25/02/2016 2:50 PM AEDT | Updated 15/07/2016 12:51 PM AEST

The Diet Myth: Your Gut Bacteria Play A Bigger Role In Weight Loss Than You Think

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Tomato dissection: from plant to ketchup.

More than half of the Australian population is overweight or obese. Yet, despite all the steps we’re taking to battle the bulge, we are still fat.

We’ve tried cutting carbs, switching sugary drinks for diet varieties, saying no to fat and banning dairy. That, combined with vigorous exercise -- and still nothing.

As a result, researchers are looking for clues elsewhere to what controls our weight.

And the answer that keeps popping up? Our guts.

“The evidence is rapidly showing that the hundred trillion microbes [or bacteria] that live in our gut are crucial to body size and metabolism,” Professor of Genetic Epidemiology, Kings College London and author of The Diet Myth, Tim Spector told The Huffington Post Australia.

But even more surprisingly, research shows that our microbiome (where the gut bacteria live) may hold the key to why some people aren’t able to shift excess weight.

A new SBS documentary called The Diet Myth airing on Thursday night examines this concept by looking at overweight people who have struggled to shift weight for years and the state of their microbiome.

“The documentary shows how obese people have a less diverse microbiome compared to normal people,” Spector said.

Basically they have less species of good bacteria in their gut caused by a combination of things.

“A diet consisting of a lot of junk food which starves the good microbes in the gut, years of dieting where particular food groups are cut out and also, overuse of antibiotics,” Spector said.

What all of this does is spin your microbiome into a poor state which may contribute to someone's inability to lose weight.

Spector likens your microbiome to a rainforest in that, if one species becomes extinct, it throws the rest of the stable ecosystem out.

The documentary includes a mini Supersize Me experiment to demonstrate the effect junk food has on our gut.

To illustrate this, he used his own son for an experiment where he put him on a diet of only burgers and chicken nuggets for three weeks to see the effect it would have on his microbiome.

“The idea was to recreate Supersize Me with the microbiome on a slightly smaller scale,” Spector said.

Fast food is a feast for the upper intestine but it leads to famine down below where your gut bugs are waiting to be fed.

The result? The microbes in his system significantly reduced.

But it’s not just a crappy diet. Our genes play a role too. Researchers have discovered that some people posses fat busting microbes, just as some possess microbes that puts them at higher risk of being overweight.

“Studies have clearly shown if you take microbes from fat people and put them into skinny mice, you can make them fatter. In a way, it’s an infectious cause of obesity,” Spector said.

As a whole, most of the microbes researchers have found are protective against obesity which means the more you have, the better chance you have of maintaining a healthy weight.

What Spector found in his research was that after looking at years of data, most of the things he believed relating to calories in, energy out had no evidence.

“This idea of 'eat less fatty foods, exercise more and you’ll be fine' has failed,” Spector said.

Professor Tim Spector said avoiding processed food is crucial for a diverse microbiome.

Instead, it should be about eating a diverse range of food including lots of fibre so as to give your gut a wide range of microbes.

“It’s not what you should cut out, it’s what you should eat instead. This obsession about fructose or certain types of saturated fats is missing the point,” Spector said.

Experts agree the key is to increase diversity in our food intake, not reduce it.

“It’s about thinking, this isn’t just about the scales -- it’s about how you can get the hundred trillion microbes in your gut healthier by growing more species from a range of healthy food,” Spector said.

To do that, we should be avoiding processed food, minimising antibiotic intake and following a Mediterranean-style diet.

“If we can have a good baseline diet, people can have the odd burger. There’s evidence that if we increase our fibre levels and diversity our guts are much more robust,” Spector said.

So is it really this simple? And is it time to say goodbye to diets once and for all?

“Diets don’t work, they are psychologically unsustainable and a one size fits all regime doesn't work because everyone is different."

"The only 'diet' I agree with is some intermittent fasting like the 5:2 plan, while keeping with your diverse food intake with a very high fibre content,” Spector said.

The Diet Myth airs on Thursday, 8.30pm on SBS