Investigating the interplay of hormones and the microbiome

The microbiome

The human body hosts a vast ecosystem of bacteria, with trillions crawling on our skin, colonising our gut, and living throughout our bodies. Most of these microbes serve to protect us against infections influencing our metabolism and even our behaviour. However, scientists have started to question the mechanisms by which these bacteria affect our bodily functions and characteristics. Scientists have studied these communities of microorganisms residing within our bodies and the genes they contain, yielding new and exciting perspectives…

…Welcome to the human microbiome. 

The microbiome is the dynamic community of microorganisms (like fungi, bacteria and viruses) that exist in a particular environment. In humans, the term is most often used to describe the collection of microorganisms that inhabit a particular body area, such as the gastrointestinal tract, mouth or skin. While a person’s core microbiome is established within the first few years of life, its composition can shift over time in response to factors like medication, such as potent antibiotics and environmental factors.

Researchers have uncovered that the gastrointestinal microbiota can influence some physiological processes, including a direct line of communication between the gut and the brain. But what facilitates this dialogue? What mechanisms enable the gut to relay signals to the brain?

The answer is hormones.

Hormones and the endocrine system

The endocrine system is a network of glands that produce and release chemical messengers known as hormones. They travel via the bloodstream and bind to specific receptors on their target tissues. This binding of hormones to their receptors triggers a response in the target tissue. 

For instance, during stressful situations, epinephrine (also known as adrenaline) is produced by the adrenal medulla, the inner region of the adrenal glands. This hormone, released into the bloodstream, acts on target tissues such as the heart, where it increases heart rate. 

Hormones regulate most of the body’s vital functions through their release. Some of these crucial processes include growth, metabolism, and reproduction. In the following sections, however, we specifically focus on how hormones influence the microbiome.

The interactions between hormones and the microbiome

Exploring the relationship between hormones and the microbiome is known as microbial endocrinology; it is a complex field because there are numerous interactions to account for, and the effects of each one can have lasting impacts on human physiology. 

For example, epinephrine and norepinephrine can lead to more bacteria, notably E. coli and Pseudomonas aeruginosa, signifying that imbalance could harm humans. Also, parts of the host, ranging from mood to gender, impact hormones, bacterial presence and activity (Figure 3).   

An emerging area of microbial endocrinology is how the microbiome and sex hormones engage with each other in disease and female health. One paper noted that disorders from metabolic syndrome (MetS) to type 2 diabetes (T2D) have distinctions in the levels of sex hormones and gut microbiota, indicating that they are essential to understanding in developing those conditions. 

The influence of gut microbiota on sex hormones can occur through various mechanisms, such as bacteria controlling the activity and expression of endocrine receptors and even bacteria metabolising sex hormones; this knowledge can help create treatments against polycystic ovarian syndrome and ovarian cancer, among other diseases that usually impact females due to gut microbiome imbalances (Figure 4). 

Another part of microbial endocrinology being researched is how the microbiome impacts human growth. In one study involving adult male mice, decreased growth hormone (GH) led to undeveloped microbiomes, while surplus GH was linked to an expanded microbiome; this depicts that bacteria influences development via the growth hormone-insulin-like growth factor 1 (GH-IGF-1) axis; maintaining a steady dynamic between the microbiome and this axis is vital for development (Figure 5), particularly in children. 

In puberty, hormones and the gut microbiome interact, as observed in obesity and precocious puberty. Hence, a deeper awareness of the bacteria and sex hormones during puberty is crucial to designing targeted medicines for growth disorders. Moreover, patients with GH-secreting pituitary adenoma (GHPA) have modified gut microbiota, like increased Alistipes shahii and Odoribacter splanchnicus. Still, more research is needed to investigate this.    

Conclusion

The microbiome refers to the millions of microorganisms on and within the human body that influence various physiological functions ranging from digesting food to outcompeting pathogens for resources. Also, the microbiome can affect the endocrine system, which consists of hormones that control glucose and reproduction, among other processes. This bridge, known as microbial endocrinology, has critical applications for understanding women’s health and growth disorders; this emerging area is growing, so it can address knowledge gaps in diseases like cancer and even improve other medical treatments.    

Written by Sam Jarada and Fozia Hassan

The interactions between hormones and the microbiome, and Conclusion sections by Sam

The microbiome, and Hormones and the endocrine system sections by Fozia

Related articles: The gut microbiome / Dopamine and the gut / The power of probiotics / Vitamins

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