A calorie may not be a calorie

We often read how estimating calories is an inexact science and calorie counts can be off by as much as 20% this article goes some way to explaining this.

Ref: http://www.smh.com.au/lifestyle/proof-is-in-the-eating-all-kilojoules-not-equal-20130220-2ervg.html

When it comes to weight loss, a calorie is a calorie; a kilojoule is a kilojoule. That has been the mantra of nutritionists, dietitians, and food regulators for more than a century. But when it comes to comparing raw food with cooked food, or beans with breakfast cereals, that thinking may be incorrect. That is the consensus of a panel of researchers in Boston who list the many ways the maths doesn't always add up correctly on food labels.
In a wide-ranging discussion of how food is digested in everything from humans to rats to pythons, the panel reviewed a new spate of studies showing foods were processed differently as they moved from our gullet to our guts and beyond. They agreed net caloric counts for many foods were flawed because they did not take into account the energy used to digest food; the bite oral and gut bacteria took out of various foods; or the properties of different foods themselves that sped up or slowed down their journey through the intestines, such as whether they were cooked or resistant to digestion.

The process used to estimate calories for food was developed at the turn of the 20th century by Wilbur Atwater. It was a simple system of calculating four calories for each gram of protein, nine calories for each gram of fat, and four calories for each gram of carbohydrate (modified later by others to add two calories for a gram of fibre).
One key area where the system is inaccurate, Wrangham reports, is in estimating the calories for cooked food. Cooked items often are listed as having more calories than raw items, yet the process of cooking meat gelatinises the collagen protein in meat, making it easier to chew and digest - so it takes fewer calories to eat. Heat also denatures the proteins in vegetables such as sweet potatoes, says Harvard University evolutionary biologist Rachel Carmody, who studies the energetics of digestion.

The way foods are processed can also make them easier to digest. Take ''resistant'' starch in cereal kernels, such as barley grain, or beans, which take a long time to digest. Grind the same cereals into flour or process it into breakfast cereal or instant oatmeal, and it becomes easy to digest, says biochemist nutritionist Klaus Englyst of Englyst Carbohydrates, a carbohydrate chemistry firm in Southampton, in Britain. This is why ''bread is more rapidly digested; beans more slowly,'' he says.


Conclusion

''But why are you doing this? Will it make a real difference? If you want to lose weight, you still have to cut back on calories.''