CALORIE levels printed on food packaging are wildly misleading and should not be relied upon by dieters, nutritionists claim.
Manufacturers' measurements of energy levels in their food do not include fibre, which accounts for about five per cent of our calorie intake.
Dieters who eat Muesli for breakfast, for example, may wonder why they are struggling to lose weight because the packaging ignores "invisible calories" contained in its high fibre content.
In contrast, those eating large amounts of protein may be taking in less energy than they realise because the current system overestimates the number of calories it contains by 20pc.
To add to the confusion, nutrition advice also fails to account for whether the food is raw or cooked and processed or unprocessed.
Cooking and processing food can alter calorie levels by up to 30pc because we burn more energy digesting things which are hard and uncooked, experts explained.
It means people trying to stick to the daily recommended calorie limits of 2,500 for men and 2,000 for women may be taking in significantly more or less energy than they realise, researchers said.
Speaking at the annual meeting of the American Academy for the Advancement of Science in Boston on Monday Dr Geoffrey Livesey, an independent nutritionist, explained: "In Britain we have not assigned a value for fibre, so calorie counts have normally been lower - on average around five per cent of energy in food is fibre.
"So consumers have been eating more calories than they thought they were, particularly if the food was high-fibre....when people eat muesli, it is a healthy food but they are often putting on lots of weight."
For decades calorie levels in Britain have been calculated according to the "general factor system", a simple formula based on how many grammes of fat, protein and carbohydrates there are in the food.
Each gram of protein or carbohydrate contains four calories, according to the system, while a gram of fat contributes nine calories to the total displayed on the package.
But research in the early 1990s established that each gram of fibre is worth another two calories, and this additional information has yet to be included on food in Britain.
Nutritionists recommend that an adult should consume 18g of fibre per day, the equivalent of an additional 250 calories per week.
Rules introduced by the European Commission last year require food manufacturers across the continent to include fibre in their calorie calculation, but Dr Livesey said it remains unclear how many are complying with the new system.
Studies have also established that each gramme of protein contains 3.2 calories rather than four, but there is currently no move to update the general factor system to take this into account.
Prof Richard Wrangham, of Harvard University, added that nutrition advice should also factor in the difference in energy contained in raw and cooked foods.
"There are two basic reasons why raw food provide less calories than cooked foods – they are less digestible and also the bits that can be digested cost more to break down," he explained. "We are talking at least a difference of between 10 to 30pc.
"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."