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Am J Clin Nutr. 2010 Feb;91(2):343-8. doi: 10.3945/ajcn.2009.28315. Epub 2009 Nov 18.

Reproducibility of ad libitum energy intake with the use of a computerized vending machine system.

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National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases, Phoenix, AZ, USA.



Accurate assessment of energy intake is difficult but critical for the evaluation of eating behavior and intervention effects. Consequently, methods to assess ad libitum energy intake under controlled conditions have been developed.


Our objective was to evaluate the reproducibility of ad libitum energy intake with the use of a computerized vending machine system.


Twelve individuals (mean + or - SD: 36 + or - 8 y old; 41 + or - 8% body fat) consumed a weight-maintaining diet for 3 d; subsequently, they self-selected all food with the use of a computerized vending machine system for an additional 3 d. Mean daily energy intake was calculated from the actual weight of foods consumed and expressed as a percentage of weight-maintenance energy needs (%WMEN). Subjects repeated the study multiple times during 2 y. The within-person reproducibility of energy intake was determined through the calculation of the intraclass correlation coefficients (ICCs) between visits.


Daily energy intake for all subjects was 5020 + or - 1753 kcal during visit 1 and 4855 + or - 1615 kcal during visit 2. There were no significant associations between energy intake and body weight, body mass index, or percentage body fat while subjects used the vending machines, which indicates that intake was not driven by body size or need. Despite overconsumption (%WMEN = 181 + or - 57%), the reproducibility of intake between visits, whether expressed as daily energy intake (ICC = 0.90), %WMEN (ICC = 0.86), weight of food consumed (ICC = 0.87), or fat intake (g/d; ICC = 0.87), was highly significant (P < 0.0001).


Although ad libitum energy intake exceeded %WMEN, the within-person reliability of this intake across multiple visits was high, which makes this a reproducible method for the measurement of ad libitum intake in subjects who reside in a research unit. This trial was registered at as NCT00342732.

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