DEAR DOCTOR K >> You recently wrote that the microbes that live on and within us might be a cause of disease. A friend told me she heard they might cause heart disease and autism. Is there any truth to that?
DEAR READER >> For a long time we've known that defects in how our genes are built, and defects in whether our genes are appropriately turned on, powerfully influence whether we develop diseases. However, our human genes may not be the only genes that affect our health.
Trillions of germs live on and within us, all of our lives. They live on our skin, in our mouth, in our gut and elsewhere. And they have genes, too. We call their genes, collectively, our "microbiome." Indeed, our microbiome contains about 400 times more genes than we have human genes. These microbial genes make molecules that could well affect our health, perhaps profoundly.
You asked about whether our microbiome could affect our risk of getting heart disease. It may do so in several ways. First, as I discussed in an earlier column, our microbiome may affect our risk of becoming obese. It may also affect our risk of developing Type 2 diabetes (the most common kind). Both obesity and diabetes increase our risk for getting heart disease.
In addition, certain species of bacteria that live in our gut make a molecule called TMAO. People with high blood levels of TMAO have an increased risk for heart disease. In animal experiments, high blood levels of TMAO were abolished by probiotics or antibiotics that reduced the number of bacteria that make TMAO. It is far from certain that such approaches would reduce the risk of heart disease in humans. However, it is not implausible.
You also asked about whether our microbiome could affect a child's risk of developing autism. While I was skeptical about such a possibility, a study from a research team at Caltech in 2013 was provocative. The team studied mice that behave in a way that is similar to the way autistic children behave. The mice were found to have unusual amounts of certain bacteria in their guts. They also have a gut condition that makes it easier for substances produced in bacteria to enter the bloodstream.
The unusual gut bacteria in the mice make a molecule that is similar to a molecule that is at high levels in the urine of many children with autism. The scientists injected this molecule into normal "non-autistic" mice: The mice developed autistic behavior. Then the scientists treated the "autistic" mice with a probiotic that reduced the number of unusual bacteria in their gut: The mice stopped their autistic behavior.
So I think it is possible, but unproven, that the trillions of bacteria that live on and within us could affect our risk for developing heart disease and autism. Our microbiome may well also increase our risk for many other diseases. Over the past decade, microbiome studies have become one of the most exciting fields of research.