Calorie count system is well past sell-by date
Cooking raises calorific content by 15 per cent in meats and by 39 per cent in the case of sweet potatoes
The calorie counts displayed on many foods are incorrect and give wrong figures. In some cases they significantly underestimate the calories present and in others overestimate, nutritional experts have claimed.
Most of the errors arise because the most commonly used system for calculating calorific energy in foods is out of date.
It does not take account of the latest biochemistry research and our deeper understanding about human metabolism, a session at the American Association for the Advancement of Science heard on its closing day.
Today’s calorie counting is based on the work of a 19th-century chemist, Wilbur Olin Atwater from the US. He burned various foods to calculate how much energy they released. His figures were acknowledged as estimates, but the system stuck and is now in widespread use.
Unfortunately, no effort has been made to bring this old system up to date on the basis of new science. For example, Atwater assumed that the calorie count for raw food was the same as in cooked foods, but these numbers can vary widely, said Prof Richard Wrangham of Harvard University.
Cooking food increases the amount of energy that our bodies can get out of it but this is not reflected in the figures, he said. “The Atwater system fails to take this into account.”
Many dutifully count their calories to maintain or reduce weight, but the numbers can be out by as much as 25 per cent. Dr Rachel Carmody, also of Harvard, described experiments with mice who were fed raw and cooked meats and sweet potatoes.
Raw cuts calories
Cooking raised calorific content by 15 per cent in meats and by 39 per cent in the case of sweet potatoes, she said.
“The mice that were eating the processed foods were fatter,” she said. “We are not suggesting consumers go on a 100 per cent raw diet. That kind of diet can actually be harmful.” But the more refined and processed the food, the more bioavailable the energy becomes. Virtually all the energy available in the processed foods we eat are absorbed by the body, while less-processed foods reduce the amount of energy captured.
A good example of this is dietary fibre, according to studies by Prof Geoffrey Livese of Independent Nutrition Logic Ltd in the UK. We process food ourselves by chewing and breakdown in the stomach and gut, but fibre is more resistant to this mechanical and chemical breakdown.
The microbes that help do this for us also take a share of the energy available before we get ours, he said. This means fibre may deliver between 5 and 25 per cent fewer calories than the number printed on the packaging.
The errors in the Atwater system can go up or down depending on the food, but it makes it more difficult for the consumer to make accurate calculations of their energy balance. Men need about 2,500 calories and women 2,000 calories a day, but just 20 calories too many a day will build up to a kilogram of fat over a year.
All of the panel agreed that a new, more accurate system was needed, but there were considerable challenges, Prof Livese said. Efforts to change the system were put to the UN Food and Agriculture Organisation in 2003 and, despite backing the proposals, no progress has been made. “Industry may have a hand in this and makes it a little bit more difficult,” he said.
“We are hoping that this is the start of a process that will bring together recommendations,” Dr Carmody said.