Why are some people “carboholics”?


By Stephan J. Guyenet, PhD.  Adapted from stephanguyenet.com.  If you enjoy this article and live in the San Francisco area, come see my talk at the Commonwealth Club on July 18th: “Seduction and Satiety: Under the hood of the brain circuits that drive us to overeat”.

Most of us have had the experience of staring at a delectable dessert like pecan pie or chocolate ice cream, telling ourselves we shouldn’t eat it, and then eating it anyway.   And for some of us, once we’ve partaken in a food we generally avoid, we may continue to crave that food for days afterward.

The nutritional wisdom du jour tells us we’re addicted to sugar, perhaps as a result of some ill-defined effect on our metabolism.  The real explanation for food cravings, however, is much more interesting.  To understand how food cravings work, let’s start with a related, but simpler phenomenon—drug addiction.

An analogy from drug addiction

To understand why we crave food, sex, gambling, winning, catching fish, driving fast cars, playing video games, and everything else we crave in our lives, it’s helpful to start with drugs of abuse.  This is because the way in which drugs create cravings is very simple, and we can consider it without burdening ourselves with all of our preconceived notions and feelings about food.

Habit-forming drugs, including drugs of abuse, are habit-forming because they stimulate the dopamine signaling pathway in the brain.  This pathway determines your gut level of motivation for things– what we often call a craving.  It evolved to motivate us to get food, water, sex, shelter, social status, and other things that were important for the survival and reproduction of our distant ancestors, but drugs of abuse artificially tap into it.  So when a drug increases the level of dopamine in key structures of your brain, you will become more motivated to take the drug– you will crave it.  This is called reinforcement.

Some drugs, like amphetamine and injected or smoked (crack) cocaine, are very good at increasing dopamine levels in the brain.  This means they’re also very good at increasing your motivation to take them.  If you take a drug repeatedly and your level of motivation/craving crosses a certain threshold, your brain begins to implicitly prioritize drug-seeking behaviors over constructive behaviors like keeping a job and obeying the law.  We call that addiction.

When a person with an addiction doesn’t use a drug for a long time, his craving for it will slowly subside.  This is why a person who quits smoking will initially feel very strong cravings, but these will diminish to a low level over a period of months to years.  Yet when that person exposes himself to cigarettes again, or cues associated with it– for example, by taking a puff of a cigarette– it often re-awakens those latent brain pathways and the addiction comes roaring back.  This can leave a person struggling with strong cravings once again.  Note that this abstinence -> cue -> relapse cycle is precisely what I described at the beginning of this article, only for desserts instead of cigarettes.  How does it work for food?

From drugs to food

Like drugs of abuse, food is reinforcing.  In fact, the brain systems that drugs tap into were “built” to drive us toward food, not drugs.  We actually have a pretty good idea of how food reinforcement works, thanks in large part to the research of Anthony Sclafani, which I describe in more detail in my book The Hungry Brain.  As with drugs, food reinforcement works by spiking dopamine in the brain.  But how does this happen?

Research by Sclafani and others shows that food reinforcement primarily occurs when the upper small intestine detects carbohydrate, fat, and protein, causing a spike in brain dopamine levels via a pathway called the vagus nerve.  This is our first clue that sugar might not be the only problem.  In fact, fat is also highly reinforcing.

In a nutshell, the brain is wired to be motivated by the food properties that kept our distant ancestors alive and fertile.  When you eat food, your brain analyzes its composition via receptors in your mouth, upper small intestine, and perhaps liver that measure the amount of fat, sugar, starch, protein, and salt it contains.  This all happens on a nonconscious level.  If the brain “likes” what you just ate, it will release dopamine.  The more fat, sugar, starch, protein, and salt your food contains (up to the “bliss point”), the more dopamine your brain releases.  The more dopamine your brain releases, the more your cravings increase.  This is why we crave cookies and bacon but not lentils and plain celery.

Foods that are skillful combinations of fat, sugar, starch, salt, and delightful flavors likely cause your brain to release high levels of dopamine, sometimes provoking addiction in susceptible people.  And when a person excludes a problem food for a long time, then suddenly has it in front of them at the dinner table, those latent craving pathways are reactivated– just as they are for drugs.  And as it turns out, this can happen even when a food doesn’t contain sugar at all.

