Why Do We Love Sugar?

There’s one thing we all look forward to on Halloween: candy. Every year, we come home with buckets full of sugar. But what about these treats makes them so addictive? The answer can be found in our brain’s mesolimbic dopamine system, our body’s built-in reward system.

When you take a bite of a piece of candy, your body boosts dopamine levels, which can be traced back to the ancient practice of hunter-gatherers developing high sugar diets to increase energy and their chances of survival. Most of these dopamine molecules are produced in the ventral tegmental area (VTA) located in the middle of the brain. The molecules travel along the mesolimbic dopamine pathway, which goes from the VTA up to the nucleus accumben located in the ventral striatum, a region associated with motivation and rewards. There, dopamine receptors receive the dopamine and produce a feeling of pleasure, leaving you to crave another bite. Your body’s excess of dopamine in response to sugar triggers the desire to repeat this behavior. However, doing so leads to the brain producing less dopamine, causing you to ingest more sugar to regain the initial feeling of pleasure.

Interestingly, our body also has a built-in craving-suppressing counterpart in the same way our body has a built-in reward system to induce sugar cravings. Your liver produces a powerful hormone called fibroblast growth factor 21 (FGF21) that has been linked to the suppression of sugar cravings. In a 2015 study, when the researchers increased plasma FGF21 levels in the mice and monkeys, both species willingly turned down sweets and alcoholic beverages. Though there are many pathways in the central nervous system that alter sweet and alcohol preference, FGF21 stands out for its powerful impact from just one dose, triggered right when we ingest sugar.

By experimenting with β-Klotho, the co-receptor that FGF21 works with to produce its effect on sugar, researchers have identified the specific mechanism of FGF21. In response to sugar intake, FGF21 is released into the bloodstream where it specifically targets the hypothalamus, an area of the brain responsible for energy maintenance and regulating macronutrient intake. FGF21 signals to the glutamatergic neurons, which produce the important neurotransmitter glutamate in the ventromedial hypothalamus, an area associated with satisfaction, to lower sugar intake by increasing their sensitivity to sugar. This lowers the threshold for sugar consumption to reach that “high” and greatly reduces sugar intake.

Another study by researchers at the University of Southwestern Medical Center takes a closer look at what happens in the brain during this phenomenon. When FGF21 reaches neurons in the central nervous system, dopamine levels drop in the nucleus accumbens. When researchers gave mice the choice of either sweetened water or plain water, they found that the hormone suppressed their desires for the sweetened water. They further showed FGF21’s ability to counter dopamine’s effect, this time on alcohol rather than sugar, by conducting a similar experiment with alcoholic beverages that the mice had already shown interest in. Here, the mice chose plain water after receiving a dose of FGF21, showing its ability to reduce sugar or alcohol preference and overturn already established addictions to substances like alcohol.

Not only does our brain have an ingrained system to promote sugar cravings, but it also has a simultaneous counterpart working to suppress them. However, there is still more work ahead in identifying the effects of FGF21 in areas beyond sugar consumption. Research has already begun on the role of FGF21 on diseases including Type 1 diabetes and obesity, which may uncover new treatments for addiction in general. This Halloween, when you contemplate whether to take the next bite of something sweet, think about all the simultaneous molecular processes swaying you to either side.