Health & Wellness

Gut health and weight loss

Can gut bacteria affect weight loss? We look at the importance of intestinal health and how changes in your microbiome may be linked to weight gain.
Published 31 July 2018 | Updated 14 September 2023

The role of gut bacteria

Over the past decade, science has made an extraordinary discovery: gut health, it turns out, may play a more important role in maintaining our health than we ever knew. Everything from energy levels to mental health can depend on those trillions of bacteria living in your digestive system. It’s now becoming clear how vital gut health is, not only to digestion but to many other health issues. “We are only just beginning to realise the gut’s marvellous complexity and the way it not only governs the absorption of nutrients, but also regulates the immune system, influences brain function and affects our skin, muscles and joints,” says Professor Kerryn Phelps, one of Australia’s best-known doctors who recently co-authored The Mystery Gut with Dr Claudia Lee and Jaime Rose Chambers.

“One way to think about it is to consider these microbes as our friends on our life journey with us,” says Associate Professor David van der Poorten, Staff Specialist Gastroenterologist and Hepatologist at Sydney’s Westmead Hospital. “They can actually make a big difference to our health, and doing things to keep them healthy and in good order makes a lot of sense.” There’s plenty you can do to boost the good bacteria in your gut and possibly help your health and wellness journey. And it turns out that a diet featuring plenty of – you guessed it! – vegetables, fruits and wholegrains are central to supporting a diverse, happy microbiota.

Here are a few questions and answers about the microscopic critters that call your body home, and how they might affect weight gain.

What is the gut microbiome?

Your body is teeming with trillions of bacteria, viruses, fungi, yeast and other microscopic organisms that live in and on the body, including the lungs, skin and intestines, which is all referred to as the human microbiome. Most of these are found in the large intestine and this is called the gut microbiome. “It is estimated that around 100 trillion microbes live inside the human gut, producing a range of different enzymes, chemicals, hormones and vitamins,” says Dr Jane Varney, research dietitian with the Department of Gastroenterology at Monash University. Weighing up to 2kg, the microbes are key to a number of vital metabolic, immune and nutritional processes. They help to make a variety of vitamins and amino acids, as well as digest carbohydrates and produce short-chain fatty acids. They also aid in metabolising glucose and cholesterol.

Can my gut health affect my weight?

Scientists are only starting to understand the role that gut bacteria may play in weight gain, but animal and human studies are pointing to the importance of a balanced gut microbiome. When the balance is disrupted – a process called dysbiosis – this is thought to affect metabolism, and may also play a role in health conditions as varied as autism, anxiety and depression, inflammatory bowel disease such as Crohn’s and obesity. The balance of bacteria in the gut can be altered through factors such as diet, antibiotic use or illness.

There are two main phyla or families of bacteria found in the gut, Bacteroidetes and Firmicutes. One theory is that the balance of gut microbes differ between people in a healthy weight range and people with obesity. Studies have found that lean people tend to have more of the Bacteroidetes, and their microbiome absorbs less kilojoules. On the other hand populations with obesity tend to have more Firmicutes, leading to the body storing more energy. Associate Professor van der Poorten says people with a healthier bacterial balance may actually absorb fewer kilojoules from their food. “If the gut bacteria is shifted to include more of the healthy range of microbes, the person might absorb less energy from food, and shed more through bowel movements,” he says.

Associate Professor van der Poorten treats patients who suffer from a chronic bowel infection caused by Clostridium difficile bacteria with faecal transplants, which involves transferring faecal matter from a healthy person into the bowel of an infected person. He points to a case study reported in scientific journal Open Forum Infectious Diseases in which a 32-year old lean woman , who received a faecal transplant from someone with overweight, now has obesity after the transplant. At the time of the faecal transplant, the woman weighed 62kg and had a BMI of 26. However, one year after having the transplant, she weighed 76kg and had a BMI of 33. The woman still has obesity, even though she is on a supervised diet and exercise program.

