Digestion and Development: The Emerging Connection

It may seem like an odd—even bizarre—connection, but the bacteria in your child’s gut (his stomach and intestines) that help him digest his food may also influence how his brain develops. Though perhaps it’s less surprising when you realize that the gut is lined with neural tissue housing some 100 million neurons. What’s more, scientists recently discovered that communication between the brain and gut (or “second brain,” as it’s often referred to) goes both ways: The vagus nerve has been found to carry information from the gut bacteria to the brain, indicating that our state of mind may be influenced—even driven—by the state of our gut. This is especially intriguing in light of some preliminary new research regarding the role of gut bacteria in brain development during the critical early years of life.

Getting to Know the Good Bacteria

The average adult harbors roughly 100 trillion microorganisms in the gut, mostly bacteria. In fact, our bodies contain 10 times as many microorganisms as they do our own human cells. Although we normally associate bacteria with illness and disease, these are friendly germs that play a critical, positive role in many of our most basic biological processes—from helping us utilize the nutrients in the food we eat to fighting off dangerous bacteria. In recent years, researchers have discovered that these naturally occurring microbes may also have a profound influence on the brain’s development.

In a Japanese study, mice that were bred to be germ-free showed an exaggerated release of stress hormones when confined to a small space in which their movement was restricted, compared with normal mice. This effect was partially reversed when the germ-free mice were allowed to develop a normal colony of gut microbes—as long as the bacteria was introduced at an early enough stage in the mice’s development. If researchers waited too long to introduce the bacteria, the stress response didn’t change, which suggests that there’s a critical stage during which gut microbes must be present in order for the stress response system to develop and function normally.

In a similar study, Swedish researchers performed a brain analysis on germ-free mice that revealed alterations in nearly 40 genes, as well as an impact on the signaling between nerve cells. The genes involved—associated with anxiety, emotion, and motor control—correlated directly with discrepancies in behavior between the germ-free mice and the mice with a normal population of gut microbes. And, again, when the germ-free mice were exposed to gut bacteria at an early stage of development, they behaved like normal mice by the time they reached adulthood.

Could Bacteria Hold Developmental Answers?

How these effects translate to humans isn’t absolutely clear at this point. But we do know that infants leave the womb with a sterile gut, which immediately becomes colonized by a complex population of microbes. These organisms play a key role in the programming of the immune system. They also produce the neurochemicals the brain uses to regulate physiological and cognitive processes, including learning, memory, and mood. In addition, there’s evidence of their involvement in neurodevelopmental disorders, such as autism and schizophrenia. Some research has found that stool from autistic children tends to have a different bacterial composition from that of normal children. However, this research is very preliminary, and the relationship needs to be further studied.

Researchers in this growing field hope that someday they’ll be able to prevent or treat disorders like autism or severe anxiety by simply altering the composition of beneficial bacteria in a child’s gut. But that’s still a long way off—there’s a great deal that’s still unknown about how friendly bacteria influence the brain and development, let alone what we can do to influence that process. For now, it’s an area to watch and a reminder that so much of the human body is connected in ways that researchers are still uncovering.

References:

“Brain–Gut–Microbe Communication in Health and Disease” by Sue Grenham et al., Front Physiol (2011).

“Postnatal microbial colonization programs the hypothalamic–pituitary–adrenal system for stress response in mice” by Nobuyuki Sudo et al., J Physiol (2004).

“Normal gut microbiota modulates brain development and behavior” by Rochellys DH, et al., Proc Natl Acad Sci U S A. (2011).

Autism Speaks Study: Kids with Autism Have Fewer Kinds of Gut Bacteria, Available at: http://www.autismspeaks.org/science/science-news/studykids-autism-have-fewer-kinds-gut-bacteria

“Voices from within: gut microbes and the CNS” by Forsythe P and Kunze WA, Cell Mol Life Sci (2013).

“The brain-gut interaction: The conversation and the implications” by Prins A, S Afr J Clin Nutr (2011).