Airing on ABC’s catalyst series last month, Gut Revolution was a two-part feature seeking to sort the facts from the faeces. Hosted by Dr Joanna McMillan, we followed the path of two people with debilitating gut issues, on their quests for better health. Food and Mood Centre director, Professor Felice Jacka, featured and provided her expertise on the role of the gut and diet, to mental wellbeing. In this two-part recap, we describe the science that has made poo trendy and how we may be one step closer to understanding two rampant health conditions  – irritable bowel syndrome and obesity.

Part 2: Obesity (Garry’s story)
Getting to the guts of weight control
By Meghan Hockey

Episode 2 highlighted Garry’s battle with his weight. As one of the 28% of Australians who are obese, Garry had experimented with many diets, but none that had focused on helping his gut bugs thrive. Researchers now speculate that an imbalance in our gut bugs, known as the gut microbiota, may be implicated in the development of obesity and associated metabolic diseases.

This theory first originated in studies conducted in mice. When the gut bugs of obese mice were transferred into lean mice that had no gut bacteria, the lean mice gained an extra 60% body fat in a mere two weeks. How? It is thought that the gut bacteria of these obese mice metabolised food differently. This meant that more energy was harvested from food, which subsequently increased fat deposition in the body. Such studies are profound as they imply that our gut bugs may be the cause, and not just a consequence, of metabolic diseases.

If you aren’t convinced yet, studies in humans have also demonstrated metabolic changes after the transfer of gut bacteria. Six weeks after men with metabolic syndrome received gut bugs from lean donors, their insulin sensitivity improved. These improvements were proposed to be due to increases in butyrate, a short chain fatty acid (SCFA) that is produced by gut bacteria. SCFA’s confer numerous health benefits and are suggested to play a role in decreasing our appetite and influencing fat storage. While there are still very few studies containing large numbers of participants, research is clearly pointing to the importance of gut bacteria in influencing our weight.

How diet can shape our microbiota – a focus on obesity

We now know that our gut bugs may be able to influence obesity, but if this is the case, how do we change our gut bugs? Our diet can leave an imprint on our microbiota, altering its composition in as little as 24 hours. More specifically, three components of our diet – fat, fibre and periods of intermittent fasting – have been reported to be influential in altering our microbial composition. We turned to the literature to understand how ‘eating for the microbiome’ may help Garry and others, shed the extra kilos.

Dietary fibre

Much of the attention on the gut microbiota and weight loss has focused on fibre and in particular, the action of prebiotic fibres. The consumption of these non-digestible carbohydrates promote the growth of beneficial bacteria in the gut with studies conducted in humans suggesting that this can alter the composition of the gut microbiota, improve insulin sensitivity and alter levels of inflammation in the body. These all have favourable outcomes on obesity, with a recent review supporting that prebiotics can influence obesity via multiple pathways.

In a recently published randomised controlled trial, a prebiotic supplement was found to selectively alter the gut microbiota and significantly reduce body weight in obese and overweight children. Specifically, children in the prebiotic group were found to have significant increases in the bacterial specifies Bifidobacterium, which has been associated with leaner body types. However due its small sample size we require further studies to determine whether this inexpensive and non-invasive intervention may help combat one of the most costly health conditions in society.

Luckily for us, many foods are naturally high in prebiotics meaning we can skip this added step of taking a supplement! Monash University offers a comprehensive list of high fibre, high prebiotic foods with garlic, onions, chickpeas, cashews and barley reigning at the top of this list.

High fat diets

High fat diets, particularly those high in saturated fats, have been incriminated as a primary culprit in the development of obesity. More so, evidence now suggests that high fat diets can alter the gut microbiota leading to decreased abundance of bacteria that are associated with leaner body types. These changes in the microbiota can occur in as little as 24 hours after eating, and are suggested to lead to increased inflammation and insulin resistance –both of which play a role in the development of obesity.

It was proposed that Garry’s difficulties in keeping the weight off might be due to his ‘microbiome’s memory’. In animal studies, mice were shown to retain a memory of their previous high fat diet; this accelerated weight gain when the mice returned to their high fat diet after having their intake restricted. Interestingly, when researchers supplemented the mouse’s diet with polyphenols, the same re-gain in weight was not seen. Although mechanisms are yet to be elucidated, this may suggest that consuming adequate polyphenols may reverse the high fat phenotype and help prevent weight re-bounding.

In short, keep your gut bugs happy by steering clear of high fat foods (pastries, deep-fried foods, fatty meats etc.); selecting healthy fats such as olive oil over butter, in small amounts; and adding vegetables to your meals to get that polyphenol boost.

Intermittent fasting

Fasting has been in vogue in recent years for its beneficial effects on weight loss. As with fibre and fats, fasting can also alter the microbiota, which reflects the complexity of the diet-microbiota relationship.

Interestingly, animal studies are now suggesting that fasting, through its modulation on the gut microbiota, influences the levels of brown and beige adipose tissue in the body. These storage sites of fat burn more energy compared to their hefty cousin- white adipose tissue. Because of this, it is suggested that the weight loss associated with fasting may be partly due to our gut microbiota activating these beige fat storage sites. This provides novel insights into how the microbiota may be a therapeutic approach for obesity although, it is important to point out, that at this stage these observations have only been in a small number of animal studies.

Like most diets restricting energy, intermittent fasting can result in weight loss. However, the intermittent nature of the fasting periods may be a more sustainable alternative to weight loss for many. A further benefit of this approach is that periods of intermittent fasting have also shown improvements in insulin resistance and cardiovascular health; although further research is required to establish whether these effects are due to changes in the microbiota.

What does this all mean?

For Garry, increasing his fibre intake, reducing his fat consumption and including intermittent periods of fasting dramatically shifted his microbiome. Whether or not these microbiome changes were responsible for his weight loss – and improvements in health – still needs to be answered by large-scale human interventions. At present, there is still a lot more research to be done before we can understand what all these changes to the microbiome mean. Irrespective of this, it is clear that eating more fibre, reducing saturated fat intake and achieving a weight within the healthy weight range is beneficial for a myriad of health conditions. While we wait for some more conclusive evidence to emerge, sit back and watch this space, with some high prebiotic cashews and some polyphenol-rich vegetables in hand of course.

Meghan Hockey is an Accredited Practising Dietitian and a PhD candidate with the Food and Mood Centre

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