The reinforcement-enhancing effects of nicotine: Implications for the relationship between smoking, eating and weight
Research highlights
► Food reinforcement may underlie eating, especially in the absence of satiety. ► Nicotine acts through non-associative mechanisms to enhance reinforcement. ► The reinforcement-enhancing effects of nicotine generalize to food reinforcers. ► Addressing changes in food reinforcement may be important for cessation efforts.
Introduction
Concerns about body weight represent an important barrier to public health efforts aimed at reducing smoking. Epidemiological studies have found that current smokers weigh less than non-smokers and weight restriction is commonly cited as a reason for smoking among adolescents [1], [2], [3]. Furthermore, following smoking cessation, ex-smokers typically gain weight. Often, the weight gain is less than 6 kg, although a significant minority gain as much as 7–11 kg [4], [5], [6], [7]. Regardless, even the potential for weight gain after cessation is a motive for continued smoking [8], [9].
The mechanisms underlying the relationship between smoking and weight are complex and may involve a number of factors including changes in caloric intake, activity, metabolic rate, and lipogenesis [10]. Despite this complexity, much attention has focused on the effects of nicotine on food consumption. In free-feeding rats, nicotine produces anorectic effects, decreasing total food intake and meal size [11], [12], [13], [14], [15], [16]. However, although a common perception is that nicotine reduces eating behavior in humans, these effects are often not observed in smokers. In fact, although smokers generally weigh less than non-smokers, they tend to eat either the same amount or more [17]. Withdrawal from nicotine in chronically treated rats increases food consumption [18], [19]. Likewise, numerous studies have reported increases in caloric intake during smoking cessation that account for a substantial portion of the variance in weight gain [20], [21], [22], [23].
Much like the determinants of body weight, multiple behavioral processes and neurobiological mechanisms that could be differentially affected by cigarette smoking underlie eating behavior. One determinant of food intake that may be particularly important is the reinforcing efficacy of food [24]. Indeed, individual differences in food reinforcement predict food intake amongst smokers enrolled in a cessation trial [25], suggesting that the effects of cigarette smoking on the incentive and reinforcing properties of food and food-related stimuli should be a focal point in efforts to understand the relationship between smoking, eating, and weight [26]. Furthermore, of the thousands of constituents in tobacco, substantial data suggest that nicotine, the primary psychoactive constituent, is of central concern [17].
The hypothesis that the effect of cigarette smoking on food intake may be mediated by the impact of nicotine on the reinforcing efficacy of food is consistent with a growing literature describing the effects of nicotine on reinforced behavior. The reinforcement-related effects of nicotine include the widely acknowledged ability of nicotine to act as a primary reinforcer capable of establishing conditioned reinforcers, and a second, powerful ability of nicotine to non-associatively enhance other reinforcers in the environment. In this paper, we first review data from our laboratory that highlights these distinct ways in which nicotine impacts reinforced behavior. We then elaborate on the reinforcement-enhancing effects of nicotine, focusing on behaviors and stimuli related to food. Finally, we discuss the theoretical and clinical implications of this perspective for understanding and addressing the apparent tradeoff between smoking and weight gain.
Section snippets
Nicotine and reinforced behavior
Nicotine, like other drugs of abuse, functions as a primary reinforcer. Numerous studies have shown that nicotine is self-administered by a variety of animal species [27], [28], [29], [30]. Self-administration of nicotine varies as a function of dose and schedule of reinforcement, extinguishes when nicotine is replaced by saline or pharmacologically blocked by a nicotinic antagonist, and, in the absence of other reinforcing stimuli, is dependent on nicotine delivery being response-contingent
Effects of nicotine on operant behavior associated with palatable reinforcers
Although much of the preclinical literature supports the notion that nicotine has anorectic effects, this work is based largely on conditions in which food is freely available [18], [61], [62], [63]. Free access conditions (or rich schedules of reinforcement) are insensitive measures of reinforcing efficacy, particularly when motivation can be reduced through consumption. Hence, a better assessment of the effects of nicotine and nicotine withdrawal on the reinforcing efficacy of food would
Theoretical and clinical implications
Comparison of the effects of nicotine on behavior reinforced by food and food-related stimuli with its effects on non-food reinforcers, reveals both similarities and differences. Acutely, nicotine appears to produce similar enhancing effects on behaviors related to a wide range of reinforcing stimuli, including sensory reinforcers, food, sucrose, and conditioned reinforcers associated with food, sucrose, and nicotine [46], [49], [64], [65], [66], [68]. The effects of nicotine on food
Conclusions
This review highlights an apparent paradox between the proposed ability of nicotine to enhance food reinforcement on one hand and the known weight suppressing effects of smoking on the other. As noted above, both weight and feeding behavior are likely to be controlled by multiple mechanisms. Under some conditions (e.g., food restriction), the ability of nicotine to enhance reinforcement may dominate the effects on caloric intake. However, under other conditions (e.g., satiation), other effects
Acknowledgements
This manuscript was supported by a grant from the National Institute of Health (DA 10464; ECD). The authors have no conflicts of interest to report.
References (104)
- et al.
Meal patterns in male rats during and after intermittent nicotine administration
Pharmacol Biochem Behav
(2003) - et al.
Meal patterns in female rats during and after intermittent nicotine administration
Pharmacol Biochem Behav
(2005) - et al.
Nicotine alters the usual reciprocity between meal size and meal number in female rat
Physiol Behav
(2001) - et al.
Effects of nicotine on body weight, food consumption and body composition in male rats
Life Sci
(1990) - et al.
Chronic nicotine and withdrawal effects on body weight and food and water consumption in female rats
Physiol Behav
(1987) - et al.
Effects of smoking cessation on weight gain, metabolic rate, caloric consumption, and blood lipids
Am J Clin Nutr
(1986) - et al.
Cessation from cigarette smoking: changes in body weight, body composition, resting metabolism, and energy consumption
Metabolism
(1991) - et al.
Food reinforcement
Appetite
(2006) - et al.
Relation between food reinforcement and dopamine genotypes and its effect on food intake in smokers
Am J Clin Nutr
(2004) - et al.
Control of behavior by intravenous nicotine injections in human subjects
Pharmacol Biochem Behav
(1983)