It changes your body clock.
That means you start to crave good more, and at times when you normally wouldn't have.
"If mice eat a high-fat diet, they actually wake up during what is nighttime for them and eat," says Dr. Joe Bass, a Northwestern University endocrinologist and molecular biologist who has published numerous studies about the body clock and mice. "It would be as if you were waking up every night during holiday season and eating all the sweets in your refrigerator."
It can make you addicted to eating.
Like many pleasurable behaviors—including sex and drug use—eating can trigger the release of dopamine, a feel-good neurotransmitter in the brain. This internal chemical reward, in turn, increases the likelihood that the associated action will eventually become habitual through positive reinforcement conditioning. If activated by overeating, these neurochemical patterns can make the behavior tough to shake—a result seen in many human cases, notes Paul Kenny, an associate professor in the Department of Molecular Therapeutics at The Scripps Research Institute in Jupiter, Fla., and co-author of the new study. "Most people who are overweight would say, 'I would like to control my weight and my eating,' but they find it very hard to control their feeding behavior," he says.
...and that addiction can make you feel depressed.
"Food highs" that come from over eating can actually cause the same vicious cycles in the brain that highs from addictive drugs can cause, according to research from the University of Montreal.
Overeating junk food can turn you completely off healthy food.
In one study, rats who were given junk food food (like sausages, bacon and chocolate) for a long period of time weren't interested in healthy food.
The new study showed that after eating a diet full of sausage and sweets for 40 days—even though regular lab rat chow was available—the obese rats had little interest in reverting to the more healthful diet when the tasty stuff was taken away. In fact, after depriving the high-fat habituated rats of their human junk foods, the rats would refuse to eat their standard chow for an average of 14 days. "I was really shocked at the magnitude of the effect," Kenny says. "They basically don't eat anything. If that translates over to us as a species, that's a major problem."
It lowers your body's "pleasure receptors" overall.
"Pleasure receptors" is how a University of Texas study referred to dopamine, the chemical that makes you happy.
Evidence shows obese individuals have fewer dopamine (D2) receptors in the brain relative to lean individuals and suggests obese individuals overeat to compensate for this reward deficit...
"Although recent findings suggested that obese individuals may experience less pleasure when eating, and therefore eat more to compensate, this is the first prospective evidence to show that the overeating itself further blunts the award circuitry," said Stice, a senior scientist at Oregon Research Institute, a nonprofit, independent behavioral research center. "The weakened responsivity of the reward circuitry increases the risk for future weight gain in a feed-forward manner. This may explain why obesity typically shows a chronic course and is resistant to treatment."
It can make you stop being able to tell when you're full all-together.
If you consistently overeat, you'll trigger changes in your stomach, the doctor says. The neurological tissue at the top of the stomach, which signals the brain that the stomach is full, starts to malfunction.
Additionally, drinking cold liquids while you eat only make this worse.
It can also make you feel tired and dizzy.
This is all caused from the over production of insulin and low blood sugar.
Excess food can trigger an unfortunate cycle: The pancreas produces extra insulin to process the sugar load and remove it from the bloodstream. It doesn't stop producing insulin until the brain senses that blood sugar levels are safe. But by the time the brain stops insulin production, often too much sugar is removed. Low blood sugar can make you feel tired, dizzy, nauseous, even depressed — a condition often remedied by eating more sugar and more carbohydrates.
It can cause heart burn and acid reflux.
You're especially at risk if you eat too much before bed. The sphincter muscle at the top of your stomach controls what goes up and down the esophagus. When it's weak (or you're laying down), acid can wash back through the esophagus during digestion which can cause anything from coughing to heartburn.
It can make you more susceptible to diabetes.
According to a study by Mt. Sinai School of Medicine, this is because overeating causes malfunction in brain insulin signaling.
In previous research Dr. Buettner's team established that brain insulin is what suppresses lipolysis, a process during which triglycerides in fat tissue are broken down and fatty acids are released. When lipolysis is unrestrained, fatty acid levels are elevated, which can initiate and worsen obesity and type 2 diabetes.
BONUS: Fructose may promote overeating.
Fructose (as in 'high fructose corn syrup') found in many sweeteners can spur overeating, according to the findings of a Yale study released last month.