Introduction

‘Microbiota’ is the term used for the diverse microbial community living in your body, especially in the gut.^1,2 Each individual has a unique gut microbiota that plays an important role in their health.¹ Consequently, any disruption in the composition of the microbiota can lead to several problems like weight gain, diabetes, and intestinal disorders.^2,3

In this article, we'll take a closer look at how these microorganisms impact your health and what you can do about it.

What is the gut microbiota?

The gut microbiota is composed of various microorganisms like bacteria, yeast, and viruses. The most studied components of the microbiota are bacteria, which can be classified into different groups, based on their characteristics:^1,3 

• Firmicutes 
• Bacteroidetes 
• Actinobacteria 
• Proteobacteria 
• Fusobacteria 
• Verrucomicrobia 

Among these, the Firmicutes and Bacteroidetes make up around 90% of the gut microbiota.¹

The role of the gut microbiota in human health

The gut microbiota helps with several functions in your body, including:^3,4

• Digesting molecules we cannot digest, like dietary fibre. These bacteria break down complex sugars, which help make compounds that are good for the intestinal wall
• Metabolism of proteins, fats, and fatty acids. For example, some bacteria help control how our bodies absorb fats by activating certain receptors in the small intestine
• Synthesising vitamin K and B vitamins
• Modulating mucus secretion by intestine cells
• Supporting the growth of cells in your intestines
• Helping the integrity of the epithelial barrier in the intestine
• Protecting us from harmful bacteria by making germ-killing molecules, or producing substances that stop them from growing
• Stimulating local and overall immune responses by triggering the production of certain immunological proteins and by activating immune cells such as macrophages, lymphocytes, and dendritic cells
• Influencing bone metabolism by making certain acids that change the pH level in the gut, which helps the body absorb more calcium and reduces the breakdown of bones

Impact of gut microbiota on health conditions

Gut dysbiosis and disease

‘Dysbiosis’ refers to an imbalance in gut bacteria that frequently occurs when the growth and division of gut bacteria slows down.⁴ This can be caused by many factors, like diet, age, and medication (such as antibiotics or chemotherapy).³

Many diseases, like inflammatory bowel diseases, liver problems, obesity, diabetes, osteoporosis, heart issues, and certain cancers, are associated with a decrease in the variety of gut microbes and an increase in harmful bacteria.³ Understanding how bacteria in our gut interact with the immune system is important for understanding how dysbiosis can affect us.⁴

Gut microbiota and metabolic disorders

Dysbiosis can impact how our body reacts to signals from important systems like the hormones, immune system, and nerves. This can play a part in causing metabolic diseases like obesity and diabetes.¹

Animal studies have found that different and specific types of bacteria are different in obese animals compared to non-obese. For example, obese mice tend to have more Firmicutes and fewer Bacteroidetes bacteria in their gut compared to lean mice. These changes in gut bacteria can affect how the body processes food, leading to increased fat storage and weight gain.¹

Studies about type 2 diabetes have also shown that people with type 2 diabetes have different gut bacteria compared to those without the condition. In particular, type 2 diabetes is associated with a decrease in bacteria that produce butyrate, a beneficial short-chain fatty acid, and an increase in opportunistic pathogens.¹ Another study showed that in people with type 2 diabetes, changes in the gut bacteria were closely linked to insulin resistance and some research suggests that the bacteria in our gut can affect how our body controls sugar levels.²

Gut microbiota and intestinal disorders

The gut microbiota has been associated with different intestinal disorders.^1,2

Irritable bowel syndrome (IBS): IBS is a common gut disorder, and researchers have found differences in the types of bacteria present in the gut of people with IBS compared to those without it. For example, there's a reduction in beneficial bacteria like Lactobacillus in people with IBS, while other bacteria groups, like Proteobacteria and Firmicutes, are more common.

Some studies show that when there's less diversity in gut bacteria, it might affect how our body makes amino acids, keeps our cells connected, and deals with inflammation, which could be one reason why people experience symptoms of irritable bowel syndrome (IBS).¹

Inflammatory bowel disease (IBD): IBD includes conditions like ulcerative colitis and Crohn's disease, which cause inflammation in the gut. Research has shown differences in gut bacteria between IBD patients and healthy people. For example, there's a decrease in certain bacteria families like Lachnospiraceae in IBD patients. In Crohn's disease, some bacteria like Faecalibacterium prausnitzii are reduced, along with a reduced overall diversity. Studies suggest that restoring the balance of gut bacteria, as a probiotic, could help treat IBD.¹

Coeliac disease (CD): CD is a gut inflammatory disorder triggered by gluten in the diet, affecting people genetically predisposed to it. Studies have found differences in gut bacteria between CD patients and healthy individuals. For example, beneficial bacteria like Bifidobacterium are reduced in CD patients, while potentially harmful bacteria like Bacteroides and Escherichia coli are increased. Even after adopting a gluten-free diet, some imbalances in gut bacteria may persist in CD patients.¹

Gut-brain axis and mental health

The gut and the brain are connected through the gut-brain axis, a pathway that enables the brain to affect gut functions, and vice-versa.¹

