We’re technically more “other” than we are human. Our bodies contain vastly more microbes than there are stars in the Milky Way—about 100 trillion microbes just in the gut of any single person. This huge and dizzying ecosystem of tiny critters, mostly bacteria, is referred to as the gut microbiome, and it helps make us who we are.

It may also help heal our bones.

Our diet, antibiotic use, where and how we were born, and personal hygiene all affect the gut microbiome. This is important, because even slight imbalances in the composition of our microbiomes can lead to dramatic effects on disease.

In fact, the microbiome is now known to influence heart disease, diabetes, arthritis, Alzheimer’s disease and obesity - and scientists are discovering new links every year. The microbiome is no longer just a preoccupation of microbiologists. It’s finally getting the attention it deserves across all areas of medicine.

One of these new areas is its link with bone health. This link seemed surprising at first, and most scientists viewed these two systems as quite separate. But exciting new research now shows that the gut microbiome can affect bone strength in both animal models and humans.

Some of the best evidence for this link comes from studying mice that were raised in sterile environments. These “germ-free mice” are known to have greatly reduced microbiomes, and have been an extremely useful model system for studying the effects of the microbiome on various diseases.

In the context of bone health, these germ-free mice have increases in bone volume and density. Interestingly, if you introduce a microbial community to germ-free mice at young ages, these effects can be reversed, suggesting that the microbiome can regulate bone health. 

These interesting results seem to be related to the immune system, and the microbiome's influence on maturation (or abnormal maturation) of immune cells. Different cells in the immune system can regulate bone density, and an imbalance in this regulation leads to diseases such as arthritis, cancer and osteoporosis (a disease in which bones become weak and brittle).

What these findings are also telling us is that by optimizing the microbiome, we may be able to treat or prevent a number of diseases associated with changes in the immune system, including osteoporosis.

Indeed, studies testing the effects of probiotics and prebiotics on bone health have already begun to support this approach. Probiotics are live microbes (such as those found in yogurt) thought to confer specific health benefits. Prebiotics are ingredients thought to stimulate the growth of beneficial bacteria.

Treatment with probiotics has been shown to improve bone mass in rodent models of bone loss, and treatment with prebiotics can modify bone density in other types of rodent models. Collectively, these studies indicate that treatments that affect the composition of the microbiome can also regulate bone health.

We can now begin thinking of the microbiome as a new “target organ” for future therapies. The more we learn about how deeply the microbiome influences diseases all over the body, the more important it will be to optimize its function and minimize its negative effects on the immune system. Whether it’s through diet, exercise, or changing our relationship with antibiotics, the microbiome could hold the key to multiple diseases with minimum intervention.

In fact, changes in our diet and environment that influence our microbiomes may be linked to increases in the rates of conditions like asthma, autism, and inflammatory bowel disease. This so-called “hygiene hypothesis” proposes that our lifestyle choices, use of antibiotics, and diets high in processed food has altered our “healthy” microbiome and ability to naturally prevent these conditions.

We have a long way to go in terms of understanding the exact mechanisms for how the microbiome interacts with bones and other biologics systems, but we have great tools at our disposal. Raising animals in germ-free environments has been an extremely valuable way of understanding the effects of the microbiome on the immune system, and at Charles River Labs we are pursuing a number of research questions using these mouse models.

If we could find ways to optimize our microbiomes through environmental and behavioral changes, in theory we could prevent many diseases and strengthen our bones naturally. While designing compounds that target the microbiome may be years off, we are getting closer to using those 100 trillion critters to our advantage.