Intermittent access to preferred food reduces the reinforcing efficacy of chow in rats

Abstract

Intermittent, extended access to preferred diets increases their intake. However, the effects of such access on the acceptance and reinforcing efficacy of otherwise satisfying alternatives is less known. To investigate the role of nonnutritional contributions to the hypophagia that follows removal of preferred food, male Wistar rats were fed a chow diet (Chow A/I), preferred to their regular chow (Chow), which was equally consumed under 1-choice conditions to an even more preferred chocolate-flavored, sucrose-rich diet (Preferred). Rats then learned to obtain Chow A/I pellets under a progressive ratio schedule of reinforcement and were assigned to two matched groups. Each week, one group (n = 15) was diet-cycled, receiving Chow A/I for 5 days followed by the Preferred diet for 2 days. Controls received Chow A/I daily (n = 14). Progressive ratio sessions were performed daily during the 5 days that all subjects received Chow A/I in the home cage. Across 5 wk, diet-cycled rats progressively ate less of the otherwise palatable Chow A/I diet. Hypophagia was not due to greater prior intake or weight gain, motor impairment, or facilitated satiation and was associated with changes in progressive ratio performance that suggested a reduced reinforcing efficacy of the Chow A/I diet in diet-cycled animals. By week 4, diet-cycled animals began to overeat the preferred diet, especially during the first 6 h of renewed access, resembling a deprivation effect. The results suggest that intermittent access to highly preferred food, as practiced by many restrained eaters, may progressively decrease the acceptability of less palatable foods, and may promote relapse to more rewarding alternatives.

Fig. 1.

Effects of repeated cycles of 5-day access to Chow A/I (C phase) alternated with 2-day access (P phase) to either Chow A/I (Chow A/I / Chow A/I, n = 14) or highly preferred chocolate-flavored sugary diet (Chow A/I / Preferred, n = 15) in male Wistar rats. A: average daily food intake. B: cumulative food intake (note that error bars are smaller than symbols). C: time course of food intake during 2 days of access to the preferred diet. Food intake was measured 1, 3, 6, 24, and 48 h after restoration of preferred diet access after 20 wk of diet cycling. Inset depicts food intake of the first (0–24 h) vs. second (24–48 h) day of access to preferred diet. Inset scale differs from main panel. D: average daily change in body weight during each diet phase of each week. *Significant difference from Chow A/I / Chow A/I, P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 2.

Effects of repeated cycles of 5-day access to Chow A/I alternated by 2-day access to either Chow A/I (Chow A/I / Chow A/I, n = 14) or highly preferred chocolate-flavored sugary diet (Chow A/I / Preferred, n = 15) on progressive ratio responding for Chow A/I pellets in male Wistar rats. Rats were tested daily at the dark cycle onset during the C (Chow A/I) Phase. Values represent the average of all 5 days from each phase. Values are presented as means ± SE. A: breakpoint, the last ratio completed by a subject prior to the end of the session. B: total responses, the number of reinforced and nonreinforced responses. *Significantly different from Chow A/I / Chow A/I P < 0.05, **P < 0.01. ##Significantly different from Chow A/I / Preferred baseline (week 1) P < 0.01, ###P < 0.001.

Fig. 3.

Effects of repeated cycles of 5-day access to Chow A/I alternated by 2-day access to either Chow A/I (Chow A/I / Chow A/I, n = 14) or highly preferred chocolate-flavored sugary diet (Chow A/I / Preferred, n = 15) on the time course by which 45-mg Chow A/I pellet reinforcers were obtained by male Wistar rats under a progressive ratio schedule of reinforcement. Testing occurred on the first day of the Chow A/I (C) phase of week 5. Values are presented as means ± SE. *Significantly different from Chow A/I / Chow A/I, P < 0.05, ***P < 0.001. The rate at which subjects earned reinforcers per unit time is shown. Each symbol represents the mean number of cumulative reinforcers earned by subjects through that time point. The time course is continued until each subject in the group has stopped obtaining reinforcers, and the graph therefore does not represent average session durations.

Fig. 4.

Effects of repeated cycles of 5-day access to chow alternated by 2-day access to either Chow A/I (Chow A/I / Chow A/I, n = 14) or highly preferred chocolate-flavored sugary diet (Chow A/I / Preferred, n = 15) on the duration of interresponse intervals (IRIs) following reinforced (A) or nonreinforced responses (B) during progressive ratio responding for 45-mg Chow A/I pellets in male Wistar rats. Values are presented as means ± SE. IRI durations were ln-transformed for statistical analysis, and bars represent back-transformed values. Data were cumulated from 5 sessions during each of weeks 1 and 5. *Significantly different from Chow A/I / Chow A/I, P < 0.05, ***P <0.001.

Fig. 5.

Effects of repeated cycles of 5-day access to chow alternated by 2-day access to either Chow A/I(Chow A/I / Chow A/I, n = 14) or highly preferred chocolate-flavored sugary diet (Chow A/I / Preferred, n = 15) on the duration of interresponse intervals (IRIs) following reinforced responses during progressive ratio responding for 45-mg Chow A/I pellets in male Wistar rats. Relative frequency histograms (bars) and fit log-Gaussian distributions (lines) (22, 23), show the relation of diet schedule to the duration of IRIs after Chow A/I-reinforced responses. Before diet cycling (left, week 1), distributions were similar between groups, whereas after 5 wk of diet cycling (right), the relative frequency histogram was shifted to the right in diet-alternated rats compared with controls, indicating longer postreinforcement intervals. Nonreinforced intervals were not similarly shifted (not shown). The frequency histogram shows IRIs of between e^-1 and e³ s in duration (0.4–20.1 s) using a bin width of e^0.2 (note ln-scale of x-axis). The time scale shown accounts for a similarly large majority of IRIs in diet-cycled rats (98%) and controls (98%). Tick labels indicate the beginning of the bin. Data were cumulated from 5 sessions during each of weeks 1 and 5.