When Australian actor Damon Gameau started adding almost a cup of extra sugar a day to his diet for a new documentary, the results weren't surprising: his belly grew, he gained weight and he felt rotten.

"For 60 days, I ate 40 teaspoons of sugar a day, which is what the average Australian eats every day," he told CTV's Canada AM Tuesday.

But the twist of the experiment was that Gameau wasn't eating a "Supersize Me"-style diet of shakes, pop and junk food; in fact, all those foods were off-limits. He was eating so-called "healthy" foods that just happened to be loaded with sugar.

His resulting health woes are all documented in a new eye-opening movie called "That Sugar Film," which seeks to expose just how much sugar we're all eating and what it's doing to us.

Gameau says many of the "healthy-but-not-healthy" foods he ate are those that parents often feed their kids, including granola bars, cereal, yogourts, juice, canned soups and sauces.

"A lot of us have them with lunch during the day, thinking we're making a smart choice without realizing that some of them have as much -- if not more -- sugar in them as junk foods," he said.

The high-sugar diet devastated Gameau's health pretty quickly. Before the experiment, he ate a clean, healthy diet, but when he suddenly shifted into a high-sugar mode, "the results were really drastic, which none of us expected.”

He gained 15 pounds (6.8 kg) in just two months, as well as 10 cm of visceral fat around his waist. He also developed the early signs of pre-diabetes, heart problems, and something called non-alcoholic fatty liver disease.

It's a condition that Gameau says currently affects about one in four people in Western countries and that virtually didn't exist 35 years ago. A diet high in sugar can cause fat build-up in the liver, which is exactly was happened to Gameau -- in just 18 days.

Beyond all the physical woes, Gameau says the worst effect was the mood swings.

Eating a meal high in sugar causes the body to release insulin into the bloodstream. But a few hours later, the body can go through a "sugar crash" called reactive hypoglycemia that leads to sudden weakness, lightheadedness, headaches and feelings of anxiousness or irritability.

Gameau hated that feeling but he says it's probably one that many that many of undergo every day.

"Think about children who are having these perceived health breakfasts that are very high in sugar and then struggling to concentrate in the day," he said.

Gameau realized that what we eat has a huge role in our mental state and ability to focus. He also learned that the sugar habit is tough to break.

When he did break the habit, he lost 90 per cent of the weight gain in a few weeks and got back his liver health.

"Once I stopped the experiment and went back to drinking water, eating real foods as much as I could, shopping the perimeter of the supermarket, all my symptoms reversed in 60 days," he said.