News

Fat or thin: How gut bacteria affects our eating

When we can't lose weight, we tend to want to blame something outside our control. Could it be related to the microbiota - the bacteria and other organisms - that colonise your gut?

You are what you eat

Our gut harbours some trillion microorganisms. These are key in harvesting energy from our food, regulating our immune function, and keeping the lining of our gut healthy.

The composition of our gut microbiota is partly determined by our genes but can also be influenced by lifestyle factors such as our diet, alcohol intake and exercise, as well as medications.

What is the human microbiome?

The bacteria in the gut obtain energy for growth when we metabolise nutrients from food. So our diet is a crucial factor in regulating the type of bacteria that colonise our gut.

One key role of the gut microbiota is degrading the carbohydrates we can't digest into short-chain fatty acids. These help regulate our metabolism and are also important for keeping our colon cells healthy.

Changes in our diet can rapidly change the gut microbiota. Generally, a high-fibre diet which is low in saturated fat and sugar is associated with a healthier gut microbiome, characterised by a greater diversity of organisms.

On the other hand, diets high in saturated fat and refined sugars with low fibre content reduce the microbial diversity, which is bad for our health.

Our animal studies have shown that consuming an unhealthy diet for only three days a week has detrimental effects on the gut microbiota, even when a healthy diet is eaten for the other four days.

This may be because the gut microbiota are under selective pressure to manipulate the hosts' eating behaviour to increase their own fitness. This may lead to cravings, akin to your system being "hijacked" by your microbiota.

Can gut microbiota changes lead to obesity?

Bacteria in humans fall into two major classifications: bacteroidetes and firmicutes. Obesity is associated with a reduction in the ratio of bacteroidetes to firmicutes but weight loss can reverse this shift.

Many studies have found that the gut of an obese person is more likely to contain bacteria that inflame the gastrointestinal tract and damage its lining. This allows the bacteria in the gut to escape.

We still don't know definitively if changes in the gut microbiota from an unhealthy diet can contribute to obesity.

Most evidence supporting this hypothesis comes from animal studies; for instance, the transfer of faecal material from an obese human can lead to weight gain in a recipient mouse.

One possibility is that the obese microbiota may be more efficient in harvesting energy, in part, by influencing the host to eat foods which favour its growth. This could ultimately contribute to weight gain.

Gut changes after weight-loss surgery

Bariatric surgeries such as gastric bypass, are one of the most effective treatments for obesity because they reduce the size of the stomach.

This limits how much food can be eaten and has also been shown to promote the release of hormones which make us feel full.

But other factors may be involved. Intriguingly, some patients report a shift in food preference away from energy-dense foods after surgery. This may contribute to the success of the procedure.

Gastric bypass-induced weight loss has also been associated with increased diversity of the gut microbiota. But how much this contributes to the success of the procedure remains to be determined.

One possibility is that the changes in food preferences reported in bariatric patients may relate to changes in the composition of their gut microbiota.

How gut microbiota affect our behaviour?

Apart from regulating gut health, there is compelling experimental evidence that gut microbiota play a role in regulating mood.

Several studies have shown that depression is associated with changes in the gut microbiome of humans.

Depressed patients showed changes in their abundance of firmicutes, actinobacteria and bacteroidetes.

When these patients' gut microbiota was transferred to mice, the mice showed more depressive behaviour than mice that received biota from healthy people.

More work still needs to be done as it is unclear whether this may indicate a causal relationship, or be related to other factors associated with depressive disorders such as a poor diet, changed sleep patterns and drug treatment.

Emerging evidence suggests that gut microbiota can influence other behaviours through the "microbiota-gut-brain axis".

Put simply, the gut and the brain communicate in part via the microbiota, which links the emotional and cognitive centres of the brain with our intestinal functions.

Recent work from our lab showed that rats consuming diets high in saturated fat or sugar, for just two weeks, had impaired spatial memory.

These rats consumed the same amount of energy as the control rats (those on a regular diet) and were also a similar body weight.

We found that the memory deficits were associated with changes in the gut microbiota composition and genes related to inflammation in the hippocampus, which is a key brain region for memory and learning.

Similar memory deficits have also been reported when healthy mice were transplanted with microbiota from overweight mice who had been fed a high-fat diet.

Together, studies such as these suggest the gut microbiota could play a causal role in regulating behaviour. This may, in part, be due to the different microbiota profiles influencing the production of key transmitters such as serotonin.

What can you do now?

