Is it all about calorie counting or is focusing on the glycaemic index the way to go? Peta Bee tackles the food-lover's great debate

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One of the longest standing truisms of dieting is that a calorie is a calorie, that the more of them we consume (and the less we expend) the fatter we will get. Countless diets have based themselves on this premise and millions have bought into the calorie crusade. But what if everything we thought we knew about calories wasn’t quite as it seems? Scientists looking at the value of calorie counting in our efforts to shed weight are beginning to think we’d be better off forgetting calories which, they say, affect the body in different ways when certain foods are consumed.

How calories work

Calorie tables as we know them were the result of work carried out more than a century ago. Using a device called a “bomb calorimeter”, chemists literally burned samples of food and measured the amount of energy released from the heat this produced. It was rudimentary. Many of the measurements were based on foods in their raw state and experts now believe that calorie values are more complex and should take into account variations in food make up, preparation methods and processing techniques as well as the energy required by the body to consume a food.

Why consistency matters

“It’s true that the texture and consistency, even the temperature of what you eat influences the amount of energy you need to digest it,” says Louise Sutton, dietician at Leeds Metropolitan University. “A food that is highly processed takes less effort to chew and digest than something high in fibre.”

Protein foods are estimated to use 10-20 times as much energy to digest as fats, losses that are not accounted for in calorie tables or on packaging. Similarly, your body needs to warm frozen and very cold products before they are digested which requires extra energy. “If you were to measure calories taking this into account, then a product such as a chocolate bar that appears on labels to have fewer calories than a fruit flapjack may actually have more because it is easier to digest,” Sutton says. “We now know that no calorie is the same and the calculations for certain foods such as vegetables and high fibre foods are thought to be inaccurate.”

When a calorie is not a calorie

Researchers are investing time and effort into the new science of calories. A study by the US department of Agriculture Iooked at peanuts, pistachios and almonds and found they are less completely digested when eaten than previously thought, mostly because of their tough cell walls. They concluded that while labels might list a 30 gram handful of pistachios as providing 170 calories, the reality is a more waist-friendly 160. Have a handful of almonds and you’ll consume just 128 calories rather than the 170 in calorie count books.

Others have shown how cooking a food changes its calorie content. Researchers at Harvard University have shown that sweet potatoes contain more calories when they are cooked because the starch they contain can be more easily digested by the body. In some cases the differences are negligible. But even a small calorie inaccuracy can have a big effect on weight. Researchers at the University of California showed that subjects who consumed just 19 more calories a day than usual gained two pounds of weight during the course of a year.

What’s the alternative?

If straightforward calorie counting misses the mark, what should we do instead? The answer could be to focus on the glycaemic Index (or GI), a figure that indicates how fast and how much a food raises blood sugar levels. “What calorie research has shown us is that foods with a low GI - rye bread, bran muffins and porridge - require more chewing and are more difficult to digest, so you use up more energy eating them,” Sutton says. With many low GI and high fibre foods, only about three-quarters of the calories they contain are absorbed. On the other hand, many highly processed foods and sugary foods, such as cookies, smoothies and honey, seem to barely tax the digestive system at all, meaning no extra calories are needed to process them.

Calorie Counting vs GI

David Ludwig, the professor of nutrition at the New Balance Foundation Obesity Prevention Centre at the Boston Children’s Hospital is among the many experts who have compared different calorie sources. In one of his studies, Professor Ludwig asked subjects to follow either a low fat diet that limited fats to 20 per cent of total calories, a low carbohydrate, Atkins-style diet that cut carbs to 10 per cent of total calories or a low GI diet containing 40% fat, 40% carbohydrate and 40% protein for four weeks. All the dieters ate the same amount of total calories; only the nutrient composition of the food they ate was different.

When the results were analysed, he found that high GI foods - such as sugar, bread and potatoes - seemed to spike blood sugar and stimulate hunger and cravings, which can drive people to overeat. By far the most effective overall when it came to weight loss was the low-glycaemic approach which led to an extra 150 more calories being burned than on the low fat diet, but with no negative impact on hormone or blood fat levels. Ludwig suggested the beneficial effects boiled down to the type of carbohydrates consumed in the low-GI diet: all were slow to be digested, from beans, pulses, non-starchy vegetables and other minimally processed sources. In short, they were hard work to eat.

Should you tear up the calorie manual?

Popular diet plans like Weight Watchers have dropped conventional calorie counting because of the new findings about calories, and obesity researcher Zoe Harcombe says “There is a lot seriously wrong with the calorie counting advice.” Most experts do concede that some people do initially lose weight through calorie counting. But the results are usually temporary. The common explanation is that motivation dwindles as it becomes harder.

But Dr Ludwig said, “Another possibility is that highly processed foods undermine our metabolism and overwhelm our behaviour.” Food selection according to GI rather than calories is certainly more scientifically sound. “The important thing to think about is balancing out the foods you eat so that there is less refined produce, more fresh and plenty of fibre,” says Bridget Benelam, a nutrition scientist at the British Nutrition Foundation. “Food is a natural substance and its calories are not always what they seem.’