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July 17, 2009

The costs of digestion

New Scientist has a fascinating article on the energy in food from digestion rather than the energy from incineration which is what is on food labels: The calorie delusion: Why food labels are wrong.

It taught me two cool facts and reminded me of one on the difference between the calories in food and the calories we get out of food:

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
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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).
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n a study published in 2003, for example, a team led by Kyoko Oka at Kyushu University in Fukuoka, Japan, investigated the effect of food texture on weight gain. They fed one group of rats their usual hard food pellets, while a second group received a softer version. Both pellets had exactly the same calorie content and flavour. The only difference was that softer ones were easier to chew. After 22 weeks, the rats on the soft food diet were obese and had more abdominal fat. "Food texture might be as important a factor for preventing obesity as taste or food nutrients," Oka and his colleagues concluded (Journal of Dental Research, vol 82, p 491).

Which suggests an alternative explanation for a couple of different weight loss methods. Instead of the ketosis hypothesis, perhaps the Atkins diet makes followers loose weight by replacing 4 calorie a gram carbohydrates with 3.2 calorie a gram protein and increasing the average toughness of the food you eat.

It also suggests that traditional healthy diets high in fiber and lean protein work not so much by preventing hunger by improving blood chemistry, but instead by giving us lower energy foods that are also harder to digest.

Posted by OneEyedMan at July 17, 2009 11:17 AM

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Posted by: giblfiz at July 19, 2009 8:42 AM

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