CONSUMERS have been under-estimating their calorie consumption for decades as the system of assessing the energy content of food is seriously flawed, say scientists.
Food manufacturers and government agencies have "consistently misled the public over the accuracy of calorie counts", nutritional experts warned.
Some high-fibre foods may contain up to 25pc more calories than the label suggests, because producers do not count the energy provided by fibre.
This means high-fibre foods like muesli, targeted at people on a diet, are more fattening than they are led to believe, said Dr Geoffrey Livesey, an independent nutritionist who has advised the UN's Food and Agriculture Organisation.
"In Britain, we have not assigned a value for fibre, so calorie counts have normally been lower – on average around 5pc 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 problem has existed for decades because the system for assessing calories goes back to the 1970s.
It 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.
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 upsets 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 Professor 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 is cooked or raw, but scientists know raw food provides fewer calories as the body uses energy breaking it down, Prof Wrangham said.