Humans experience many drives that prompt them to perform certain behaviors, the most powerful of which is the drive to eat.
This multidimensional need has many components and is not controlled by one area of the brain or one isolated organ in the body; rather it is all-encompassing. Older views on hunger and appetite maintained that the reason animals eat certain foods at certain times of day were entirely explained by the body’s need to maintain homeostasis, but current research shows that higher levels of processing are involved in appetite control as well as the important role of timing.
The urges, the cravings and even the physical sensations of hunger are experiences that all people can relate to since the drive for food is necessary for survival. Hence, nature has composed many mechanisms to ensure the activation of these desires with the use of higher-order brain signals and hormone release throughout the body.
“Brain areas that are more complex and involved in other processes like reward and cognition control the desire for food, not only those dealing with bodily energy balance as was previously thought,” said Gene-Jack Wang of Brookhaven National Laboratory’s Center for Translational Neuroimaging.
“People are actually eating more and beyond what is necessary for their bodies to function.”
This excess of consumption, Wang said, is due to the brain’s ability to override the body’s biology and the drastic changes in the environment humans inhabit.
Human life today is full of cultural and social situations where overeating is welcome and food is readily available. Combine a vastly sedentary lifestyle with mainly office-based jobs that do not require physical activity, and the recipe for growing numbers of obese individuals becomes evident, Wang said.
Since humans evolved in an environment of scarcity, the need for high-energy and high-fat food storage was promoted to prevent starvation and hunger, said Kate Baicy, a neuroscience graduate student. Now we live in a time where food is plenty but our biology still reacts by storing food and fat, as was naturally selected for in the past, she added.
But the brain can still override the body and continue to send signals to continue eating when the body is satiated, Wang said.
This occurs when eating certain foods becomes a conditioned response to certain outside stimuli such as smells.
“We either eat when we are hungry or when we smell something that increases our desire for that particular food and as a result causes the release of dopamine in the brain,” Wang said.
When the neurotransmitter dopamine is released, the pleasure-processing center of the brain lights up, Baicy said.
Research has uncovered that this center for motivation and conditioned response is also overextended and abused by those with drug addictions as well as compulsive eaters, Baicy added.
“But food and appetite are more complex than drug addictions,” Baicy said.
“Drugs only act through the central brain system whereas appetite acts on the whole body.”
The hunger message travels from the brain to the digestive acids in the stomach and the activation of other organ systems, releasing a cascade of events that signify preparation for feeding time.
“All of the body’s organ systems must work together for this array of activities to occur successfully,” said Christopher Colwell, professor of psychiatry and biobehavioral sciences.
This teamwork is controlled by a set of internal mechanisms ruled by the body’s circadian clocks, Colwell said. They regulate certain activities dependent on different times of day and recent research has found that all of the body’s organs have their own clocks, which must work in sync with one another to regulate appetite, he added.
“If you regularly have breakfast at a certain time, then your stomach will start growling at that time every day,” Colwell said.
If this synchronicity is lost, things can get out of whack and cause drastic changes in how food is broken down, digested and processed, causing diseases such as diabetes or obesity if chronic.
Sleep deprivation is the most common cause of this lost timing and, if severe, lack of sleep has been shown to mimic the symptoms of diabetes, Colwell said.
In an experiment where subjects were only allowed four hours of sleep each night, it was found that there were hormonal changes and increases in the amount of stress hormones circulating in their bodies. There were also changes in insulin and appetite control.
This deprivation also caused a change in the psychological preferences for food, making subjects crave foods with high carbohydrate and salt content, Colwell said.
But it is evident that, when it comes to food, there are multiple components that can simultaneously act and determine the type and amount of food one will ingest at any given time. The key is, the brain can override the body’s needs and go beyond these natural needs since we are not slaves to our biology, Colwell said.