Food Addiction and Dopamine

Why your brain likes sweets.

The brain's ability to sniff out calories in the form of sugar depends upon sugar's drug-like effect on the dopamine-rich reward center known as the nucleus accumbens, according to a study published in the March 27 issue of Neuron. This tiny structure in the mid-brain is also the locus of reward activity for all addictive drugs.

In the study, Ivan de Araujo and colleagues at Duke University and the Universidade do Porto in Portugal demonstrated that lab mice lacking the ability to taste sweet foods still preferred sugary water to regular water. The genetically altered mice, lacking functional taste receptor cells for bitter and sweet, consistently chose to consume sugar water--even though they could not sense the sugar. (The lab animals were also prevented from smelling or sensing textural differences in the offerings.)

"Our findings suggest that calorie-rich nutrients can directly influence brain reward circuits that control food intake independently of palatability or functional taste transduction," the researchers wrote.

The findings offer new clues to obesity, and also bolster the contention that simple carbohydrate foods--because of their effect on reward pathways in the brain--can be addictive for certain people. As Tamas Horvath of Yale University's School of Medicine told Science News (sub. required): "This is a very exciting new element in how you get addicted to food. It doesn't even matter how it tastes."

In the same article, written by Amy Maxmen, study author de Araujo said: "The animal's reward processing systems were sensitive to changes in metabolism, not just flavor. This is a new system."

The "sweet-blind" animals did not go for the low-cal alternative, when they were offered water mixed with sucralose, otherwise known as Splenda. Low-cal sweeteners did not result in a similar dopamine boost along the reward pathways of the brain.

The brain's ability to "sense" calories may help explain why diet foods are often ignored in favor of sweets. As Ewan Callaway of New Scientist put it, "Anyone who has devoured a tub of ice cream in one sitting knows that delicious foods can override our body's pleas of 'enough.'" We have increased levels of dopamine in the nucleus accumbens to thank for that.

As De Araujo explained to New Scientist, "even when you do not stimulate the sensory pathways in the mouth you still have this reward signal in the brain."

In a preview of the article in the same issue of Neuron, Zane Andrews and Tamas Horvath speculate that "high-fructose corn syrup is an ubiquitous sweetener in American society.... It may be that fructose produces stronger activation of the reward system and that removing high-fructose corn syrup as a sweetener will curb some desire for these products."

Photo Credit: Biz/Ed

dopamine addiction

Food Addiction: Snacking on Serotonin

The neurology of carbohydrate craving.

Eighteen years ago, Richard and Judith Wurtman, a husband and wife research team at the Massachusetts Institute of Technology (MIT) reported in Scientific American:

"We wondered whether the consumption of excessive amounts of snack carbohydrates leading to severe obesity might not represent a kind of substance abuse, in which the decision to consume carbohydrates for their calming and anti-depressant effects is carried to an extreme--at substantial cost to the abuser’s health and appearance."

In the case of certain carbohydrate cravers, the Wurtmans discovered, dietary tryptophan was being converted into serotonin, like always—but this concentrated serotonin surge acted like a powerful mood-booster. It acted like medicine.

The Wurtmans had hit on something important. People who tended to binge late in the day on carbohydrate foods, particularly simple sugars, got a drug-like “buzz” that was highly reinforcing. In the experiments, these people quite specifically, if unconsciously, selected the kinds of foods richest in serotonin-building compounds.

The serotonin-boosting effects of carbohydrates may explain why addicts in recovery, as well as carbohydrate cravers and PMS sufferers, show a tendency to binge on sugar foods. Abstaining addicts apparently turned to the overconsumption of carbohydrates as a means of attempting to redress the neurotransmitter imbalances at the heart of their disorder. Perhaps some addicts discover early in life that carbohydrate-rich foods are their drug of choice.

None of this should be taken to mean that large doses of supplemental tryptophan constitutes some sort of easy remedy for serotonin-mediated disorders. It isn’t that simple. However, many drug treatment experts are convinced that dietary alterations, vitamin therapy, and nutritional supplements--as well as strenuous exercise, which also has a marked effect on neurotransmission—play vital roles in addiction treatment programs.

Dopamine is involved with eating behaviors, too. A Princeton University animal study compared dopamine levels while rats were experiencing stimulant drugs, and while they were eating preferred foods. The researchers found that “amphetamine and cocaine increase dopamine in a behavior reinforcement system which is normally activated by eating. Conversely, the release of dopamine by eating could be a factor in addiction to food.”

The idea of a link between addiction and appetite control is not new, and the controversy over the addictive properties of sugar foods is an old one. Heroin addicts, alcoholics, and cigarette smokers, when deprived of their drug of choice, will sometimes binge on sugar foods in a pattern highly suggestive of cross-addiction or substitute addiction. Old-time alcoholics tell stories of pouring bottles of pancake syrup down the gullets of colleagues in dire need of sobering up. (Fructose does indeed speed the elimination of alcohol.) The practice of referring to drug cravings as “drug hunger” may be closer to the mark than we realize. Intense physical hunger may be as close as any non-addict ever comes to experiencing the mind/body sensations of drug craving.

Many addicts with alcoholic relatives report that they have experienced substitute addictions and multiple addictions repeatedly—and sometimes, these substitutions and additions center on food.

As usual, some of the best hard evidence comes from rats. In a study by Dr. Neil Grunberg at the Uniformed Services University of the Health Sciences in Bethesda, Maryland, rats were regularly injected with nicotine over long periods. When the injections were suddenly withdrawn, the rats chose sweetened food over regular food--a complete reversal of the food preference they had previously shown.

Not all overeaters are abstaining drug addicts, of course. Obesity, like drug addiction, comes in a variety of forms, and is influenced both genetically and environmentally. But the spotlight is now on a subset of obese people in which obesity does not seem to be a behavior learned from obese parents, any more than alcoholism is inevitably a learned behavior picked up from alcoholic parents.