Corn oil, but not cocaine, is a more effective reinforcer in obese than in lean Zucker rats

There is little argument that obesity is driven at least partly by the over-consumption of energy-dense, highly palatable foods. But we are hardwired to seek pleasure, and not everyone who overeats becomes obese. It has thus been suggested that persons predisposed to becoming obese demonstrate a disruption in both the neurobiological processes that underlie sensitivity to reward and those that underlie inhibitory control, which in turn leads to compulsive food intake and weight gain. In support of this, it has been shown that MRI-assessed brain activity of our “reward” region to high-calorie foods increases significantly with increasing BMI, while the “control” region shows less activity.

Outside of brain activity and subconscious drives, research also shows that obese people subjectively enjoy high-fat and high-sugar foods more than normal weight individuals. Interestingly, there is no genetic component because research of monozygotic (genetically identical) twins that are discordant for weight status shows the fatter twin to prefer fatty foods three times more frequently than the lean co-twin. However, although the twin study provides strong evidence that food preferences lead to weight outcomes, little is known about how obesity influences food choices.

By studying the reinforcing effects of sugar and fat under discrete conditions, a more precise assessment of the relative contributions of these macronutrients on the reinforcing effects of food is possible. As such, researchers from The University of Mississippi Medical Center publishing in Physiology & Behavior recruited the help of the Zucker rat, a genetically obese rat in which a mutation of the leptin receptor renders leptin useless, resulting in hyperphagia and excessive weight gain. These rats were compared against lean controls in their drive for corn oil and cocaine.

Yes, cocaine. There exists an interesting drug-food hypothesis that suggests a hyposensitive dopamine system, such as that seen with drug tolerance, to be responsible for the increased drive for highly-palatable and rewarding foods. However, in the present study there was no difference between the lean and obese rats in their drive for cocaine. Conversely, the obese rats “wanted” significantly more corn oil than the lean rats, suggesting that obesity may be specifically maintained by a heightened reinforcing effect of fat.

And this is certainly supported by other research that uses humans and dairy fat, which circumvents the “corn oil in rats” argument that may come up. For example, an early experiment of normal weight and obese women had the subjects consume various milk beverages with differing amounts of sugar and fat in order to discern how weight influences the taste response. Ultimately, it was found that while normal weight women preferred the beverages that were composed of 20.7% fat and 7.7% sugar, the obese women preferred the beverages that were composed of 34.4% fat and only 4.4% sugar. In fact, the obese women hated sugary, fat-free beverages.

Bottom line

The present study provides support to the notion that obesity drives a preference for fat, which may lead to increased caloric intake and maintenance of obesity, which continues to drive the want for fat in a viscous cycle of subconscious rewards and drives. How this cycle can be broken has yet to be determined, and while I’m sure there are those who disagree, self-control will likely play no small part. Manipulating the environment to reduce temptation and trick the brain in beneficial ways can help.

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