Last night, while stuck in an airport (the inevitable delay), I decided to get a Wendy’s milkshake. Not a particularly noteworthy decision – when traveling, I like to subsist entirely on fast food – but it occurred to me, while standing in line, that I wasn’t actually hungry. At all. (I’d just finished a greasy combo meal.) So why was I lining up to pay $4 for ice cream? Over at Mind Matters, we’re discussing a new paper that sheds some light on the issue. The problem turns out to involve the dopamine reward pathway (not so surprising), which responds not just to the delicious taste of a milkshake but to the caloric energy in the food. As a result, we still like to eat, even when the hypothalamus is sated:
Although the hypothalamus will direct intake based on the metabolic value of the food–when you’re very hungry, you seek out food with lots of calories–it remains to be determined whether the dopamine reward system can also sense a food’s energy content. In other words, does the dopamine system care about calories, or is it just concerned with taste and pleasure? Neuroscientist Ivan de Araujo and colleagues at Duke University, explored this question by using a line of mice genetically engineered to lack a functional receptor essential for detecting the taste of sweetness. In these mice, any change in reward behavior cannot be due to food palatability or the sensation of sweetness. If these mice prefer sweetness, thus, it is because sweeter foods have more calories, implying that there is something inherently rewarding about the consumption of calories.
In the first set of behavioral experiments, the authors showed that the genetically altered mice were completely insensitive to the “sweet” rewarding properties of sucrose (table sugar) and showed no preference for sucrose compared with water. In contrast, control mice without the genetic mutation strongly preferred the sucrose solution.
The scientists then exposed the different strains of mice to a “conditioning protocol” in which the rodents received alternating access to water or sucrose for six days. During these conditioning sessions, the genetically altered mice were able to associate the sweet solutions with caloric load post-ingestion, as the sugar water has more calories than plain water. Interestingly, both strains of mice now consumed significantly more sucrose. Although the genetically altered mice couldn’t taste the sweetness, they learned to prefer the sweeter water. This finding suggests that mice without functional sweet taste receptors were able to detect the reinforcing caloric properties of sucrose in the absence of sweet taste receptors. There seems to be something inherently pleasurable about ingesting food that contains calories.
As a critical control, the experiments were then repeated with sucralose (a.k.a. Splenda), an artificial sweetener that tastes sweet but contains no calories. Although normal mice consumed more sucralose than water during the conditioning period–they still preferred the sweet taste–the genetically altered mice did not.
Read the whole thing.