by Dinsa Sachan
(BrainWorld Magazine) – Eating comes to us naturally. Initiated by hunger, a basic human drive, it helps us survive. But why do people overeat? According to the World Health Organization, there are more than 1 billion overweight adults worldwide, at least 300 million of them obese. That can’t just be about hunger.
Understanding Basic Hunger
Feeding is hard-wired into the brain. The hunger loop revolves around the lateral hypothalamus, an important center for hunger. “A lesion of this area reduces 
feeding,” says Sabrina Diano, associate professor of neurobiology at the Yale University School of Medicine. However, the most studied area of the hypothalamus that seems to play a major role in the regulation of metabolism is the arcuate nucleus. The hypothalamus is also concerned with maintaining homeostasis (a state of balance) with regards to temperature, weight, pressure, etc.
“The hunger system wants to make sure each cell in the body has the energy [generated from food nutrients] to perform its task,” says Larry McCleary, author of Feed Your Brain Lose Your Belly (Teocalli, 2010). “The hypothalamus coordinates this by functioning as the central clearing house of energy information. The nerve cells in the hypothalamus act as energy sensors for the body. When they are short on fuel, they assume the rest of the cells in the body are and they generate a hunger signal. When they sense no energy shortfall, the hunger signal stops.”
Typically, the hunger process will start with an internal stimulus like low blood glucose. Upon detecting low blood glucose, a hormone called ghrelin (popularly called the hunger hormone) rises through the bloodstream. It relays these signals to the hypothalamus, which in turn triggers hunger.
As we eat, blood glucose levels rise and the stomach is distended. This initiates release of certain chemicals—primarily leptin, a hormone present in fat cells—which send “fullness” signals to the hypothalamus, which produces a feeling of satiety.
Why We Give in to Temptation
Physical hunger is not all there is to it. What we eat and how much we eat depend on many factors. Appetite is complex, with the brain playing the boss, helped along by hormones, neurotransmitters and sensory cues.
It doesn’t help that the hypothalamus isn’t really good at discerning between physical hunger and cravings. A brain-imaging study by researchers at the Montreal Neurological Institute, McGill University, suggested that ghrelin acts on specific brain regions to enhance our response to food-related cues. In fact, the mere sight or whiff of food is enough to provoke a feeling of hunger.
Researchers at Northwestern University’s Feinberg School of Medicine tried to lure participants with Krispy Creme donuts, the sight of which activated the limbic region in the brain’s important region involved in motivation.
Whether we actually eat in response to the sight of food depends partly on how hungry we are in the first place, but food definitely tempts.
Food, like sex, is associated with pleasure. This is nature’s way of encouraging us to eat. The feel-good aspect of food is one important cause of overeating. The link between the brain’s reward mechanism and overeating is something that scientists are still trying to figure out.
Researchers at the Monell Chemical Senses Center and the University of Pennsylvania School of Medicine conducted an fMRI study on human volunteers and found that food cravings activate the brain areas associated with reward, emotion and memory. Participants were subjected to a monotonous diet prior to an imaging session where they were shown names of the foods they liked. As the researchers had predicted, the subjects experienced cravings. Also, ghrelin works very closely with the reward system, and that makes it susceptible to possible misuse by sensory cues.
Why are most of us tempted by calorie-rich foods but not by lentils or broccoli?
A 2009 University of Cincinnati study showed that ghrelin levels rise when mice are exposed to fatty foods. This might be an evolutionary strategy to store fat to survive in times of famine.
The Science of Comfort Foods
Why do some women starve their way through divorce? Why does exam stress make students binge? Scientists have only begun to comprehend the complex relationship between food and emotion.
“Different kinds of stress can have different results,” says Timothy Moran, co-director, Center for Metabolism and Obesity Research, Johns Hopkins University School of Medicine. “Usually, stress reduces food intake. In studies on mice, acute stress in the form of tail pinch has been shown to increase eating, but whether this is a stress or activational response is not clear.”
Researchers at the US Department of Energy’s Brookhaven National Laboratory tried to unravel the link between overeating and negative emotions. They found that similar mechanisms are found underlying drug abuse. Moreover, the tendency to eat in response to negative emotions is found to be particularly prevalent among the obese. Emotional states induce fight-or-flight responses, and in these states blood goes to areas other than the brain to prepare surges of energy to fight or flee. This in turns leaves the area in the brain responsible for self-regulation deficient in blood and oxygen supply.
Even positive emotions can lead people to overeat. “When people are happy and binge it is because they have associated celebrating with eating, because of having had experiences in childhood where they go out for ice cream or other fun food to celebrate an occasion,” says Carole Lieberman, assistant clinical professor of psychiatry, University of California, Los Angeles. Over time, the neuronal paths are strengthened by sensory cues.
A study published in Nature Neuroscience reported that levels of ghrelin increase during stress, and this could explain why stress prompts us to eat when we are not hungry. “Ghrelin [produced by the stomach] rises in response to stress and sleep deprivation—which tells the brain ‘I’m hungry,’ ” says Shawn Talbott, nutritional biochemist and author of The Cortisol Connection: Why Stress Makes You Fat and Ruins Your Health (Hunter House, 2007). And cortisol (produced by adrenal glands) also rises in response to stress, which tells the brain to refuel. “The end result is that the brain gets two signals that are slightly different, but result in us eating more.”
When it comes to understanding overeating, neurobiology has just detected the tip of the iceberg. But insights are pouring in every day, and in the future we may be able to devise possible ways to curb the monster.
