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ARS Home » Northeast Area » Beltsville, Maryland (BHNRC) » Beltsville Human Nutrition Research Center » Food Components and Health Laboratory » Research » Publications at this Location » Publication #320185
Title: Walnuts consumed by healthy adults provide less available energy than predicted by the Atwater factors

Author
item Baer, David
item Gebauer, Sarah
item Novotny, Janet

Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/2015
Publication Date: 1/1/2016
Citation: Baer, D.J., Gebauer, S.K., Novotny Dura, J. 2016. Walnuts consumed by healthy adults provide less available energy than predicted by the Atwater factors. Journal of Nutrition. 146:9-13.

Interpretive Summary: Calories listed on food labels represents the metabolizable energy of a food which is the energy available to the body. The typical method for determining the metaboliazable energy of a food is by calculation using Atwater factors. These factors are the energy density of macronutrients (protein, fat and carbohydrates) and adjust for the digestibility (availability) of these macronutrients. Previous studies have shown that the metabolizable energy content of almonds and pistachios nuts is less than predicted by the Atwater factors. However, very few nuts have been investigated to date, and no information is available regarding the metabolizable energy of walnuts. A study was conducted to determine the metabolizable energy of walnuts when consumed as part of a typical American diet. Healthy adults (n=18; mean age=53.1 yrs, body mass index=28.8 kg/m2) participated in a randomized crossover study with two treatment periods (3 weeks each). The study was a fully controlled dietary feeding intervention in which the same base diet was consumed during both treatment periods; the base diet was unsupplemented during one feeding period and supplemented with 42 g/d walnuts (1.5 servings) during the other feeding period. Base diet foods were reduced in equal proportions during the walnut period to achieve isocaloric food intake during the two periods. Administered diets, walnuts, fecal, and urine samples were subjected to bomb calorimetry to determine their gross energy, and the resulting data were used to calculate the ME of the walnuts. One 28-g serving of walnuts contained 146 kcal (5.22 kcal/g), 39 kcal/serving less than the value of 185 kcal/serving (6.61 kcal/g) currently used for food labeling. The ME of the walnuts was 21% less than that predicted by the Atwater factors (P <0.0001). Consistent with data from other tree nuts, Atwater factors overestimate the metabolizable energy value of walnuts. These results could help explain the observations that consumers of nuts do not gain excessive weight, and improve the accuracy for food labeling. These results are of interest to health professional involved in making dietary recommendations, consumers interested in determining their caloric intake, and food industry and regulatory agencies interested in accuracy in food labeling.

Technical Abstract: Previous studies have shown that the metabolizable energy content (ME; energy available to the body) of certain nuts is less than predicted by the Atwater factors. However, very few nuts have been investigated to date, and no information is available regarding the ME of walnuts. A study was conducted to determine the ME of walnuts when consumed as part of a typical American diet. Healthy adults (n=18; mean age=53.1 yrs, body mass index=28.8 kg/m2) participated in a randomized crossover study with two treatment periods (3 weeks each). The study was a fully controlled dietary feeding intervention in which the same base diet was consumed during both treatment periods; the base diet was unsupplemented during one feeding period and supplemented with 42 g/d walnuts during the other feeding period. Base diet foods were reduced in equal proportions during the walnut period to achieve isocaloric food intake during the two periods. After a 9-day diet acclimation period, subjects collected all urine and feces for approximately 1 week (as marked by Brilliant Blue fecal collection marker) for analysis of energy content. Administered diets, walnuts, fecal, and urine samples were subjected to bomb calorimetry, and the resulting data were used to calculate the ME of the walnuts. One 28-g serving of walnuts contained 146 kcal (5.22 kcal/g), 39 kcal/serving less than the value of 185 kcal/serving (6.61 kcal/g) currently used for food labeling. The ME of the walnuts was 21% less than that predicted by the Atwater factors (P <0.0001). Consistent with other tree nuts, Atwater factors overestimate the metabolizable energy value of walnuts. These results could help explain the observations that consumers of nuts do not gain excessive weight, and improve the accuracy for food labeling.