Small changes in meal patterns lead to significant changes in total caloric intake. Effects of diet and social status on food intake in female rhesus monkeys

Abstract

Social subordination in macaques is a well-established model to study the adverse effects of psychosocial stress on a number of health outcomes, including stress-induced eating. The present analysis was conducted to empirically define a meal among free-feeding female rhesus monkeys and to examine the roles of meal patterning (e.g., meal size, meal frequency, and snacking patterns) in findings from a previous study demonstrating that psychosocial stress increases overall caloric intake among subordinate animals with access to a highly palatable diet. Results indicate that all animals, regardless of social status, consumed more frequent meals, larger meals, and more calories in the form of snacks when a highly palatable diet was available. Additional findings suggest that subordinate animals consumed significantly larger meals compared to their dominant counterparts regardless of the dietary environment. Additionally, subordinate females with a history of exposure to the palatable diet consumed significantly more snack calories than both dominant and subordinate animals without previous exposure to the palatable diet when these females were returned to a standard laboratory diet. These findings illustrate how small changes in meal patterns can lead to significant increases in total caloric intake, which if prolonged, could promote the emergence of an obese phenotype.

Fig. 1

Logarithmic post-meal time interval distributions during the 2-week counterbalanced dietary conditions when meals were defined as ingestion of a minimum of 70 kcal, separated from a previous feeding bout by a minimum time interval of 1300 s. Condition 1 represents the initial 2-week LCD-only phase among animals with no history of HCD exposure. Condition 2 illustrates the subsequent 2-week choice phase. Condition 3 represents the 2-week choice phase among animals that received this condition first. Condition 4 represents the 2-week LCD-only phase among animals with previous exposure to HCD. Based on this definition, each condition produced primarily unimodal distributions with slight right tail oriented bimodal or truncated distributions indicative of an extended break in feeding due to the absence of nocturnal feeding.

Fig. 2

Linear (left) and logarithmic (right) frequency distributions of 12 h diurnal (0700–1900 h) and nocturnal (1900–0700 h) feeding for all four feeding conditions collapsed. Linear frequency distributions show a greater post-meal interval during nocturnal feeding while log transformed distributions show uniform, unimodal distributions. The presence of snacking is represented by a slight modality of the briefest post meal intervals during diurnal feeding that is not present during nocturnal feeding.

Fig. 3

Meal ± SEM of meal number, meal size in kcals, kcals consumed outside of defined meals (“snacks”) for all females during the LCD-only condition and the choice condition when both LCD and HCD were available. An asterisk indicates the difference between the two dietary conditions were significant.

Fig. 4

Mean ± SEM meal parameters for dominant and subordinate females during the LCD-only condition and the choice condition when both LCD and HCD were available. Panel A shows meal number, panel B shows meal size in kcals, and panel C shows kcals consumed outside of defined meals (“snacks”).

Fig. 5

Mean ± SEM kcals consumed outside of defined meals (“snacks”) for dominant and subordinate females during the LCD-only condition prior to any exposure to the dietary choice condition (“No choice history”) or following exposure to the choice condition (“Choice history”). Different letters indicate groups are significantly different from one another.

Fig. 6

Mean ± SEM percentage of HCD vs. LCD kilocalories contributed to total snack kilocalories and total meal kilocalories. An asterisk indicates a statistically significant preference for one diet over the other.

Fig. 7

Mean ± SEM ratio of kilocalories contributed to mixed meals by HCD vs. LCD among dominant and subordinate females. An asterisk indicates a statistically significant difference between proportions of the two diets based on status.