Do microbiomes cause chronic diseases?

Human microbiomes are ecosystems in their own right. The way microbiomes function, especially in the gut, brings important benefits to digestion, nutrition and immune function. Microbiome disruption has been linked with a growing list of chronic diseases such as inflammatory bowel diseases (IBD), rheumatoid arthritis and multiple sclerosis—even depression and anxiety. But to what extent is this link to chronic disease a causal one?

Some, like Stanford University researcher Jason Sonnenburg, believe that  modern eating habits, notably decrease in consumption of dietary fibres, reduce microbial production of certain metabolites—short-chain fatty acids. This, he argues, “results in more simmering inflammation which, over decades, leads to most of the chronic diseases that plague the modern world”.

The evidence can be hard to read, however, and correlation between microbial shifts and disease does not establish that one causes the other. A paper from Harvard published last month, for example, showed that a drop in the diversity of microbes in the gut precedes appearance of Type 1 diabetes in children. The fact that the children under study were known to be genetically susceptible to diabetes complicates interpretation, however—microbial shifts and destruction of insulin-producing cells could both be encouraged by other factors, for example.

Others caution against what some have dubbed “microbiomania”. Epidemiologist Bill Hanage, also of Harvard, sees current results as  suggestive, no more. “There is very good evidence that the microbiome could be involved in a lot of things. There is very little evidence that it is definitely one of the things—let alone the most important thing—influencing almost any of them.”

He grants that there is good reason to believe that the microbiome is involved in development of IBD, for example. Tests on mice, in fact, seem to back this intuition. But getting a clear picture of what could benefit patients is still difficult. “IBD is a variable disease, which may well have multiple causes. This leads to problems with inclusion criteria. An intervention might work well in one setting, and not others,” says Mr Hanage.

The complexity of the microbiome means the number of things that might cause a particular effect, and the number of ways in which it could do so, increases significantly. Clear evidence of positive effects from microbial treatments, such as have been shown with total microbiome transplants for patients suffering from Clostridium difficile infections, will not be available to us for other conditions for some time.

The safest conclusion for now is that much more detailed research is required—research of the kind that will be discussed this week at the fifth International Human Microbiome Congress in Luxembourg. John Cryan of University College in Cork, Ireland, is a strong advocate for the importance of microbiome studies in many conditions, including some affecting the brain—where chains of cause and effect are likely to be longer. But for now, he urges, “we need to get away from the hype into the hard science”.

This article is published in collaboration with GE Look Ahead. Publication does not imply endorsement of views by the World Economic Forum.

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Author: Jon Turney writes for GE Look Ahead. 

Image: A researcher uses a microscope after a laser bio- 3D printing of human cells in the laboratory Biotis at INSERM. REUTERS/Regis Duvignau

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