IT'S an oldie, but a goodie.
And for people wanting to lose weight, the old mantra of "energy in, energy out" is still the best method to lose fat and reach a healthy weight, physicist Ruben Meerman said.
Mr Meerman, known as the ABC's "Surfing Scientist", late last year co-authored a new study on what actually happens to the body when people lose fat - confirming the age-old mantra he used to lose, and keep off, 16kg of fat.
His study, published in the BMJ in December last year, showed that when we lose fat, we are essentially breathing out more carbon atoms than the number of carbon atoms in the food we eat.
"Our calculations show that the lungs are the primary excretory organ for fat," the study reads.
"Losing weight requires unlocking the carbon stored in fat cells, thus reinforcing that often-heard refrain of "eat less, move more".
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While the "energy in, energy out" mantra was correct, he said the fat people should be losing was "subcutaneous fat" - or fat that was stored in specialised body cells called "adipocytes" - the fat cells which create beer bellies and dad's "stealth abs".
Mr Meerman said that even people's slimmest friends had the other, essential type of fat; visceral fat, which acted as a "reservoir of energy".
"You really want to empty the fat substances of those (adipocyte) cells - so when you're gaining weight, you're adding fat to those cells and they get bigger," Mr Meerman said.
"But when you lose weight, you're basically taking it out of those cells and it goes into your muscles and is metabolised.
"What you're left with is carbon dioxide and water, with the water escaping through sweat and urine among other ways, and the final step to losing weight is breathing out that carbon dioxide, which comes out through your lungs and that's really where the kilograms go."
Mr Meerman said the way to lose weight and keep it off was to balance the number of kilojoules eaten to the amount of exercise you did.
"When you're counting kilojoules, you're really counting the number of atoms going into the body, so counting kilojoules is really the way to go - you do get a lot of people who've lost faith in this method, but it is 100% correct," he said.
While he said it was possible to lose weight by simply not eating and not exercising, Mr Meerman said the evidence showed that people should get the minimum amount of exercise - or at least half an hour of moderate exercise a day - to stay in optimal health.
"Scientifically speaking that is correct, but we know exercise has so many other benefits, so cutting out exercise would be the last thing I would recommend," he said.
Arnold Schwarzenegger was technically obese
AT THE very peak of Arnold Schwarzenegger's body-building career, he was, technically, obese.
At least, according to his body weight and height when measured using the body mass index (BMI), he was.
It is an example that Ruben Meerman uses to show that the BMI that is a good measure of the weight status of a population can be misleading when applied to individuals.
Mr Meerman said he would not use it as an indicator of general health but it can be useful for people "as long as they use their commonsense".
It is more useful as a measure of population health than as a measure of individual health.
He said Schwarzenegger was 113kg and 1.88m tall at his bodybuilding best - giving a BMI of 31.6, or technically obese.
"Anything over 25 is overweight and anything over 30 is obese, but Arnie clearly wasn't obese at that time." Mr Meerman said.
Fad diets offer empty promises to the desperate
THE shelves of bookstores around the world are chock full of diet plans making outlandish claims and promising easy ways to lose weight.
But a host of experts in nutrition and diets recommend against these fads - opting instead for long-term changes towards regular exercise and a healthier diet.
Both Ruben Meerman, who has studied the science of weight loss, and nutritionist Rosemary Stanton urge people to put in place more sustainable, healthier lives.
In a piece on The Conversation website in 2011, Dr Stanton wrote that the best solution to the obesity epidemic was "to prevent it with healthier eating patterns and more physical activity".
"If it's too late for that, the solution is to make gradual changes to poor eating and exercise habits - and to make changes you can live with forever," she wrote.
Mr Meerman said almost any diet would work "if it is one you can adhere to for long enough". "When I was losing weight, I didn't realise I was having basically an extra meal each day just in my coffee," he said. "But by switching to a long black, I helped cut my fat - people should look at ... potential savings in their diet."