Results of Experiment 1 support the hypothesis that food variety slows the rate of habituation, which is related to greater energy intake than when the same food is presented repeatedly. Experiment 2 was designed to extend the analysis of the influence of variety on habituation of salivation to motivated behavior and to test the influence of ED on habituation.
Participants met the same criteria and were recruited using the same methodology as those used in Experiment 1.
General procedures were similar to Experiment 1, with the exception that LED foods (peaches, pineapple, mandarin oranges, yogurt, baby carrots, fat free pudding, and grapes) were studied in addition to the HED food studied in Experiment 1, and children chose their favorite LED and HED foods from the lists of foods available in the experiment.
Participants were randomly assigned to the Same or Variety food conditions within the HED or LED conditions, which were run in separate cohorts. HED foods were the same as those used in Experiment 1 and all had an ED of > 2.3 kcal/g. All LED had an ED of < 1.0 kcal/g (). In Experiment 2, children worked for access to the food stimuli, and changes in rate of motivated behavior across presentation of the same or a variety of HED or LED foods were determined.
Low and High Energy Density Foods Used for Experiment 2
The participant’s favorite food out of the four was presented for the first trial in both the Same and Variety conditions. Participants in the Variety condition never worked for access to the same food consecutively during the motivation task trials. In the HED food condition the foods used were the same as in Experiment 1, 40 ± 3 g portions of Wendy’s® Junior Hamburger, Wendy’s® Chicken Nuggets, Wendy’s® French Fries (Wendy’s; Columbus, OH), and Domino’s ® Cheese Pizza (Ann Arbor, MI). Food was wrapped in foil and kept warm at around 200° F in a convection oven. The LED foods included 40 ± 3 g portions of Dannon® Light n’ Fit Yogurt (Minster, OH), Hunt’s® Fat Free Pudding (ConAgra Foods; Mason, OH), baby carrots, grapes, Delmonte® Extra Light Syrup Peaches (San Francisco, CA), Dole® Mandarin Oranges served in light syrup, and Dole® Pineapple served in fruit juice (Westlake Village, CA). The LED food stimuli were presented in paper bowls and kept refrigerated (4° C and 30% humidity).
Measurement Food reinforcement task
A computer generated task was used to assess motivated responding for food. The reinforcement schedule was a variable ratio 100 (VR 100) with a range of ± 25%, that is, a point was earned after 75 to 125 button presses. This schedule of reinforcement remained the same throughout the duration of the testing session. The task consisted of two squares, one that flashed red every time a mouse button was pressed and another square that flashed green when a point was earned. The number of points earned per trial and a description with a picture of the food stimulus was presented on the computer monitor. The progression to the next trial would not occur until points were earned for the current trial. To receive the food stimulus, the participant had to earn a total of five points per trial. The game ended after 30 minutes, during which time children had the opportunity to play for 10 trials. Total number of responses per trial served as the primary outcome measure. After the participant earned five points the experimenter brought in the food for consumption. Water was also provided ad libitum throughout the experimental session. If the participant no longer wanted to earn points to eat food the child could engage in other activities (i.e., magazines, cross word puzzles, word finds) located at a table next to the computer station. This was to ensure that responding to the computer task was not out of boredom.
Between group differences in motivated responding were assessed using ANCOVA with group (Same, Variety) and energy density (HED, LED) as the between-subjects factors and Trials (1 to 10) as the within-subjects factor. Group differences in energy consumption (Kcal), amount of food consumed (grams), liking of study foods, and baseline and posttesting hunger were analyzed using ANCOVA. The influence of favorite food on the rate of responding was analyzed using an ANCOVA with favorite food as the between-subjects factor and Trials (1 to 10) as the within-subjects factor. In addition, the influence of favorite food on the amount of food and energy consumed was analyzed using ANCOVA with favorite food as the between-subjects factor. For the latter analyses, the HED groups and LED groups were analyzed separately. All analyses included sex, BMI percentile, and same-day energy intake as covariates.
Participants were 18 males and 17 females, 10.8 ± 1.1 years of age with a BMI of 18.6 ± 2.1, and a BMI percentile of 63.5% ± 21.4. Participants were 83% non-Hispanic Whites, 3% Asian, 3% Native American, 8% African American, and 3% reported other for race, with a socioeconomic status of 46.7 ± 12.7. Participants consumed an average of 637.6 ± 335.9 calories prior to visiting the laboratory (see for descriptive data by group). There was a significant difference among the groups for BMI percentile F(3, 33) = 3.10, p = .04, and for total energy consumed that day prior to participating in the experiment F(3, 33) = 4.50, p = .01. There were no differences among the groups for baseline or posttesting hunger baseline = F(3, 30) = 2.70, p = .06 and post = F(3, 30) = 1.39, p = .26; liking of study foods, pizza = F(1, 13) = 0.71, p = .42; hamburger = F(1, 13) = 0.71, p = .42; french fries = F(1, 13) = 0.04, p = .85; chicken nuggets = F(1, 13) = 2.4, p = .15; pudding = F(1, 14) = 0.67, p = .43; yogurt = F(1, 14) = 0.67, p = .43; carrots = F(1, 14) = 0.32, p = .58; grapes = F(1, 14) = 0.43, p = .53; peaches = F(1, 14) = 1.1; p = .30; pineapple = F(1, 14) = 0.002, p = .97; mandarin oranges = F(1, 14) = 1.49, p = .24.
Characteristics of Participants in the Variety Food and the Same Food Conditions for Experiment 2
ANCOVA revealed a significant interaction between variety and trials. Participants in the Variety group maintained responding for significantly longer than those in the Same group F(9, 297) = 2.35, p = .04, ES = 0.25 (), with no interaction of variety with HED/LED food status (p > .05). There were main effects of food variety F(1, 30) 3 11.67, p = .002, ES = 0.65 and ED F(1, 30) = 181.45, p < .0001, ES = 2.12 on energy consumption as well as an interaction between these factors F(1, 30) = 7.85, p = .009, ES = 0.36 (). Post-hoc testing revealed that in participants receiving HED foods, variety significantly increased energy consumption, F(1, 16) = 12.75, p = .003, but in those receiving LED foods, there was no significant effect of food variety on energy consumption F(1, 16) = 1.39, p = .256. Participants in the LED and HED Variety groups consumed 45% and 46.9% more energy, respectively, than their Same food counterparts. There was a main effect of food variety on grams of food consumed (F(1, 32) = 7.15, p = .012, ES = 0.46), but no effect of ED and no interaction between the two (). Dietary variety increased the grams of food consumed by 32.1%. There was also no effect of favorite food on the rate of motivated responding (F(90, 216) = 0.79, p = .90). Nor did favorite food have any influence on the amount of food LED = F(6, 12) = 0.25, p = .95; HED = F(3, 14) = 0.38, p = .77 or energy consumed LED = F(6, 12) = 0.30, p = .92; HED = F(3, 14) = 0.34, p = .80, within each ED group.
Figure 2 Dietary variety decreases the rate of habituation for motivated responding for food. A. (Top graph) Mean ± SEM number of responses made in each trial for access to food. ANCOVA revealed that subjects working for access to the Same food repeatedly (more ...)