YOU CAN’T GO ANYWHERE WITHOUT being confronted by calories. Some restaurants now print calorie counts on menus. You go to the supermarket and there they are, stamped on every box and bottle. You hop on the treadmill and watch your “calories burned” click upward.

But just what are calories? The more calories we take in, the more flab we add – and if we cut back on them, the flab starts to recede too, right? After all, at face value, calories seem to be the factor by which all foods should be judged. But if that were true, 500 calories of sweet potato would equal 500 calories of Double-Chocolate Tim Tams.

Not quite. There’s nothing simple about calories. So learn the distinctions and lose the lard.

CALORIES FUEL OUR BODIES

A calorie is simply a unit of measurement for heat – in the early 19th century, it was used to explain the theory of heat conservation and steam engines. The term entered the food world around 1890 in America, when the US Department of Agriculture appropriated it for a report on nutrition. Specifically, a calorie was defined as the unit of heat required to raise one gram of water by 1°C.

To apply this concept to foods like sandwiches, scientists would set food on fire (yep, really), then gauge how well the flaming sample warmed a water bath. The warmer the water, the more calories the food contained. (Today, a food's calorie count is estimated from its carbohydrate, protein and fat content.) In the calorie’s leap to nutrition, its definition evolved. The calorie we now see cited on nutrition labels is the amount of heat required to raise one kilogram of water by 1°C.

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Here’s the problem: your body isn’t a steam engine. Instead of heat, it runs on chemical energy, fuelled by the oxidation of carbohydrates, fat and protein that occurs in your cells’ mitochondria. “You could say mitochondria are like small power plants,” says Dr Maciej Buchowski, a research professor of medicine at Vanderbilt University. “Instead of one central plant, you have several billion, so it’s more efficient.”

Your move Track carbohydrates, fats and protein – not just calories – when you’re evaluating foods.

ALL CALORIES ARE CREATED EQUAL

Our fuel comes from three sources: protein, carbohydrates and fat. “They’re handled by the body differently,” says nutritionist Alan Aragon. So that old “calories in, calories out” formula can be misleading, he says. “Carbohydrates, protein and fat have different effects on the equation.” Example: for every 100 carbohydrate calories you consume, your body expends 5-10 in digestion. With fats, you expend slightly less (although thin people seem to break down more fat than heavy people do).

The calorie-burn champion is protein: for every 100 protein calories you consume, your body needs 20-30 for digestion, says Buchowski. Carbohydrates and fat give up their calories easily: they’re built to supply quick energy. In effect, carbs and fat yield more usable energy than protein.

Your move If you want to lose weight, make protein a priority at every meal.

A CALORIE INGESTED IS A CALORIE DIGESTED

Just because the food is swallowed, it doesn’t mean it'll be digested. It passes through your stomach and then reaches your small intestine, which slurps up all the nutrients it can through its spongy walls. But 5-10 per cent of calories slide through unabsorbed. Fat digestion is relatively efficient – fat easily enters your intestinal walls. As for protein, animal sources are more digestible than plant sources, so the protein from a steak will be better absorbed than tofu’s protein.

Different carbs are processed at different rates, too: glucose and starch are rapidly absorbed, while fibre dawdles in the digestive tract. In fact, the insoluble fibre in some complex carbs, such as that in vegetables and grains, tends to block the absorption of other calories. “With a very high-fibre diet, say 60 grams a day, you might lose as much as 20 per cent of the calories you consume,” says Dr Wanda Howell, a professor of nutritional sciences at the University of Arizona.

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So a useful measure of calories is difficult. A lab technician might find that a piece of chocolate and a piece of broccoli have the same number of calories. But in action, the broccoli’s fibre ensures that the vegetable contributes less energy. A study in the Journal of Nutrition found that a high-fibre diet leaves roughly twice as many calories undigested as a low-fibre diet does. And fewer calories means less flab.

Your move Aim to consume at least 35-40g of fibre every day.

EXERCISE BURNS MOST OF OUR CALORIES

Even the most fanatical fitness nuts burn no more than 30 per cent of their daily calories at the gym. Most of your calories burn at a constant simmer, fuelling the automated processes that keep you alive – that is, your basal metabolism, says Warren Willey, the author of Better Than Steroids. If you want to burn fuel, hit the accelerator in your everyday activities.

“Some 60-70 per cent of our total caloric expenditure goes toward normal bodily functions,” says Howell. This includes replacing old tissue, transporting oxygen, mending minor shaving wounds and so on. For men, these processes require about 11 calories per pound of body weight a day (about 100 kilojoules per kilogram), so a 90kg man will incinerate about 9240kJ a day – even if he sat in front of the TV all day.

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And then there are the calories you lose to NEAT, or non-exercise activity thermo-genesis. NEAT consists of the countless daily motions you make outside the gym – the calories you burn while making breakfast, walking from the office to the coffee shop, or chasing the bus.

Your move Take frequent breaks from your desk (and couch) to move your body and burn bonus calories.

LOW-CALORIE FOODS HELP YOU LOSE WEIGHT

Processed low-calorie foods can be weak allies in the weight-loss war. Take sugar-free foods. Omitting sugar is perhaps the easiest way to cut calories. But food manufacturers generally replace those sugars with calorie-free sweeteners, such as sucralose or aspartame. And artificial sweeteners can backfire.

One University of Texas study found that consuming as few as three diet sodas a week increases a person’s risk of obesity by more than 40 per cent. And in a Purdue University study, rats that ate artificially sweetened yoghurt took in more calories at subsequent meals, resulting in more flab. The theory is that the promise of sugar – without the caloric payoff – may actually lead to overeating.

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“Too many people are counting calories instead of focusing on the content of food,” says Alderman. “This just misses the boat.”

Your move Avoid artificial sweeteners and load up your plate with the bona fide low-calorie saviours: fruits and fresh vegetables.