In a 2006 study, researchers from Washington University School of Medicine transferred gut microbes from obese mice into the gut of rodents who were naturally lean. They found that the lean mice became obese after the transfer of gut microbes from the overweight mice. In a recent human study in the journal Cell Host & Microbe, scientists found that a higher fibre intake could lead to changes in the bacterial balance, the diversity in the gut and body weight. “This recent observational study showed that higher fibre intakes and greater species diversity were associated with less long-term weight gain,” says Dr Varney. However, she adds that research in this area is still only in its early stages, so while scientists have found links between dietary fibre intake, certain bacterial species, diversity in the gut and body weight, further studies are needed to tell us how the gut microbiota affects weight.

What is leaky gut?

It’s a controversial diagnosis, but some experts suggest it could have a role to play in weight gain. Leaky gut syndrome is said to involve increased permeability or leakiness of the gut wall. Dr Varney explains this is thought to allow microbes to pass from the intestine into the bloodstream, and the disorder is implicated in an array of conditions, including obesity and diabetes. However, she adds, the existence and significance of the condition remains uncertain. “For instance, there are problems with the accuracy of tests used to diagnose leaky gut syndrome and limited scientific evidence to support either the efficacy of therapies proposed to treat the condition, or the role of leaky gut syndrome in the onset of disease,” she says.

What should I eat to keep my gut healthy?

Three main dietary factors appear to promote a healthy gut: eating lots of fibre-rich foods (known as prebiotics), consuming probiotics which contain active bacteria similar to those living in your digestive tract, and eating a wide range of foods. Healthy bacteria feed on prebiotics – the fibrous, undigested matter from fruit and vegetables that passes into your large bowel – in order to survive. “We need to be eating foods to help enrich the beneficial population of the bowel, and that is the foods that get to the colon largely unchanged,” van der Poorten says.

“Processed foods, fast foods and products made of simple carbohydrates, such as biscuits, are mostly broken down in the small bowel, so by the time it gets to the colon, there’s little left for the microbes in your gut. “Gut bacteria are living inside you, and if you eat foods that never get to the colon, the good population actually starve.” Diversity of microbes is also a factor in gut health, so it’s vital to eat a range of foods to encourage the growth of many varieties of bacteria in your gut. “There’s a high chance all the different bacteria will get what they need if you’re eating a range of foods,” says van der Poorten. “At the end of the day, the diet that is best for your microbiota is lots of fruit and vegetables, moderate amounts of meat, and not too many sugary foods.”

What are probiotics?

Probiotics are foods containing ‘good’ bacteria or yeasts which are similar to those living in your digestive tract, such as yoghurt and other fermented foods. “When taken in adequate amounts, they can improve the balance of the gut microbiota,” says Charlene Grosse, advanced accredited practising dietitian and spokesperson for the Dietitians Association of Australia. They’re found in everyday foods like yoghurt, milk drinks such as kefir, and other fermented foods including kombucha, kimchi, miso, tempeh, sauerkraut and sourdough bread.

How can I increase the good bacteria in my gut?

To boost your gut health, aim for a healthy, balanced and varied diet based around whole foods and foods rich in fibre, says Grosse. “Try to choose those that are high in probiotics, such as yoghurt, milk drinks like kefir, fermented foods and prebiotics including cereal grains, vegetables, legumes, fruit and nuts, as well as resistant starch which is found in firm bananas, lentils, peas, potatoes that have been cooked and cooled, cold pasta, and certain wholegrain products. These can all help to promote a positive gut microbiome.”

What are good gut foods I can eat?

A range of factors are thought to influence the makeup of your gut microbiota, but diet is believed to have the greatest impact, says Dr Varney. “In particular, eating a high fibre diet that includes foods rich in natural prebiotics may be important,” she says. Stock up on these gut-friendly foods in your next shop.

- Baked beans
- Barley
- Beetroot
- Cashews
- Chickpeas
- Dates
- Dried figs
- Garlic
- Grapefruit
- Green peas
- Jerusalem artichokes
- Leek

- Lentils
- Onion
- Pasta
- Pistachio nuts
- Pomegranate
- Red kidney beans
- Rye bread
- Shallots
- Spring onion
- Snow peas
- Watermelon
- Wheat bran