THE calorie content of some types of food has been systematically underestimated by retailers using a system for assessing food energy that is out of date and has not kept up to date with new scientific findings, according to researchers who have investigated the accuracy of calorie labelling.

Dieters who eat high-fibre foods such as vegetables and muesli are consuming more calories than they think, for example, because the current food labels do not take into account the calories in fibre. The scientists also said that consumers could reduce their calorie intake by eating raw rather than cooked foods. They argue that the way calories are assigned to foods by retailers needs a significant overhaul.

“There is a lot of misinformation around calories, and it is crucial for the consumer, whether they are on a diet or not, to have the correct information about what they eat,” said Prof Richard Wrangham, a primatologist at Harvard University. “We believe that it is time for a high-level panel to consider how best to improve the quality of information provided to the public about the real energy value of their foods.” Wrangham convened a session to raise the issues around measurement of calories on Monday at the annual meeting of the American Association for the Advancement of Science in Boston.

Apart from underestimating the calorific value of fibre, the standard system also does not take into account the way that foods are prepared and eaten. The calorie contribution from raw and cooked versions of the same food are different for example, but that is not reflected on food packaging.

For more than a century, the energy value in foods has been calculated using the Atwater system. In general terms, this system means that a gram of protein or carbohydrate provides four calories, while a gram of fat provides nine calories. Food manufacturers work out the portion of protein, carbohydrates and fat in a food and multiply up using the Atwater factors to get the total calories.

“The Atwater Convention yields realistic values for foods that are highly digestible, such as white bread,” said Wrangham.

But the system leaves out fibre — assuming that this component of food has no energy value to the body. “At one time fibre used to be called crude fibre and it was mostly cellulose and it was mostly difficult to digest and it mostly passed straight through,” said British nutritionist Geoffrey Liveley. Nowadays, scientists know that fibre is not only indigestible cellulose but also pectins and soluble fibres that are beneficial to health. These are fermented by micro-organisms in the large intestine and broken down into elements that can provide energy for the body — equivalent to around two calories per gram of fibre.

Raw foods, he said, were also systematically less energy-producing than the same foods cooked. “There are two basic reasons why raw foods provide less calories than cooked foods — they are less digestible and also the bits that can be digested cost more to break down. So eating raw food is a good way to lose weight, but you need to be careful about it long-term and it would not be advisable in children.” — The Guardian, London