The issue of sugar and the role it plays in our health is an ongoing and all-important discussion.
We've all heard the stats before: nearly two in three Australian adults are overweight or obese, and 25 percent of children are overweight or obese.
But what is it about sugar that causes such dramatic weight gain, year in and year out? Is all sugar dangerous? Or is there a way to make eating sugar safe?
Robert Lustig, Professor of Paediatrics in the Division of Endocrinology at University of California in San Francisco, can answer all of these questions.
Leptin (the 'satiety hormone')
The reason sugar is so detrimental, even toxic, to our bodies has all to do with its effect on leptin (the 'full' hormone) and insulin.
"Leptin is a hormone that is made by fat cells in response to energy deposition," Lustig told The Huffington Post Australia.
"It is released in the bloodstream and goes to various organs in the body to tell the organs there is enough energy on board to engage in energy-expensive metabolic processes. In other words, leptin ensures the body is not starving."
For instance, puberty, pregnancy and making strong bones are "expensive" bodily process. If you don't have leptin, you don't go through puberty, you can't get pregnant and you can't make strong bones.
"Leptin allows all these things to occur because the rest of the body, whether it's the brain or bones, seize leptin."
The way leptin works is when leptin rises, your appetite diminishes, which means leptin is crucial in controlling how much you eat.
"When your brain sees the leptin signal, it says 'I'm not starving, I don't need to eat so much' and your body can procreate, menstruate, have a baby, build strong bones, have a normal immune system and so on," Lustig said.
"When leptin does not transduce a signal, then the brain says 'I'm starving, I need to ratchet down my expenditure and ratchet up my food intake in order to source energy and be able to make leptin'."
Basically, leptin regulates your metabolism and the rate of fat breakdown. As leptin levels rise, your metabolic rate increases. As leptin levels fall, your metabolism slows.
"It's like a thermostat. The thermostat reads the temperature in the house -- if it's too cold, it turns on the heat, and if it's hot enough it turns off the heat."
What we've learned from our research is that the hormone insulin blocks leptin.
So what does leptin have to do with sugar and insulin? A lot. Because what we eat (sugar) can negatively affect our leptin (so, your metabolism and appetite) levels.
"Leptin resistance. That is, there's plenty of leptin but the brain is not responding to it," Lustig explained.
"That could either be because the receptor has a mutation, or there's something downstream of the leptin receptor that's causing something to not work right. That's what the rest of obese people have.
"So what do we do? What we've learned from our research is that the hormone insulin blocks leptin."
Insulin (the hormone that converts food into glucose)
The reason insulin blocks leptin is because insulin tells your fat cells to store energy (weight gain).
"Insulin shunts the energy that you eat (sugar, if you will) into fat. The fat cells turns that energy into fat, fatty acids and triglycerides, and then the fat cell makes leptin," Lustig said.
But too much insulin can stop leptin from doing its job -- to keep us feeling full.
"If insulin's job is to store energy, then it makes sense then that it's the thing that stops leptin from working. Otherwise you wouldn't be able to store the energy, as you would stop eating because the leptin is working."
"The question is what made the insulin? Why is everybody's insulin 2-4 times higher today than it was 40 years ago? That's the problem."
And according to Lustig, leptin resistance isn't the problem. Leptin resistance is the symptom of the problem. What made the leptin resistant?
"That's where sugar comes in."
Sugar (glucose and fructose)
Sugar. It's in all our favourite foods -- biscuits, doughnuts, cakes, pastries, lollies, chocolate.
"Sugar is made of two molecules -- one's called glucose and one's called fructose," Lustig told HuffPost Australia.
"Glucose is the energy of life. Every cell on the planet needs glucose for energy. Glucose is so important that if you don't consume it your body makes it.
"For instance, there are no plants in the north pole. Eskimos eat whale blubber -- protein and fat. They don't eat carbohydrates, but they still have a normal glucose level."
This process of turning fat into glucose is called gluconeogenesis and occurs in the liver.
"Glucose is in starch. It's what starch is made of. Bread, rice, pasta, potatoes, lentils, quinoa, farro, beans. That's all glucose.
