Re-examining the Energy Value of Food

It has been 125 years since Wilbur Olin Atwater, an agricultural chemist with the U.S. Dept. of Agriculture, published his findings from more than 500 studies that attempted to understand the potential energy value of food (Atwater, 1887). Leveraging earlier calorimetric work by Frankland, Rubner, and Stohmann, Atwater reported the potential energy value of protein, fat, and carbohydrate (Hargrove, 2007). Since the time of Atwater, new research data have provided insights into how energy values are calculated and suggest that there is an opportunity for more accurate caloric labeling of food products.

Atwater reported food energy availability and digestibility data, typically based on very few male subjects who were research associates in their late 20s or early 30s. These data were calculated following the consumption of mixed diets, not single foods, for a few days. Importantly, the intent of Atwater’s early studies was to define economical diets that could satisfy the nutritional needs of people with various occupations and social status.

Jaffa (1903) reported the digestibility of individual foods from mixed diets, primarily composed of fruit and nuts, based on 28 studies among 31 fruitarians for various time periods. The digestibility of protein from nuts was typically 70%, yet the individual digestibility values of various nuts were quite variable. Atwater’s studies suggested 78% digestibility for nut protein, whereas Holmes’ research indicated an average digestibility for peanut protein of about 93% (Holmes, 1920). Similar investigations indicated a digestibility range of carbohydrates from nuts to be 96–99%.

Remarkably, Jaffa’s studies that assessed the digestibility and potential energy value of almonds, Brazil nuts, pecans, walnuts, and peanuts only included data from 2–7 adult males. Advances in analytical chemistry applied to foods and contemporary approaches to clinical studies (including statistics-based power calculations) suggest that not all foods have the same digestibility. These differences became apparent based on recent publications (Novotny et al., 2012; Baer et al., 2012). These studies indicate the metabolizable energy values using traditional Atwater factors overestimate the energy value of almonds and pistachios by 32% and 5%, respectively. Similar assessments applied to peanuts, peanut butter, and peanut oil clearly indicate the energy availability from the whole legume is less than the other two peanutderived foods (Levine and Silvis, 1980).

Research by Ellis et al. (2004) and Hollis and Mattes (2007) indicate that upon microscopic examination of fecal samples, significant portions of almonds are not digested. This examination indicated that a significant fraction of the cell walls remained intact, that is, they were not hydrolyzed through normal digestive processes. Thus, the nutrients within these cells and the respective energy values of the macronutrients were unavailable.

It appears that these lower energy values reflect a better understanding of three important areas: 1) chewing and subsequent nutrient availability; 2) cell structure of the nuts; and 3) more accurate clinical data. A better understanding of the digestibility of foods and the impact of these three areas could contribute to more appropriate energy values of foods and more accurate product labeling.

However, several recent reports strongly support the continued use of the Atwater system  to estimate the energy value of foods (LSRO, 1983; FAO, 2003; Nestle & Nesheim, 2012). On the other hand, Moe (1994) noted that  the traditional Atwater factors may not be applicable to formulated foods composed of new ingredients. Zou et al. (2007) reported that Atwater-based energy values were inaccurate with low-fat, high-fiber diets. And data from Merrill and Watt (1973) indicate a spectrum of Atwater values applied to macronutrients. For example, values for total carbohydrates from a variety of foods varied from 2.48 Kcal/g (lemon, lime) to 4.16 Kcal/g (rice). The Life Sciences Research Organization report notes that the energy value of nuts using the Atwater system may be overestimated. Charrondiere et al. (2004), who investigated energy conversion factors, concluded that at least an 8% difference could significantly impact programs directed to the undernourished. As the federal government food labeling guidelines attempt to educate the public on energy values of foods, and the food industry attempts to provide practical information consistent with those guidelines, it has become more apparent that a better understanding of the potential energy values of food is essential.

References cited in this column are available from the authors.

Karen Lapsley, D.Sc.,
Chief Scientific Officer, Almond Board of California, Modesto, Calif.
[email protected]

Roger Clemens, Dr.P.H.,
Contributing Editor
Chief Scientific Officer, Horn Company, La Mirada, Calif.
[email protected]