Why do we get food cravings?
—J. Shelton, Ogden, Utah
Peter Pressman of the Cedars-Sinai Medical Center in Beverly Hills, Calif., and Roger Clemens of the University of Southern California School of Pharmacy reply:
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HANKERINGS for certain foods are not linked to any obvious nutrient insufficiency. But other biological factors appear to be at work.
Researchers have employed functional magnetic resonance imaging (fMRI) to explore the neural basis of such appetites. The imaging data suggest that when somebody is pining for a certain fare, components of the amygdala, anterior cingulate, orbital frontal cortex, insula, hippocampus, caudate and dorsolateral prefrontal cortex are activated in the brain. A network of neural regions may be involved with the emotion, memory and chemosensory stimuli of food yens.
Desire for chocolate offers an example. Constituents in chocolate may influence satiation or alter our longing for the treat by affecting mood-influencing chemicals in the brain, such as phenylethylamine, tyramine, serotonin, tryptophan and magnesium. Other foods contain these compounds at higher concentrations but tend to be less appealing than chocolate.
Some investigators have proposed that additional factors, such as simple carbohydrate content, may amplify a food’s appeal or even attenuate depression. More support for a nutrition-neurological connection comes from research that shows that administration of naloxone, which blocks opiate receptors in the brain, appears to inhibit the consumption of sweet, high-fat foods such as chocolate. Studies of cannabinoids, commonly occurring in marijuana, in the brain have shed more light on the complex neurochemistry of selective appetite. In addition, research on satiety, or hunger-control mechanisms residing in the gastrointestinal tract, has led to the identification of an entire spectrum of gut neuropeptides with elaborate central nervous system feedback and influence on satiety.
Some studies suggest that chocolate craving, especially among women, may partly result from a sense of deprivation or a reaction to stress, perimenstrual hormonal fluctuation and modulation of neuropeptide concentrations. But culture has an influence as well.
Why do we yawn?
—A. Wong, Berkeley, Calif.
Mark A. W. Andrews, professor of physiology and director of the Independent Study program at the Lake Erie College of Osteopathic Medicine, provides this explanation:
THOUGH NOT FULLY UNDERSTOOD, yawning appears to be not only a sign of tiredness but also a much more general sign of changing conditions within the body. We yawn when we are fatigued and during other times when the state of mental alertness is changing.
Yawning involves interactions between the unconscious brain and the body, although the mechanism remains unclear. For many years, it was thought that yawns served to bring in more air because low oxygen levels were sensed in the lungs. We now know, however, that the lungs do not necessarily sense oxygen levels. Moreover, fetuses yawn in utero, even though their lungs aren’t yet ventilated. In addition, different regions of the brain control yawning and breathing. Still, low oxygen levels in the paraventricular nucleus (PVN) of the hypothalamus of the brain can induce yawning. Another hypothesis is that we yawn because we are tired or bored. But this, too, is probably not the case, because the PVN also plays a role in penile erection, which is not typically an event associated with boredom.
The PVN of the hypothalamus is the “yawning center” of the brain. It contains chemical messengers that can induce yawns, including dopamine, glycine, oxytocin and adrenocorticotropic hormone. The process of yawning also appears to require production of nitric oxide by specific neurons in the PVN. Once stimulated, the cells of the PVN activate cells of the brain stem or hippocampus, prompting yawning.
Seeing, hearing or thinking about yawning can trigger the event, but there is little understanding of why. Some evidence suggests that yawning is a means of communicating to others changes in environmental or internal body conditions, possibly as a way to synchronize behavior. If this is the case, yawning in humans is most likely vestigial and an evolutionarily ancient mechanism that has lost its significance.
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