The available energy content of foods are determined by a calculation first proposed by Atwater. The metabolisable energy content of foods are still calculated in this way and the Atwater method is still widely accepted as the gold standard for this purpose. However, a number of studies have questioned the accuracy of the method for a number of foods, and it has been suggested that micronutrients provide different energy to the consumer based on the foods that they are contained within. For example, the energy from protein has been estimated to vary from 3.75 to 4.20 kcal per gram, depending on the food source, with carbohydrate ranging from 2.45 to 4.20 kcal per gram. Fats appear to be more uniform in their energy content with the content per gram ranging from 9.3 to 9.5 kcal. However, it has emerged that the fat from nuts may be much less digestible that the fat from other sources, which suggests that the energy content of nuts has been overestimated.
Research has shown previously that the energy content of nuts is not fully absorbed (here), and their calorific value might be less than stated by the Atwater method. In order to clarify the energy content of nuts, researchers1 fed healthy subjects an almond containing diet for 18 days at 0, 42 or 84 grams per day in a cross-over design study. Urine and faeces were collected in order to determine the unabsorbed energy content. The results showed that addition of almonds to the diet caused the fat digestibility of the diet to fall by 5 % as a whole, when 42 grams of almonds were included. When the almond content of the diet was increased to 84 grams, the fat digestibility of the diet fell by 10 %. The energy content of the diet fell by 3 and 5 % with the incorporation of 42 and 84 grams of almonds, respectively, suggesting that nuts provide fewer calories that estimated previously.
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