The role of the microbiome and prenatal and early-childhood epigenetic factors in lifelong gene expression has got me wondering about what inherited constitution actually is? If things like personality, strength, empathy, and intelligence are only partially heritable (30-70%), is constitutional typing ala TCM and Ayurveda truly valid?

https://health.ucsd.edu/news/releases/Pages/2018-05-15-big-data-from-worlds-largest-citizen-science-microbiome-project-serves-food-for-thought.aspx

Major aspects of how you function and behave are actually related to the mother’s diet, exercise, and stress levels, toxins in the home, whether the child was delivered vaginally and breastfed, etc. As the child grows, his own lifestyle adds additional epigenetic signals. So, while you are born with a “constitution” that was influenced by both genetic potential and environmental factors, much of your adult “constitution” was not inherited in the genetic sense. Other than genes (including mitochondrial) and that portion of the epigenome that transmits across generations (the latter still a hotly debated concept), the rest is environmental. 

If that is the case, the “constitution” is actually not fully formed until early childhood. This begs the questions of how much one’s phenotype can vary based on such factors and how fixed it is any point. For example, could someone who was born a Vata (one of the Ayurveda body types) from a combination of genes plus epigenetic signals and unique postnatal microbiome be transformed into a Pitta (another of the Ayurveda body types) through lifestyle changes that altered microbial composition and altered epigenetic signals? If so, which is the true constitution?

According to TCM, the impact of lifestyle and environment are major factors in the development of patterns (Zheng) of disease presentation. On the other hand,constitution (Ti Zhi) is supposed to be what you come into the world with. A person could be assessed as a phlegm damp constitution in TCM without presenting with a phlegm damp pattern of disease at the time of assessment. For example, a phlegm damp constitution may make it more likely that one feels congested after eating dairy products. However, it does not mean that one would be actively experiencing an upper respiratory infection or sinusitus or even something more severe that usually has a significant phlegm component, such as Ménière’s disease.
I’m not questioning whether people with the same disease can have different underlying patterns. I am wondering if there is a such thing as a fixed constitution. Or, because it is impossible to exist in the world without the interplay of genes, epigenetic signaling, and microbiomics, is what we think of as the constitution actually something that can be changed radically by altering the microbiome and epigenetic signaling postnatally?
Or, does the constitution becomes fixed (as in unchangeable not unbroken) during the first two years of life. If so, that means constitution is actually a combination of what you inherit from your parents and certain factors that affect postnatal essence during the first few years of life. It would concern me if there was no way for adult to mitigate or undo malignant epigenetic signaling or pathogenic microflora imbalances in the gut.

This makes me think that you cannot even make a valid constitutional assessment until you treat the presenting patterns AND also put the patient on a general gut healing diet and exercise regimen for a few months. Once you have balanced the microbiome and eliminated all possible malignant epigenetic signals, what is left is the basic constitution. Then, adjusting your lifestyle according to this basic constitution will hopefully prevent future dysfunction.

To return to the Ayurvedic example, people in ancient times were probably exposed to many fewer malignant epigenetic signals of the sort that we face in modern society (sugar, refined carbs, zero exercise, environmental toxins, etc.). They’re also very unlikely to have had severely unbalanced microbiomes because they were delivered vaginally, breast-fed, and never consumed antibiotics at any point in their lives. They also lived close to nature. As a result, their “types” were “pure.” 
This way of thinking about things kind of turns personalized medicine on its head. It begins with general guidelines for bringing yourself as close to balance as possible, which are then followed by personalized recommendations once we clear the excess and restore the vital substances. This all got me thinking about the Ayurvedic concept of ama. I found this article from the Maharishi Ayurveda website about the assessment and treatment of ama. As you can see, there is a generalized approach to removing ama that appears to take precedence over constitutional balancing.