"When you consume glucose (for example, a slice of white bread) only 20 percent of the glucose ends up in the liver. Eighty percent of it ends up in the other organs because all the cells of your body can utilise glucose for energy."
The major problem is the combination of glucose and fructose, of which the Western diet is full -- found in fruit juice, soft drink, lollies, cordial, sugar and syrups.
"Sugar, the sweet stuff, is glucose and fructose. One-and-one. Fructose is not glucose. Fructose is sweet and is the reason we like and crave sugar. It's the addictive compound in sugar. Glucose is not," Lustig said.
"Fructose is not metabolised by the other organs in the body. Only the liver can deal with fructose. When you consume sugar, 20 percent of the glucose will go to the liver, but 100 percent of the fructose.
"And if you overwhelm your liver's capacity to metabolise fructose, which happens relatively easily, then that extra fructose will get turned into liver fat. This is a process called de novo lipogenesis. It's how you turn sugar into fat."
Either the liver will release the fat into the bloodstream, which will raise triglycerides (a risk factor for heart disease and obesity), or the liver won't release it and "it will stay there and precipitate".
"Now you've got fatty liver disease. Your pancreas has to make extra insulin to make the liver do its job. Now you've got high insulin levels everywhere. You're gaining weight and your insulin is blocking the leptin in the level of the brain -- making you hungrier. You've got a vicious cycle of consumption and disease."
Fibre (the way to make sugar safe)
If you're freaking out about sugar right now, remember this:
"There's a way to make fructose and glucose safe. It's called fibre. In other words, fruit, which has fibre," Lustig said.
There are two types of fibre: soluble and insoluble fibre. Soluble fibre is like the pectin which holds jam together, and insoluble fibre is like cellulose (like the stringy stuff in celery). You need both -- and fruit has both.
"When you consume both (a piece of fruit), what happens is the insoluble fibre forms a net or lattice on the inside of your intestine. Then the soluble fibre plug the holes in the net," Lustig explained.
"What you end up with is a whitish gel that coats the inside of your intestine and acts as a barrier so that you don't absorb all of the sugar fast -- you absorb is slowly, and this protects the liver and so it won't make that liver fat."
And this is where fibre gets really cool: when we eat fibre, it feeds our gut bacteria. We don't absorb it for our own use or storage, Lustig explained.
"In addition, since you didn't absorb it fast, what you ate will go further down the intestine. There's something in the second part of the intestine that's not in the first part -- bacteria. The microbiome. Those bacteria have to eat to live.
"When you consume your food with fibre your bacteria get more. And that lets the good bacteria grow, which means the bacteria chew up the energy instead of you absorbing it.
"So even though you ate it, you didn't get it. The bacteria did. That's a good thing."
Essentially, fibre is the antidote to sugar. Fibre is the reason fruit is okay.
"But the problem is when you take the fibre out -- then you just have the sugar. That's what is not okay. For example, fruit juice," Lustig said.
"That's the difference between real food and processed food. Real food is low sugar and high fibre. Processed food is high sugar and low fibre.
"People have been told their entire lives that processed food is food. It turns out it's not. Processed food is how we go into this mess."
Instead, stick to whole foods, good starches, lean protein, good fats, and cut out sugar and processed foods. In Lustig's words, "It's called real food".
And thanks to fibre in fruits, vegetables and unprocessed complex carbohydrates, a calorie is not a calorie if it comes with fibre.
"We try to explain that calories are not the issue. The primary reason is because if you're consuming the food with fibre, even though you ate the calories, you didn't absorb the calories, so the mass is irrelevant because they weren't yours (it was the gut bacteria's)," Lustig said.
"That's why counting calories has been an unsuccessful way to lose weight and get healthier. It's why we have 40 years of unsuccessful obesity therapy around the world. If you believe calories are part of the problem, then you're part of the problem."
Dr Lustig will be in Australia to present at the 5th BioCeuticals Research Symposium in Sydney on 21-23 April. His new book, The Hacking Of The American Mind, will be out later this year. Available on Amazon.
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