When it comes to cravings, the evidence is clear: Nutritionally speaking, we’re not to be trusted. We may rationalize the intense desire to slurp a milk shake as the body’s desperate cry for calcium, but it just isn’t so. The longing is really for the Dionysian pleasure of satisfied taste buds. “Usually a craving is a craving for a certain taste,” says Elizabeth Somer, author of Food & Mood. ‘Collard greens are a great source of calcium, but how many times do you crave that?”

Maybe never. In fact, most of us seem to hanker for the same few foods. According to one Canadian study of more than 1,000 college students, women most frequently crave chocolate, followed by salty foods and pizza, while men wilt for pizza first, with chocolate a close second.

How to account for the near-universal jones?  One explanation gaining credibility is that foods contain substances that influence brain chemistry and create a feeling of well-being. Carbohydrates—whether simple (like candy) or complex (as in pasta and potatoes)—work as a brain drug that we use unwittingly to self-medicate, claims Ronald Ruden, MD., author of The Graving Brain. Stress can lower the brain’s levels of serotonin, the neurotransmitter that controls mood, creating a “craving brain” in search of something to raise the levels back up. “When you eat carbs, serotonin goes up, dopamine [a brain chemical linked to arousal] is constrained, and voilâ, the brain is bio-balanced,” Ruden explains.

Other research implicates hormones in the munchie machine.  A recent study cowritten by Susan I. Barr, a professor of nutrition at the school of Family & Nutritional Sciences at the University of British Columbia in Vancouver, reveals why cravings increase premenstrually:  When more progesterone is secreted, body heat goes up and increased energy expenditure may fuel a desire for more food  Like-wise, exercise, which boosts energy output, can trigger a  binge: Working out lowers liver glycogen stores.  "When glycogen stores go down, appetite goes up",  says Eric Sternlicht, assistant professor of kinesiology at Occidental University in L.A.  Drinking fruit juice after a workout helps curb the urge to gorge, since fructose restores liver glycogen to normal.

Are we destined to be driven by our chemical urges?  Not necessarily.  As kings of the food chain, we've trained our brains to overrule some of our worst instincts.  The occasional killer urge for something "good" such as a big salad or protein shake, says Somer, is the result of conditioning and good habits.  "Once you learn to replace fat and sugar with fruits, vegetables and whole grains, you can eventually reset your brain chemistry, and won't miss sweets as much."   -Samantha Dunn