A growing body of research is erasing any doubt that eating is as much about our brains as our stomachs.
Consider, for example, a study published this summer that found that ghrelin, a hormone released when your stomach is empty and growling, may help stave off depression. Researchers found that underfed mice with higher levels of the hormone showed fewer signs of depression and anxiety than well-fed counterparts, swimming longer when plunged into water and seeming more adventurous in a maze.
A paper published last month in the journal Science suggests some people may overeat because their brains simply don't get as much pleasure out of food - a chocolate milkshake, to be specific - so they keep eating in an attempt to feel satisfied.
And a study published last week in the Journal of Neuroscience found that when a mother rat ate a fatty diet, her babies' brains were wired with neurons that produce molecules that stimulate their appetite.
Using techniques from genetics to brain scans, scientists are illuminating what happens within the brain as we eat, with goals that range from creating better fat substitutes to understanding the essence of flavor.
"What's changing with all the new biomedical research capabilities is we recognize now the importance of the signaling system in the gastrointestinal tract, in the brain," said Richard Mattes, a professor of food and nutrition at Purdue University.
Much of this work is driven by interest in pure science, but food research is never far from a practical application. Battling obesity is one such application.
"How do the things we put into our mouths modify all these different hormone . . . systems that regulate metabolism? If we can begin to understand those, we can begin to understand how to modify the ingredients of what we're eating or drinking to take advantage of the basic physiology in such a way so that it doesn't produce obesity," said Charles Wysocki, a behavioral neuroscientist at Monell Chemical Sciences Center.
Earlier this year, a study in the journal Neuron found that mice that were robbed of their sweet tooth - stripped of the very ability to taste sweetness - still preferred sipping sweet, calorie-laden drinks. Certain mouse brain pathways usually associated with tasting something scrumptious were activated when they consumed the sugary drink, even though they couldn't taste it.
"The brain processes that are linked to liking a food seem to be paying attention to what happens to your body, besides your mouth," said author Ivan E. de Araujo, a neurobiologist at the John B. Pierce Laboratory, an independent research institute affiliated with Yale University.
An accompanying commentary suggested the study was provocative - raising the possibility, for example, that high-fructose corn syrup, which is virtually unavoidable in processed foods, may trigger the brain's reward circuitry more strongly than other types of sugars.
Now, de Araujo is studying whether different kinds of sugars that have the same number of calories stimulate the brains of taste-blind mice in different ways.
The chocolate milkshake study reported in Science tested 77 young women and found that compared with their lean counterparts, overweight subjects got less reward out of eating. They had a blunted response to food in a part of the brain, called the dorsal striatum, that provides pleasure. That suggests one possible reason obese people overeat. That reduced response was particularly true for people with a genetic variation that caused that part of the brain to have fewer receptors for dopamine, the neurotransmitter that triggers feelings of pleasure.
They were also more likely to gain weight over the next year.
"If you overeat and you have this" genetic variation, that part of your brain "is going to be less responsive to food," said Dana Small, a neuroscientist at John B. Pierce Laboratory. "That has relatively large public health implications."
But the studies aren't just about overeating; the brain can also be tricked by marketing. A study by California researchers published earlier this year, for example, found that the brains of people who sipped what they were told was expensive wine showed a higher pleasure response than in people who sipped the same wine when it was described as a common variety. The subjects, whose responses were measured in scanners that monitored their brains, also said the pricey wine tasted better.
Tastes of a $5 bottle of wine were given twice, once identified by its real price and once with a false price of $45. A $90 bottle of wine was given once at its actual price and another time at the false price of $10. In each case, the wine described as more expensive triggered more brain activity and got higher ratings.
The results dovetail with other mind-over-mouth studies and gustatory illusions: Amping up the crunch of potato chips tricks us into rating them as crispier, one study showed.
Calling apple juice a soup and serving it in a bowl makes it more filling than the same portion served in a cup. Dyeing a food a particular color can make it taste different.
"There's growing interest in how flavors and tastes impact the brain, because it is the brain that is in control," de Araujo said. "If you want to understand abnormalities like obesity and anorexia, you have to understand what brain practices are going awry."
Carolyn Y. Johnson can be reached at firstname.lastname@example.org.