I thought that this was a very interesting article on food types and calories.
-the-calorie-delusion-why-food-labels-are-wrong.html? here's an extract
"STANDING in line at the coffee shop you feel a little peckish. So what will you choose to keep you going until lunchtime? Will it be that scrumptious-looking chocolate brownie or perhaps a small, nut-based muesli bar. You check the labels: the brownie contains around 250 kilocalories (kcal), while the muesli bar contains more than 300. Surprised at the higher calorie count of what looks like the healthy option, you go for the brownie.................
Calorie counts on food labels around the world are based on a system developed in the late 19th century by American chemist Wilbur Olin Atwater. Atwater calculated the energy content of various foods by burning small samples in controlled conditions and measuring the amount of energy released in the form of heat. To estimate the proportion of this raw energy that was used by the body, Atwater calculated the amount of energy lost as undigested food in faeces, and as chemical energy in the form of urea, ammonia and organic acids found in urine, and then he subtracted these figures from the total. Using this method, Atwater estimated that carbohydrates and protein provide an average of 4 kcal per gram, while fat provides 9 kcal per gram. With a few modifications, these measurements of what is known as metabolisable energy have been the currency of food ever since.
We know these values are approximate. Nutritionists are well aware that our bodies don't incinerate food, they digest it. And digestion - from chewing food to moving it through the gut and chemically breaking it down along the way - takes a different amount of energy for different foods. According to Geoffrey Livesey, an independent nutritionist based in Norfolk, UK, this can lower the number of calories your body extracts from a meal by anywhere between 5 and 25 per cent depending on the food eaten. "These energy costs are quite significant," he says, yet are not reflected on any food label. Dietary fibre is one example. As well as being more resistant to mechanical and chemical digestion than other forms of carbohydrate, dietary fibre provides energy for gut microbes, and they take their cut before we get our share. Livesey has calculated that all these factors reduce the energy derived from dietary fibre by 25 per cent - down from the current estimate of 2 kcal per gram to 1.5 kcal per gram (The American Journal of Clinical Nutrition, vol 51, p 617). Similarly, the number of calories attributed to protein should be reduced from 4 kcal per gram to 3.2 kcal per gram, a 20 per cent decrease, Livesey says. That's because it takes energy to convert ammonia to urea when protein is broken down into its constituent amino acids (British Journal of Nutrition, vol 85, p 271). Put into the context of real life, these relatively small errors may make a measurable difference. In the case of the brownie versus the muesli bar, the label will overestimate the calories derived from the fibre and protein-packed muesli bar, perhaps by enough to make it lower in calories than the brownie. Just 20 kcal per day more than you need can add up to roughly a kilogram of fat over a year.
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