The system assumes that the main food components – protein, fat, and carbohydrate – have a single energy factor, regardless of the food in which they are found. Carbohydrates and proteins are considered to contain four calories per gram, fat nine calories per gram, and the system also includes a value of seven calories per gram of alcohol.
However, the researchers suggest that this system may not work for measuring how much energy people actually get from almonds – and a growing body of evidence suggests that nuts in general are a food group in which the Atwater factors may be poorly predictive of energy content.
“Our data show that the digestibility of probably fat, and probably protein and carbohydrate as well, is lower than what Atwater and his colleagues found in the late 1800s and early 1900s,” lead ARS researcher Dr. David Baer, supervisory research physiologist with the agency’s Beltsville Human Nutrition Research Center, told this publication.
“We really think that we have cracked the nut on being able to measure the energy value of foods,” he said.
Cell structure
Baer said that the rigid structure of almond cell membranes – and perhaps other plant foods – could lock in some fat, preventing it from being absorbed in the digestive tract.
“The membrane surrounding the cell wall is where the fiber is,” he said. “…We can’t digest that fiber until it gets into the lower digestive tract – into the colon – but that is beyond the point at which it can be absorbed.”
Baer and his team measured energy and macronutrient content excreted in urine and feces from 18 participants consuming one of three randomized mixed diets, which contained different amounts of almonds.
They found that almonds’ energy content was equivalent to 129 kcal per 28 gram serving, as opposed to the 168-170 kcal as determined by Atwater factors.
The effects of processing
The researchers hypothesize that macronutrients may be better absorbed when a food’s cell walls are broken down – whether through chewing or processing – so while these results apply to whole almonds, if the almonds are ground, sliced, or even roasted, this may have an impact on how available macronutrients are for absorption.
Peanuts and whole grains may be other foods for which there could be a discrepancy between measurable calorie content within a mixed diet and those allocated under the Atwater system, Baer added.
Similar research from the same research team, presented at the Experimental Biology conference in Washington D.C. last year, suggested that pistachios may contribute about 6% fewer calories than previously thought.
The Almond Board of California said in a release that the industry is now working with government agencies to determine whether these study results could impact Nutrition Facts panels.
Source: American Journal of Clinical Nutrition
First published ahead of print July 3, 2012 as doi: 10.3945/ajcn.112.035782
“Discrepancy between the Atwater factor predicted and empirically measured energy values of almonds in human diets”
Authors: Janet A Novotny, Sarah K Gebauer, and David J Baer
The researchers, from the USDA’s Agricultural Research Service, call into question the validity of the most common way of determining the calorie content of foods – the Atwater general factor system.
