Comparing the effects of food restriction and overeating on brain reward systems
Introduction
A unique situation is emerging among the aging population. Although caloric consumption has been shown to significantly decrease with age (Briefel et al., 1995), obesity is on the rise among members of this cohort (Salihu et al., 2009). Patterns of over- and under eating may have deleterious consequences on both the neurochemistry and behaviors associated with reward and reinforcement of behavior. Of particular relevance is the prediction that by the year 2020 the number of individuals over the age of 50 with substance abuse disorder will be two times higher than estimates from each year between 2002 and 2006 (Han et al., 2009). This prediction highlights the importance of better understanding behaviors, such as under- and overeating, which are known to cause alterations in brain reward functioning and therefore may contribute to the pathology of substance abuse.
Both preclinical and some clinical studies suggest that prolonged food restriction leads to heightened reward sensitivity (Carr, 2002, Frank et al., 2005, Frank et al., 2012). Studies examining the effects of overeating on reward sensitivity are mixed, and several theories have been developed to explain what appear to be conflicting findings (Verbeken et al., 2012). Before discussing the effects of food deprivation or overeating on reward sensitivity, however, it is important to review the neural components associated with responses to reinforcing and rewarding stimuli. Although the brain reward system is complex and consists of a number of different components (i.e., opioids, GABA), this paper will primarily review clinical and preclinical studies investigating the effects of differential feeding behavior on dopamine (DA). Mesolimbic DA neurons project from the ventral tegmental area (VTA) to the nucleus accumbens (NAc), and dopamine is considered to play an important role in influencing motivation for and the reinforcing and rewarding experiences of food consumption, drug use, and other stimuli (Everitt and Robbins, 2005, Schultz, 2010).
By studying the effects of food deprivation and overeating, it may be possible to gain a clearer understanding of the mechanisms that underlie changes in reward response or functioning due to non-homeostatic eating behaviors. In addition, by reviewing findings of studies using laboratory animal models, we may be able to gain insight into the associated biological factors without the psychological variables that may accompany aberrant eating behaviors.
Section snippets
Laboratory animal studies
When laboratory animals are food restricted, they show alterations in behavior that suggest increased reward sensitivity. Rats that have been even acutely food deprived exhibit higher rates of intravenous self-administration of drugs of abuse, such as cocaine and phencyclidine (Carroll et al., 1981). Further, chronic food restriction, accompanied by weight loss, has been shown to decrease the amount of drug necessary to experience its rewarding effects. For example, lateral hypothalamic
Laboratory animal studies
There appears to be a strong causal relationship between food restriction and drug use, as food deprivation has been shown to increase self-administration of a number of different drugs in animals (Carr, 2002). Interestingly, overeating (with or without food restriction) may also precipitate addictive behavior, and there have been several studies that suggest neurochemical and behavioral similarities between drug addiction and the more recently researched topic of “food addiction” (Allen et
Relationship between these findings and the aging population
The information reviewed in this paper may be particularly relevant to the aging population. First, caloric intake has been shown to decrease considerably as age increases. In fact, in the 60 years following 20 years of age, both men and women have been found to substantially decrease their caloric intake per day, with men exhibiting an even greater decrease than their female counterparts (Briefel et al., 1995). If malnutrition is not present, caloric restriction has been reported to delay
Conclusion
Reward sensitivity may be heightened when feeding behavior is reduced as an incentive to eat, thus promoting survival. Such an increase in reward sensitivity may prove maladaptive, however, especially in the absence of need, as is the case in many societies today. Instead, increased sensitivity to reward may result in increased consumption of food and drugs of abuse. Overeating on the other hand, may reduce reward sensitivity over time, potentially perpetuating a cycle of overeating that may
Conflict of Interest
Author has confirmed that there is no conflict of interest.
Acknowledgements
Supported by the University of Florida, DA-03123 (NMA) and Kildehoj-Santini (NMA).
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