As an undergraduate I took a seminar on feeding behavior in my senior year.  We covered a variety of topics during the semester, including some of the physiological mechanisms involved in feeding behavior, and emotions evoked from feeding.

I remember my professor once talking about cravings.  We of course asked her if there was any truth to the adage that pregnant women have strange cravings.  She told us that that was probably not the case, that everyone has strange food cravings from time to time, and that people just notice the cravings of pregnant women more.  This actually made a lot of sense to me.  We pursued the subject with her, and learned that research suggests that salt is the only biologically driven craving.  What this means is that when you crave something salty, your body actually needs salt.  Given what you know about how the nervous system works, this should hardly be surprising.  But we were kind of amazed at the idea that other cravings, like a craving for oranges, were probably not caused by a deficiency in vitamin C, but more likely the product of an aversion to a specific diet.  In other words, you do not crave grapefruit because you need vitamin C; you crave it because you are tired of all the stuff you normally eat.  I always meant to pursue the question of what cravings actually are, since I think they are very interesting, but, as always, it is hard to find the time.

As I was thinking about this blog entry, however, I decided to do a literature search on cravings, and discovered a 2002 article authored by the very professor I took the feeding behavior seminar with, Dr. Marcia Pelchat, who is now at the Monell Chemical Senses Center in Philadelphia, PA.    Marci played an integral role in my life in terms of my career, so it was a pleasant surprise to come across that, and a few others from her on the very topic long in the periphery of my interests.

In this particular article, Dr. Pelchat reviewed the similarities between food addictions and drug addictions, with emphasis on physiological substrates.  A craving is defined as an intense desire for a specific food or a drug, and, according to Dr. Pelchat, it is no accident that the word links two apparently different types of cravings.  Her research has shown that food cravings, while often viewed  negatively, may actually promote more variability in the diet, and thus serve an adaptive function.

Many links between food cravings and drug cravings are apparent.  Preferences for sweets and the self-administration of certain drugs are also linked.  Alcoholics prefer more concentrated sweeteners than non-alcoholics, as well as exhibit a preference for sweets, and treatment programs for people attempting to stop smoking involve chewing various types of gum.  Opioid neurotransmitters, dopamine, and serotonin are also involved in cravings.  Endogenous opioids and dopamine are most likely involved in rewarding addictive behavior (be the addiction for food or drugs) and serotonin may affect cravings for carbohydrates.  Indeed, activity of the orbitofrontal cortex is implicated in obsessive-compulsive disorder, and this area is linked to the reward circuitry of the midbrain.    Though this area has not, to date, been directly implicated in food and drug cravings, this area does receive significant sensory information associated with food intake, notably gustatory and olfactory information.  The hormone leptin may also play a role in cravings for alcohol in individuals going through withdrawal.

In general, then, cravings, regardless of what is actually being craved, all activate the same architecture in the nervous system.

References

Pelchat, M. (2002). Of human bondage:  Food craving, obsession, compulsion, and addiction.  Physiology and Behavior, 76, 347-352