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Boston - Consumers have been under-estimating their calorie consumption for decades as the system of assessing the energy content of food is seriously flawed, scientists have said.
Food manufacturers and government agencies have “consistently misled the public over the accuracy of calorie counts,” nutritional experts warne.
Some high-fibre foods may contain up to 25 percent more calories than the label suggests, because producers do not currently count the energy in fibre. This means that high-fibre foods like muesli, targeted at people on a diet, are more fattening than people are led to believe, said Geoffrey Livesey, an independent nutritionist based in Britain who has advised the UN's Food and Agriculture Organisation. Other high-fibre foods include wholegrain bread, brown rice, wholewheat pasta and bran muffins.
“In Britain, we have not assigned a value for fibre, so calorie counts have normally been lower - on average around five percent of energy in food is fibre,” Dr Livesey told the American Association for the Advancement of Science meeting in Boston.
“So consumers have been eating more calories than they thought, particularly if the food was high-fibre. When people eat muesli, it is a healthy food but they often put on lots of weight,” he told the meeting.
The problems has existed for decades because the system for assessing calories goes back to the 1970s. If means that if people follow the daily recommended intake of 18 grams of fibre, they could be consuming more than 250 extra calories each week without realising it, Dr Livesey said.
Under new European guidelines, overseen by the Department of Health and the Food Standards Agency, food companies are being asked to make more accurate assessments of calories, but it is unclear how many are doing so.
It is not only the presence of fibre that has upset the calorie estimates. Scientists have known for years the calorie counts on food labels do not take into account energy expended by the body in eating and digesting particular types of food, said Richard Wrangham of Harvard University.
Government assessments about the amount of energy in food assume that the calorific value is the same whether the food it cooked or raw, but scientists know that raw food provides fewer calories because the body expends energy breaking it down, Professor Wrangham said.
“We are talking about at least a difference of between 10 and 30 percent,” he claimed. “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.
“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,” he told the meeting.
THE ATWATER SYSTEM
Calorie counts on food labels are based on a system developed by Wilbur Olin Atwater, a late-19th-century American chemist. Under the Atwater system, carbohydrates and protein average 4 kcal per gram, while fat provides 9 kcal per gram. Fibre was assumed to pass through the body without being digested, but soluble fibre and other constituents of fibre can be broken down in the gut to release energy. In some respects, the Atwater system overestimates the number of calories provided in food by not taking into account the energy needed to break the food down during digestion.
The Atwater system is more accurate when considering the calorific value of highly digestible food, such as white bread. But it is less accurate with other kinds of high-fibre food, such as wholewheat pasta and dried figs.- The Independent