Further research is needed into the relationship between poor diet, the gut microbiota and behavioural changes. In the long term, such knowledge may be harnessed to develop targeted therapeutic interventions to replace relevant microbiota diminished by an unhealthy lifestyle.

Meanwhile, the good news is that the gut microbiota can change relatively quickly and we have the capacity to promote the growth of beneficial bacteria which may ultimately improve a range of health outcomes. Eating a healthy diet of unprocessed foods, including adequate fibre, avoiding excess alcohol and getting enough exercise are key.

Margaret Morris
Professor of Pharmacology, Head of Pharmacology, UNSW Australia

Jessica Beilharz
PhD candidate, UNSW Australia

This article first appeared at The Conversation. It is part of a series which you can check out here.

Topics:  bacteria general-seniors-news health kick the kilos obesity science weight loss wellbeing



Keeping up with an ageing population

Forum presenter, Aged Communications Consultant, Glen Sorensen.

New reforms for aged care service providers

Coast Rd traffic light stoush ahead of meeting

Aerial shot looking south from Skennars Head towards Ballina.
Photo The Northern Star Archives

Future of picturesque farm up for discussion

Grow your own dwarf citrus trees

There's a new meyer lemon called Lemonicious that produces lots of tangy, juicy fruit, has glossy green leaves and also beautifully-perfumed white flowers in spring. Plus it grows to a very manageable 1m when planted in a pot.

Tips and picks from gardening expert Angie Thomas

Local Partners

Pencil in annual leave: Jetstar flights sale on now

YOU can book a return flight to popular holiday hotspots right across the network and fly home for free, paying only for the departing flight on a return trip.


First female superhero film in 12 years

AMAZON: Actress Gal Gadot in a scene from the film Wonder Woman.

Wonder Woman opens next Thursday

Stories Troy Cassar-Daley carries around

LEGEND: Troy Cassar-Daley after receiving the Golden Guitar Award for Album of the Year during the Tamworth Country Music Festival in January.

You can see him in Ballina this weekend

What to expect at Birds of Tokyo's Ipswich gig

The band will perform at the Racehorse Hotel on Friday.

BAND member Glen Sarangapany talks music, pub grub and doing shoeys

19 dead, 50 injured in Ariana Grande 'terror attack'

NINETEEN people have been killed and at least 50 people have been injured following a terror attack in Manchester

Pitch Perfect star suing Woman’s Day over ‘liar’ articles

Actor Rebel Wilson outside court on Friday.

REBEL Wilson's career destroyed by grubby campaign, court hears.

First female superhero film in 12 years

AMAZON: Actress Gal Gadot in a scene from the film Wonder Woman.

Wonder Woman opens next Thursday

Seven Year Switch: The boner to end all boners

Johnny’s outraged over claims he has a cracked boner.

She gulps. Her face says it all.

Bay to star in Hollywood shark thriller ‘Cage Dive’

Cage Dive, written and directed by Gerald Rascionato, is now screening in the United States of America. It had scenes filmed in Hervey Bay.

And our visiting humpback whales also make appearance.

Celebrity sex tapes: Where does all that money go?

Basically, did Paris and Kim earn fortunes from their videos?

Stories Troy Cassar-Daley carries around

LEGEND: Troy Cassar-Daley after receiving the Golden Guitar Award for Album of the Year during the Tamworth Country Music Festival in January.

You can see him in Ballina this weekend

One of Maryborough's most historic homes is still for sale

FULL OF HISTORY: Trisha Moulds is owner of the historic Tinana state known as Rosehill. The beautiful home is currently for sale.

It has been the scene of both joy and tragedies over the years.

The face of the Sunshine Coast's overpriced rental crisis

Alyx Wilson had to rent a $385 unit in Currimundi because the market was too competitive for cheaper rental housing. She is now renting a room from friends who own a house in Currimundi, and says its much more affordable.

Young people feel the strain in competitive, expensive rental market

WATCH: Take a tour of a tradie's dream home

5a Bruce Hiskens Court, Norman Gardens, going for $720,000. INSET: Lea Taylor.

Huge block with potential for anything

REVEALED: Where it's cheaper to pay off a mortgage than rent

6/190 Ewing Rd, Woodridge, is listed for offers $215,000. Picture: realestate.com.au

Brisbane suburbs where it is cheaper to buy than rent

The hardest place in the state to find a rental property

RENTAL SHORTAGE: Richmond River, Ballina Bar Emigrant Creek Pacific Highway Teven Interchange. Photo Jay Cronan / The Northern Star

Not enough supply to meet insatiable demand

Ready to SELL your property?

Post Your Ad Here!