In this study, a novel carbohydrate snack displayed indications of abuse potential for rigorously defined carbohydrate cravers, when tested under laboratory conditions used to evaluate abuse liability for drugs,. After being induced into a negative mood, simulating the conditions under which they ordinarily snack on carbohydrates, the carbohydrate cravers we studied chose overwhelmingly to self-administer an unfamiliar carbohydrate-rich, protein-poor beverage in preference to a protein-rich beverage that non-carbohydrate craving taste-testing panels had rated equally palatable. Carbohydrate cravers also showed greater reduction in negative mood after drinking the pure carbohydrate beverage compared to the beverage containing both protein and carbohydrate, and they liked the carbohydrate beverage better (i.e., found it more palatable). The two beverages had no differential effect on positive mood. On three out of four indicators (behavioral choice, self-rated liking, and anti-negative mood effect, but not pro-positive mood effect), therefore, carbohydrate snacks appeared to showed evidence of abuse potential for overweight women who crave them, plausibly contributing to overconsumption and weight gain.
Consistent with contemporary best practice in abuse liability assessment, we applied a multi-method approach that integrates different indicators of abuse potential (Griffiths, Bigelow, & Ator, 2003
). Results indicated that the different abuse potential indices were related but non-redundant, as has been observed previously for addictive drugs (Johanson & Uhlenhuth, 1980
). The carbohydrate craver’s greater liking for the carbohydrate beverage was not associated with the greater reduction in negative mood produced by that beverage. Nor did the mood effects or the liking for carbohydrate fully explain the behavioral preference for carbohydrate. Interestingly, even though we asked participants to choose and self-administer the beverage that most improved their mood, the effect of the beverages on mood had no significant influence on beverage choice. In contrast, liking ratings for both beverages were associated with beverage preference, even though they did not fully explain the variance in beverage choice.
Positive reinforcing effects of carbohydrate snacks have been found also in other studies of carbohydrate craving populations that used different methodologies and did not use a negative mood induction. For example, women in nicotine withdrawal, a state also associated with carbohydrate craving, overeating and weight gain, worked harder under increasing response requirements, for carbohydrate snacks, relative to money (Spring et al, 2003
). Similarly, carbohydrates induced positive mood in several prior studies of different carbohydrate craving populations (Lieberman et al, 1986
; Sayegh et al, 1995
; Wurtman et al, 1989
; Rosenthal et al, 1989
), whereas in non-carbohydrate cravers, carbohydrates induced fatigue (Spring et al, 1982
A noteworthy finding in the present study was that, among those who consistently preferred carbohydrate, the behavioral response to the novel carbohydrate beverage changed over time in a manner suggestive of neuroadaptation. Interestingly, liking for the carbohydrate beverage grew (sensitized) over time, whereas the beverage’s impact on dispelling negative mood decreased (showed tolerance) over repeated exposures. Gradual recruitment to a pattern of increased liking and self-administration of snacks that progressively lose their potency to dispel (self-medicate) bad moods seems a likely pathway to binge eating. Investigation is warranted to determine whether the observed growing subsensitivity to dysphoria alleviating properties of a food corresponds temporally to the diminution in striatal dopamine D2 receptors that has been observed in substance abuse and obesity (Wang et al. 2004
We operationalized carbohydrate craving as snacking on high carbohydrate, low protein foods in relation to negative moods at certain characteristic times of day (late afternoon, evening). The carbohydrate snacks that the women routinely consumed were either sweet, savory or bland and either high, medium or low in fat. That is, we studied women who exhibited a particular macronutrient craving (high carbohydrate, low protein), independent of taste and independent of fat content. Study participants ate several different kinds of carbohydrate snacks: sweet, high fat foods (eg., chocolates or cake); sweet, low fat foods (e.g., gummy bears), savory, high fat foods (e.g., potato chips); savory, low fat foods (e.g., pretzels); and bland foods with preparation methods or condiments that made them either high or low in fat (e.g., pasta, bread, potatoes). Given such diversity, it is noteworthy that the pure carbohydrate test beverage lacking any protein was resoundingly preferred by carbohydrate cravers under double-blind conditions, even though both beverages were novel, sweet, lacked any fat, and were liked equally well by non-carbohydrate cravers.
The present study design improved upon prior carbohydrate craving experiments (Wurtman et al. 1981
; Wurtman et al. 1987
; Wurtman et al. 1993
) by using stringent definitional criteria that required a demonstrable pattern on more days than not of snacking on high-carbohydrate, low-protein foods at characteristic afternoon or evening times of day and either preceded by negative mood or followed by improved mood. Additionally, we employed double-blind testing, measured choice in addition to self-reported mood, controlled for intake preceding the session, and assessed self-administration against the relevant dysphoric affective state.