Diseases like Alzheimer's and Parkinson's, the most common forms of dementia in elderly individuals, have been linked to changes in gut bacteria. For example, in Alzheimer's patients, certain types of bacteria are found in higher or lower amounts compared to healthy individuals - and this dysbiosis has been associated with inflammation that causes cognitive impairment and accumulation of substances in the brain related to Alzheimer's disease. Similarly, Parkinson's patients have different gut bacteria than healthy people. This dysbiosis may contribute to the neuroinflammation that leads to the disease.¹

Children with autism spectrum disorder also show differences in gut bacteria compared to those without ASD and these differences may be related to both gastrointestinal and behavioural symptoms of autism.¹

Overall, understanding how these diseases and conditions are linked to changes in gut bacteria could help develop new treatments in the future.¹

Factors influencing gut microbiota balance

Birth

The way a baby is born affects the early development of their gut bacteria. Babies born vaginally tend to have gut bacteria similar to those found in the mother's vagina, like Lactobacillus and Prevotella. However, babies born via cesarean section get their gut bacteria from their mother’s skin, leading to different types of bacteria becoming dominant, like Streptococcus, Corynebacterium, and Propionibacterium. These early gut bacteria change over time and become more diverse and stable.⁵

Age

At one year of age, the baby's gut has lots of specific bacteria, like Akkermansia muciniphila, Bacteroides, Veillonella, Clostridium coccoides spp., and Clostridium botulinum spp. As children grow older, the variety of bacteria in their gut increases until the microbiota settles into a stable mix similar to adults. This mix is mainly made up of three types: Firmicutes, Bacteroidetes, and Actinobacteria. By around three years old, a child's gut bacteria are a lot like an adult's.¹

When people get older, especially over 70, changes in how their bodies digest food and absorb nutrients can affect their gut bacteria and eating habits, like having a less varied diet, can also make their gut bacteria less diverse.¹

Diet and nutrition

After birth, one of the first things that affect gut bacteria is what they eat, whether it's breast milk or formula. Breast milk has special sugars that are easy for certain bacteria like Lactobacillus and Bifidobacterium to digest. Formula-fed babies have different bacteria in their guts because formula doesn't contain those special sugars. The microbiota babies develop during infancy are important for their immune system. Studies show that breastfed babies have more stable gut bacteria and a better immune response.⁵

After infancy, what people eat shapes their gut bacteria. Vegetarian diets are linked to having healthy gut bacteria, while diets with lots of meat can change the gut bacteria in a bad way. These changes can happen quickly. Eating a lot of meat can make the gut bacteria produce harmful stuff, but a vegetarian diet keeps them working well by breaking down carbohydrates.⁵

Antibiotics 

Antibiotics are like a double-edged sword: whilst they kill harmful and disease-causing bacteria, they also kill the helpful gut bacteria - and can therefore cause they dysbiosis. Studies on mice have shown that antibiotics can get rid of too many good bacteria. They also mess with the body's natural way of fighting off bad bacteria, which can make pathogens like Clostridium difficile grow out of control.^1,5

Lifestyle factors

Other factors, such as geographical location, living in the city versus in a rural environment, cultural practices, and even how often a person exercises can influence the types of bacteria in their gut. For example, daily exercise increases gut microbial diversity, with bacteria involved in the intestinal barrier resistance and inhibition of inflammatory substances.¹

Maintaining a healthy gut microbiota

Here are some approaches to maintaining a healthy and stable microbiota:³

• Following a Mediterranean diet: this diet is rich in vegetables, fruits, whole grains, and fish. It supports beneficial bacteria such as Firmicutes, that help maintain a healthy gut barrier, reducing inflammation. In contrast, a Western diet high in fat and low in fibre can lead to colon inflammation and cancer
• Consuming prebiotics: prebiotics are substances that feed beneficial microorganisms in the gut. The intake of prebiotics, such as dietary fibre can reduce obesity, inflammation, and the risk of colon cancer by promoting the growth of helpful bacteria
• Consuming probiotics: probiotics are live microorganisms that offer health benefits when consumed in adequate amounts. They can help maintain intestinal barrier function, reduce inflammation, and regulate the immune system. Probiotics are found in foods like yoghurt and can also be taken as supplements

Summary

The gut microbiota is a diverse community of microorganisms living in our digestive tract that plays a crucial role in maintaining our overall health. It is predominantly composed of bacteria that aid in several bodily functions such as digestion, vitamin synthesis, and immune system regulation.^1,3,4

Disruptions in the gut microbiota balance, known as dysbiosis, have been linked to a range of health conditions including inflammatory bowel diseases, obesity, diabetes, as well as Alzheimer's and Parkinson's disease.^1,2

Understanding how factors such as diet, use of antibiotics and lifestyle influence gut microbiota and impact health is essential for developing strategies to maintain a healthy balance of these microorganisms.^1,5 By promoting practices such as a balanced diet, avoiding unnecessary antibiotic use, and adopting a healthy lifestyle, individuals can support their gut microbiota and potentially reduce the risk of associated health issues.³