By Noa Mecica

“Given that 70% of the people in the world die from what we call non-communicable diseases (NCDs), it gives you a whole different way of looking at these things,” describes Finlay in intriguing new paper published in the journal Science (1,5).  Non-communicable diseases are thought to be due to a combination of genetic, lifestyle, and environmental factors rather than being transmitted by bacteria or viruses, for example. However, in recent years the collection of microbes in our bodies, known as one’s microbiome, has come to the forefront of scientific health research. Could it be that NCDs such as diabetes, irritable bowel syndrome, and cardiovascular disease have the ability to be passed from person to person through our microbiomes? Finlay and colleagues outline a new theory suggesting the answer is yes.

Although evidence provided for this radical theory is circumstantial, researchers are successful in building a compelling case. First, they explain that individuals with chronic diseases tend to have a different gut microbiota than healthy individuals. They showed that when feces from people suffering from NCDs such as diabetes, obesity, and cardiovascular disease were transplanted into healthy mice, the mice developed the same diseases. Next, researchers found that “your microbes are more similar to the person you’re living with than the genetically related twin living on the other side of the world.” They bring further evidence to support this point through a study done in Fiji in 2019. In that study researchers collected stool and saliva samples from about 300 people living near each other in a village in order to study the bacteria and gut microbes they each carry. The data published in Nature presents distinct patterns of transmission and shared microbes between individuals living closer to one another, being more pronounced between individuals living in the same household.  The gut microbiota between spouses seemed to be the most similar, with researchers even able to predict couples based on their microbiota alone. 

The authors continue to build their case by referring to a study following about 12,000 individuals over 32 years and report that having an obese friend or sibling increases your chance of being obese by 57% and 40%, respectively (2). Furthermore, several studies found that injecting lean mice with feces from obese mice can make the lean mice fat (3). These obesity studies are of significance as obesity is a risk factor for many NCDs such as type 2 diabetes. Of course, these results do not suggest a causal effect and can be easily explained by individuals sharing the same environment and dietary habits. A similar pattern is observed between spouses, as chances of developing diabetes or irritable bowel syndrome is increased when one of the partners is already diagnosed, with mouse models supporting this hypothesis as well (4). The authors claim that this data cannot be explained by chance or genetics alone, and an additional contributing microbial component is likely. 

However, although noteworthy, all the studies brought down raise a similar issue, how can one tell which aspects of the disease are linked to one’s microbiota as opposed to diet, lifestyle, genes, and the environment?  Of course, scientists are not able to transplant feces from a diseased individual into a healthy one in order to provide more support, and distinguishing between environmental and microbial factors will be challenging. Nonetheless, Finlay and his colleagues do not intend to convince readers that NCDs are 100% transmissible but rather hope to propose a “hypothesis that stimulates additional discussion and research.”