Since we, to a great degree, receive the microbiome of our mother, and to a certain degree father and other family members, the second genome should also be considered when discussing genetics and heredity. Just like the 23 pairs of human DNA we receive from our parents determine our physiology and disease susceptibility, the microbial communities and the genetic material they hold, also have a significant impact our health. While more research in this area is needed, it’s likely that part of the hereditary component of health disorders that are characterized by alterations in the human microbiome stems from the transfer of microbes from mother to child.
It’s well established that diet, pharmaceutical use, birth method, etc. have a direct impact on the gut microbiome. However, the relationship between environment and lifestyle and the gut microbiome doesn’t stop there. Our own microbiome is also influenced by the microbiome of animals and other people. This contagious health might sound far-fetched to many, but it has been shown that household members, particularly couples, share more of their microbiota than individuals from different households (16). It has also been shown that children who grow up with animals are healthier than children who grow up without pets and that this beneficial effect stems from the transfer of microbes (17).
While it has long been known that the human body is not sterile, recent discoveries about the human microbiome have taken the idea of the human body as an ecosystem to a whole new level. Just like other ecosystems in nature, the microbial communities that live in and on this superorganism are shaped by other microbial ecosystems we encounter. This is relevant in terms of health and disease, as it could mean that disorders, such as obesity, that are generally considered non-communicable, actually are communicable in the sense that microbes are transferred between humans. To which extent the microbiome is able to affect the microbial communities of other humans is still unclear, but likely depends on several factors, such as the proximity of contact (e.g., husband and wife share more bacteria) and the original resilience of the microbiome. Is it possible that the skyrocketing rates of diseases of civilization are partly driven by the transfer of westernized microbiota between people? We don’t know. Nobody’s denying that diet and lifestyle factors play the most important roles, but it’s certainly possible that “contagious health” could also be contributing.
The obese microbiome
One of the disorders that we now know is associated with an altered gut microbiota is obesity. Almost a decade ago, researchers at Jeffrey Gordon’s lab at Washington University showed that sterile mice – mice without a microbiome – who received microbiota from lean mice stayed lean, while germ-free mice who received microbiota from obese mice gained weight (18). These were some of the first studies showing a causal relationship between gut microbes and body fat regulation, and in the decade that has passed since then, these results have been replicated and numerous other reports have investigated this relationship further. By now it’s well established that obesity is characterized by an obese microbiota and that gut microbes can influence fat storage through a variety of mechanisms (19,20,21). We’ve also learned that changes in the gut microbiota contribute to reduced host weight after gastric bypass surgery, that perturbations in the bacterial communities (e.g., from antibiotics) can cause weight gain, and that transfer of intestinal microbiota from lean human donors increases insulin sensitivity in individuals with metabolic syndrome (22,23,24).
At this point we don’t know enough to say exactly what characterizes the obese microbiota and which species are important. Some studies show a shift in the balance between the two major phylums of gut microbes, the bacteroidetes and the firmicutes, while others point to specific strains of bacteria that can initiate weight gain (25,26). However, what we do know is that there’s probably more a question of the overall community structure, rather than a few specific types of microbes. Just like the causes of obesity are considered to be both genetic and environmental, the obese microbiota is also shaped by both hereditary and environmental factors. At this point there isn’t much concrete data on the transfer of gut microbes from obese mothers to their child and the impact this inoculation has one the child’s body weight, but it’s very likely that part of the genetic component of obesity stems from the transfer of an obese microbiome.
Since we know that the human microbiome harbors 99% of the unique genetic material in the human body and that gut microbes play an important role in regulating fat storage, the role of this gene transfer from mother to child shouldn’t be underestimated. If the transfer of the microbiome from mother to child is as important as some scientists think, it’s clear that we have to rethink our perspective on obesity and genetics. While it’s well established that several genes in the human genome play a key role in obesity, we’re now learning that microbes/genes in our second genome also impact how likely we are to gain and store fat. This is also where microbiome science has a long way to go. While a lot of the studies have focused on specific bacterial strains and general community structure, the genetic material could be more important than the specific types of bacteria.
Microbes transfer DNA through other means than traditional reproduction, and in the gut it probably goes on all the time (27). This horizontal gene transfer means that gut microbes are able to pick up genes from other critters, and new species, with a somewhat different genetic material, are therefore created all of the time. This complexity and dynamic nature of the gut microbiome can help explain why it’s often estimated that humans are 99% alike in terms of our human genome, but very different in terms of our microbial inhabitants (28). It can also help explain why it’s so difficult to accurately characterize an unique makeup of the obese microbiota.
Next: Two Emerging Hypotheses of Obesity
Part I: The Second Genome
Part II: Ancestral vs. Westernized Microbiome
Part III: Two Emerging Hypotheses of Obesity
Part IV: Wrapping It Up
Like this article? For more great content, delivered right to your door (and tablet, phone & computer!) subscribe to Paleo Magazine now!