Our sample was highly selected in that participants were required to meet stringent operational criteria for carbohydrate craving. However, participants with psychopathologic comorbidities were included, since those often accompany the carbohydrate craving syndrome. We have no reason to expect results to differ among males or more obese carbohydrate cravers. Females were studied in this initial research because they are believed to comprise the majority of individuals who report carbohydrate craving (Wurtman and Wurtman 1995
). Overweight or mildly obese individuals were studied rather than heavier individuals in order to standardize the dose of carbohydrate needed to trigger an insulin response. A sweet rather than a non-sweet carbohydrate snack was studied because we previously found that sweet snacks show heightened reward value among women who exhibit dysphoric mood and overeating (Spring et al. 2003
). Liquid rather than solid snacks were studied because liquids permitted greater ease of blinding and expectancy control. Also, the use of beverages allowed for a more efficient protocol, because the time course of plasma tryptophan ratio changes after consuming a liquid is more rapid than that after a solid food.
In designing the experiment, differences in test beverage palatability were regarded as error variance that needed to be controlled. Encapsulating the two beverages to eliminate any difference in taste was not feasible for reasons of practicality and ecological validity. Consequently, we adopted the alternative approach of having adults who did not self-identify as carbohydrate cravers match the beverages as closely as possible on palatability. Interestingly, we observed on repeated occasions that beverages whose palatability was equated carefully for panels of non-carbohydrate craving adults were unmatched for the carbohydrate craving sample. Because the carbohydrate cravers’ difference in liking for the two beverages persisted at the end of the study, liking was treated as a covariate in the analyses. However, the data also reminded us that rated liking for a drug is a well-validated predictor of abuse liability (Epstein et al. 2006
). Thus trying to equate liking/palatability for two beverages, one of which has abuse potential for a population and the other of which does not, may prove to be challenging if not impossible. Liking/perceived palatability may be an inherent, integral constituent of a food’s abuse potential.
The reliability and generalizability of the present findings remains to be determined. The results warrant replication, as well as testing to see whether the findings extend to solid foods and non-sweet carbohydrates. Also needed is an examination of whether carbohydrate foods possess apparent abuse potential when tested outside of a context that involves negative mood induction. It is unknown whether self-administration of carbohydrate foods by carbohydrate cravers occurs primarily under conditions of negative affect, or whether preferential self-administration also occurs under conditions of neutral or positive mood. Indeed, elsewhere we have demonstrated that drug effects are integrally related to the affective context in which the substance is self-administered (Spring et al, in press
). Another important question is whether the current results have alternative explanations that might have greater scientific and heuristic utility. The study’s most important contribution may be to encourage re-examination of whether a meaningful distinction can be drawn between behavioral addictions and drug addictions, and what methodologies might genuinely differentiate between the two.
These findings should not be interpreted to suggest that obesity is generally attributable to food addiction, or that carbohydrate-rich, protein-poor snacks exhibit abuse potential for the average eater. We think it more plausible that gene by environment interactions convey differential susceptibility to food preferences and cravings involving different nutrients, tastes, textures, etc. (Perusse and Bouchard, 2000
). Obesity is likely a heterogeneous syndrome comprised of a number of different behavioral phenotypes. Most likely, different gene by environment interactions give rise to individual differences in susceptibility to specific food cravings and preferences. Although here we have objectively demonstrated the existence of carbohydrate preference, other phenotypes such as fat preference and sweet preference are likely important contributors to obesity. Also important are the other parts of the interaction between genes, environment, and behavior that gives rise to unhealthy eating. As noted, affective triggers, including stress, may play a pivotal role, as may traits like self-control or its converse, impulsivity.
Overconsumption of palatable, energy dense foods has adverse health consequences as severe as those of most drugs of abuse. Early theories of carbohydrate addiction posited that intake of carbohydrate-rich, tryptophan deficient foods corrects serotonergic hypofunction and dispels negative mood via an insulin-mediated rise in the brain tryptophan influx and serotonin synthesis (Fernstrom and Wurtman 1971
; Fernstrom and Wurtman 1972
). More recently, certain forms of compulsive, excessive food self-administration are being conceptualized as an addiction perpetuated by the rewarding properties of food (Lowe and Butryn 2007
) and largely mediated by the mesolimbic dopamine system (Berthoud 2002
; Kelley and Berridge 2002
), a neural network that is “hijacked” by exogenous substances of abuse (Volkow and Wise 2005
). Neither a serotonergic, dopaminergic or other neurobiological pathway for food addiction has been tested directly. The current findings indicate that for those who crave them, carbohydrate snacks display apparent abuse potential on indicators derived from a laboratory protocol that has been used to test the abuse liability of drugs. The possibility of food addiction may warrant reconsideration.