Gut bacteria – for a decidedly unsexy topic, it’s been all the rage in the health community, touted as the key to everything from obesity to autism to depression. But when we talk about gut bacteria, what do we mean? And what links are just coincidence, and what could understanding gut bacteria actually solve?

And more importantly — what does it all mean for the average person?

What is gut bacteria?

The idea of bacteria living inside your body might seem strange, but the truth is your body is full of microbes.

“We live in a symbiotic relationship with our gut bacteria,” says Dr. Ajay Jain, chief of gastroenterology, Choithram Hospital and Research Centre, Indore. In other words, gut bacteria help our bodies function, and we give them food and shelter in return.

When doctors, researchers or nutritionists talk about gut bacteria, also called gut flora, they are generally talking first about the balance between good and bad bacteria in the digestive tract, and second, about the composition of the good.

Good gut bacteria constantly work to keep your digestion smooth. They also metabolise vitamins, regulate hormones, and strengthen immunity. Your mix of good microbes is unique and diverse. (Fun fact: The average person, through these trillions of bacteria, is home to almost 100 times more genetic material than their own DNA.)

Each individual’s gut flora develops within the first year or two after birth, built from bacteria passed from the mother and absorbed from the surrounding environment. While there is similarity between the gut bacteria of a mother and newborn, no two people have the exact same composition. There are roughly 30 to 40 different species of good bacteria that, in different combinations and quantities, make up most people’s gut flora.

But bad bacteria can creep in and proliferate over the years, just like bad bacteria can get trapped in, say, your lungs and cause bronchitis. An imbalanced diet, poor exercise routine and overuse of antibiotics – which don’t discriminate between good and bad bacteria – could all be responsible for bad bacteria outnumbering good in the gut, Dr. Jain says.

History of gut bacteria

The recent craze around good gut bacteria means the microbes have been recast as a new discovery by the West. But their benefits are one of the oldest open health secrets there are – and can be traced back to India.

Good bacteria, more commonly called probiotics when referring to foods, can be found in fermented foods, like dahi, which, as a Vedic medicine, is one of the oldest known therapeutic probiotics. Nearly 1000 to 1500 years later, fermented milk was recorded by Pliney the Elder, living in ancient Rome, as a method of treating intestinal disorders. And 1500 years after that, there’s an account of a king of France having a rather disgusting intestinal disorder cured by yogurt.

In the early 20th century, we learned the reason: The probiotics in yogurt helped reset or rebalance good bacteria in the gut. The latter part of that century brought better and more innovative methods to balance gut bacteria to treat, well, the gut, from fecal transplants to treat colitis, to probiotic-infused diets to treat inflammatory bowel syndrome (IBS), says Dr. Jain.

The last decade or so, however, has put the spotlight on gut bacteria with growing insight into the variety and extent of the effects of too much bad or imbalanced good gut bacteria.

Gut bacteria and obesity

The idea that healthy gut bacteria can help with weight loss draws on research that establishes a link between gut flora and obesity. Some small studies suggest the composition of gut flora during early life predicts whether a child will grow up to become obese, while other studies suggest gut flora differs in obese and lean people.

However, such studies have either been conducted on mice or involve only a small number of humans. Also, there are conflicting studies that have found no such difference.

Yet, other studies link gut flora and obesity in different ways. One of the functions of gut bacteria is to ferment fibre into short-chain fatty acids. These then get converted to lipids in the liver and added to the body’s store of fat. Some scientists claim in the case of excess gut bacteria, lipids build up as fat in the liver – a condition called non-alcoholic fatty liver disease, which can contribute to metabolic syndrome.

It’s an important, if not definitive study, to take note of because most dietary advice calls for lots of plant-based fibre: think lentils, mung, whole grains, palak, mustard greens – staples of the Indian diet. And Indians are already predisposed to insulin resistance, a condition that falls under the umbrella of metabolic syndrome.

Gut bacteria and autism

Gut bacteria has been linked to obesity, and obesity in pregnant women has been linked to a clear increase in risk of autism in children. The underlying reason behind this chain could be the imbalance of gut flora passed from mother to child, according to a recent study conducted on mice.

Scientists involved in the study found the offspring of obese mothers in the experiment had problems socializing and had different and less diverse gut bacteria compared to pups of normal-weight mothers. Further, when exposed to the gut bacteria of normal-weight mothers, the offspring of obese mothers showed improvement in their social interactions.

The results, while far from definitive, are considered groundbreaking, and many have made calls for a study among humans.

Gut bacteria and depression

So far, gut bacteria’s relation to the brain is tenuous; much of the research into it has been conducted on mice or very small groups of humans. But the links suggested by recent studies do open up an exciting possibility for treatment of conditions like anxiety and depression.

A Canadian researcher has found that two strains of bacteria reduce anxiety-like symptoms in mice. Another study suggests that introducing a particular bacterial strain in mice has the same effect as giving them a dose of an antidepressant.

It is possible, though by no means proven, that gut bacteria could even help with run-of-the-mill stress: The use of prebiotics (a group of carbohydrates that’s food for gut bacteria) and probiotics by humans has coincided with reduced stress levels.

Gut bacteria and multiple sclerosis

Much seems to be going on between the brain and gut bacteria, and one of the areas where this might manifest, according to recent research, is in neurological disorders like multiple sclerosis, a condition in which the immune system damages neuron’s myelin coat.

In a study conducted among germ-free mice (carefully raised to lack gut flora), about 90 genes were seen to express differently than normal mice. Some of these genes are involved in the crucial process of myelination, the formation of the fatty covering around neurons.

So what does it all mean?

While research on the links between gut bacteria and the above conditions is at varying distances from finality, what’s clear is that gut flora is much more than a humble collection of bacteria.

But for the average person, so far, that doesn’t mean much beyond listening to your grandmother.

“It all boils down to a healthy lifestyle,” Dr. Jain says. “Age-old wisdom on including a bowl of curd in one’s meals, and starting the day with a glass of buttermilk, it seems, was prescient.”