The Sugar Made Me Do It

Neuroscientifically Challenged had a great post awhile back, Every Sweet Hath Its Sour, reporting on research that basically equates modern, processed food with drugs.

Why? As the Duke Health news release tells us, “Researchers at Duke University Medical Center have discovered that the brain can respond to the calorie content of food, even in the absence of taste.” An even better title summarizing this research is “Tasteless Food Reward.”

This March 2008 Neuron paper “Food Reward in the Absence of Taste Receptor Signaling” by Ivan de Araujo, Albino Oliveira-Maia and colleagues shows that high-calorie food can directly reinforce the mesolimbic dopamine system. This result overturns that common assumption that what we eat relies on conditioned preference, pairing taste with the ingestion of a particular substance, say, cops and their donuts. This assumption has been used to great effect in evolutionary medicine research—we evolved in a fat-, sugar- and salt-limited environment, and today our evolved tastes drive our excessive consumptions of fast food in the modern world.

Now the modern situation appears even more dire, for calories alone can also reinforce food consumption, at least in mice “which lack the cellular machinery required for sweet taste transduction.” The Tasteless Food Reward editorial by Zane Andrews and Tamas Horvath tells us that “de Araujo et al. show that mice lacking functional ‘sweet’ taste receptors (trpm5−/−) develop a preference for sucrose by activating the mesolimbic dopamine-accumbal pathway, solely based on calorie load.”

The press release gives us a more understandable version: “The authors also demonstrated that neurons in the same area of the brain had significantly higher responses when taste-impaired mice were consuming sucrose in comparison to sucralose. ‘This suggests that the calorie-dependent release of dopamine in the ventral striatum has an impact on the response properties of populations of neurons in the same area,’ said Miguel Nicolelis, professor in the Duke Departments of Neurobiology and Biomedical Engineering and the Center for Neuroengineering. ‘Thus, the brain dopamine pathways might also perform an action we had not previously identified, by detecting gastrointestinal and metabolic signals’.”

Why does this matter? As Andrews and Horvath tell us, research has already established that “Food palatability and hedonic value are critical to the overall regulation of food intake and significantly contribute to obesity by overriding long-term homeostatic control in today’s highly palatable, energy-rich food environment. Highly palatable foods increase dopamine concentrations in the NAc (Hernandez and Hoebel, 1988), and the hedonic value of sucrose can be attenuated by dopamine antagonists (Bailey et al., 1986).”

Now we can also add the phrase, “The sugar made me do it.” As Andrews and Horvath say, “brain dopamine reward circuits can be controlled by calorie load, independent of food palatability, hedonic value, or functional taste transduction.”

I’ll include the end to their editorial, as it summarizes well the implications of this research.

Like most important and interesting papers, the results presented here raise many more intriguing future questions. Obvious mechanistic questions need to be addressed, such as how is caloric load sensed by the dopamine reward system? Is nutritional information on caloric load conveyed via vagal afferents through the brainstem to regulate VTA dopamine neuronal function, and does caloric load affect satiety signals from the digestive tract? Additionally, do certain types of sugars affect the reward system differentially (i.e., fructose), and does the same phenomenon occur when calories come from different types of food (for example, do calories from lipids produce a stronger effect)? Finally, can caloric load also affect other cognitive functions, such as learning and memory?

All of these questions are extremely important to understanding the pathogenesis and sociology of human obesity. For example, high-fructose corn syrup is a ubiquitous sweetener in American society, and evidence suggests that fructose is not as effective as sucrose in terminating a meal. 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. Regardless, the present study alone will further galvanize the scientific community to understand how higher cognitive centers in the brain control food intake and body weight regulation. It also effectively adds to the growing body of information showing that metabolic cues are not solely the domain of the hypothalamus and that much more crosstalk occurs between metabolic cues and higher brain centers than previously believed (Figure 1). Thus, categorizing food intake as hedonic versus homeostatic may not only be redundant, but also misleading.

In earlier posts on obesity in relation to human biology and to culture & inequality, I made a similar point about how categorization limits our thinking about food intake and weight gain (see posts that also covered genetics and successful weight loss). But in many ways, this de Araujo et al. research is even more significant.

I wrote yesterday about Michael Pollan and his anthropologically-inspired approach to food and eating. In his editorials that I linked to, Pollan shows how profound the support through both agribusiness and political price supports is for the use of sugar in all types of food processing and food consumption.

With taste, it’s too easy to blame weak-willed people for eating too much “delicious” processed food. But for anyone who’s found themselves scarfing down food, that tasteless food that looks nothing like what’s featured in the pretty ads, you might look for a different explanation. We’ve created foods that carry a double-whammy, through both taste and calorie reinforcement. But it’s not “we’ve created.” Industry, supported by politics, has done that.

Sidney Mintz in his classic book Sweetness and Power: The Place of Sugar in Modern History shows how important sugar, by driving production, trade and consumption, was to the globalization that now dates back several centuries. Mintz focuses on how class identity, the meaning of sugar, and its increasing use in all sorts of food production played a fundamental role in the rise of the modern sugar industry.

Now we can add another dimension to Mintz’s argument—consumption is not just about taste, whether subjective or cultural. Building calories into our food products helps promote their consumption, even as a wealth of associations from advertising, childhood memories, and flavor-saturated food creates a food “high” that makes that hamburger or that Twinkie seem more delicious than it really is.

Calories matter, in more ways than we imagined. Does the food industry push calories just like the tobacco industry pushes nicotine? I’ll let you know after I scarf down my 1420 calorie Hardee’s Monster Thickburger, a “monument to decadence,” followed by a light 730 calorie dessert from Cinnabon. I’ve gotta live the moment.

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