What about fatoholics?

The conventional wisdom about sugar breaks down further when we consider that many of the most common food cravings aren’t for sweet foods.  Rob Markus and colleagues recently published a paper in which they identified the food types that are most commonly associated with addiction-like behavior, basically a strong craving.  Here’s what they found:

“The majority of respondents experienced these problems for combined high-fat savoury (30%) and high-fat sweet (25%) foods, whereas only a minority experienced such problems for low-fat/savoury (2%) and mainly sugar-containing foods (5%). Overweight correlated only with addictive-like problems for high-fat savoury and high-fat sweet foods, while this was not found for foods mainly containing sugar.”

Wait, what?  The most common foods that trigger addiction-like behaviors are “high-fat savory” foods that aren’t even sweet?  And only one person out of twenty reported addiction-like behavior for “mainly sugar-containing” foods?  It appears that “fatoholics” are a thing too.

This lines up with my personal experience, and I suspect, the experience of most readers.  I don’t know about you, but to me, hard candy just isn’t that appealing.  I have no desire to eat fat-free frozen yogurt.  Candy corn and Tootsie Rolls are barely better than starvation.  Throw some fat into the mix, as in chocolate, ice cream, or brownies, and now you’re talking!  Savory foods like potato chips, bacon, pizza, and ribs are also high on my list.  Given that sugar and other types of carbohydrate are the current nutritional bogeyman, we don’t often stop to think that the fat content of the foods we eat also contributes to our hard-to-control cravings.  But sugar is certainly not off the hook.

One thing that’s clear from reading Markus’s study is that different people find different foods “addictive”.  Most people have a hard time controlling their eating behavior around certain high-fat savory and/or high-fat sweet foods.  Yet a small minority have a hard time with low-fat savory foods like pasta or low-fat sweet foods like hard candy.

So why are some people carboholics and fatoholics?

The reason is simple.  The human brain is hard-wired to be motivated by calorie-dense foods rich in fat, sugar, starch, protein, and salt, because those things kept our distant ancestors alive and fertile in a difficult world.  In the modern world, the food industry and home cooks are better at pushing our brain’s reinforcement buttons than ever before in history.  Our distant ancestors didn’t have ice cream or pizza, and the brains we inherited from them may not be designed to constructively handle that kind of powerful reinforcement.  We’re so good at pushing our own buttons that some of our modern foods probably provoke very high levels of dopamine release in the brain.  Like drugs of abuse, those high levels of dopamine make us crave in ways that don’t always support our health– and sometimes lead to addiction.  That’s why we crave dessert.

Due to individual differences in how we’re wired, some of us become “carboholics”, others become “fatoholics”, others become both, and still others become neither.  We have much work to do before understanding these individual differences.  Yet regardless of whether or not we have addiction-like eating behaviors, nearly all of us eat too much when we encounter foods that are highly reinforcing.

Hello—I’m Stephan J. Guyenet.  After earning a BS in biochemistry at the University of Virginia, I completed a PhD in neuroscience at the University of Washington, then went on to study the neuroscience of obesity and eating behavior as a postdoctoral fellow.  I’ve spent a total of 12 years in the neuroscience research world studying neurodegenerative disease and the neuroscience of body fatness.  My publications in scientific journals have been cited more than 2,100 times by my peers.

Today, I continue my mission to advance science and public health as a researcher, science consultant, and science communicator.  My book, The Hungry Brain, was released in February of 2017 and was named one of the best books of the year by Publishers Weekly and called “essential” by the New York Times Book Review.  I am a Senior Fellow at GiveWell and scientific reviewer for the Examine.com Research Digest.  Past clients include the Open Philanthropy Project.  I am the primary designer of an innovative course-based body weight management program called the Ideal Weight Program, which is part of the HumanOS platform.  I periodically contribute to the scientific literature and I am a review editor at Frontiers in Nutrition.

I live in the Seattle area, where I grow much of my own food, use my bicycle to get around, and brew a mean hard cider.

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