addictive agents have been ingested in foods

Until recently in our evolutionary history,  addictive agents have been ingested in foods.  Many are secondary plant metabolites that  evolved because they discourage ingestion by  animals1. The hungers that arise from bodily  needs are non-directive; they merely encourage  us to put things in our mouth. The more acute  the hunger, the greater the range of substances  we will ingest2. We learn to return to the yellow  banana, the purple fig, the pink peach. We also  learn to chew the tobacco leaf and drink the  nectar of fermented fruits and grains. Because  of the need for the nutrients in plants containing  addictive substances, many species  have learned to accept mildly intoxicating  amounts of these compounds. Paradoxically,  some of the poisons that evolved in plants to  discourage animals from returning are—like  the nutrients that the plants offer—habitforming  in their own right.  Addiction and obesity each results from  foraging and ingestion habits that persist  and strengthen despite the threat of catastrophic  consequences. Feeding and drug use  involve learned habits and preferences that  are stamped in by the reinforcing properties  of powerful and repetitive rewards. Palatable  food activates brain reward circuitry through  fast sensory inputs and through slow postingestive  consequences (such as raising glucose  concentration in blood and brain), whereas  drugs activate these same pathways mostly  through their direct pharmacological effects  on the reward circuitry. 

Publication bibliography

Volkow, Nora D.; Wise, Roy A. (2005): How can drug addiction help us understand obesity? In Nature neuroscience 8 (5), pp. 555–560. DOI: 10.1038/nn1452.