (2018) The hologenome concept of evolution after 10 years. Microbiome 6:78. doi.org/10.1186/s40168-018-0457-9
May RM, Levin SA, Sugihara G (2008) Complex systems: Ecology for bankers. Nature, 451: 893-895