Sugar, does it make your brain hit the gas or the brake?
As I remember from my experiences in primary and secondary school, we were only allowed to take sweets into the class room during tests. Because, as teachers argued, “sugar gives you the energy your brain needs to stay focused on the test”. Really?
How does sugar work in our brain? Research suggest that sugar consumption leads to cognitive dysfunction, especially spatial learning and memory. The dopaminergic pathway is projected in brain regions important for cognitive processes. So, can we conclude that the effects of sugar on cognition is mediated by sensitivity of the dopamine receptors? Several studies have shown association between sugar intake and altered cognitive functioning in both rodents and humans (Kendig, 2014). For example, spatial memory was poorer in rats who were sugar-fed compared to honey-fed and sugar-free control rats (Chepulis, Starkey, Waas and Molan, 2009). Indeed, dopamine receptor signaling was altered in sugar-dependent rats and as a result, they showed flatter dopamine responses (Kenny, 2011). When it comes to humans, Francis and Stevenson (2011) found students who reported high intake of refined sugars to perform poorer on hippocampus-dependent memory which is required for spatial learning. As the hippocampus is a structure in the dopamine circuit, the findings in humans may also suggest a neuromodulatory role of dopamine in the relationship between the effects of sugar and hippocampus-dependent cognitive functions.
Besides memory and learning, the reward system, an important construct in cognition, is not resistant to sugar either. Again, dopamine comes into play as it is one of the most important neuromodulators for the reward system (Schultz, 1998). The dopamine system is assumed to be desensitized and down regulated as a result of chronic high sugar intake. As shown by Cottone and colleagues (2008) adolescent sucrose-fed mice developed a tendency to consume more sugar later in life even though there was no concurrent effect of sugar (i.e. weight gain and reduced preference for sugar). Moreover, the shift from goal-directed (hippocampus-dependent system) to habitual learning (striatum-dependent) is much faster in rats who were trained to press lever for pellet after 28 days of limited access to sugar. This line of research implies that frequent exposure to sweets during adolescence changes the way in which our reward system works in adulthood. The mechanism of this change can be explained by alterations in dopamine transmission in reward related brain regions such as nucleus accumbens and striatum. Due to a blunted dopamine response after exposure to sweets/sugar the organism will consume more sugar to achieve the same level of incentive motivation to respond.
Exploring this mediating mechanism of dopamine between sugar and cognition, we can gain more insight in psychopathology of eating disorders and addiction which are likely to be triggered in adolescence and persist into adulthood. When you give your child sweets as a reward next time, think twice!
Chepulis, L.M., Starkey, N.J., Waas, R., & Molan, P.C. (2009). The effects of long-term honey, sucrose or sugar-free diets on memory and anxiety in rats. Physiology and Behavior, 97, 359- 368.
Cottone, P., Sabino, V., Seardo, L., & Zorrilla, E.P. (2008). Intermittent access to preferred chow reduces the reinforcing efficacy of chow in rats. American Journal of Physiology – Regulatory Integrative and Comparative Physiology, 295, R1066 – R1076.
Francis, H.M., & Stevenson, R.J. (2011). Higher reported saturated fat and refined sugar intake is associated with reduced hippocampal-dependent memory and sensitivity to interoceptive signals. Behavioral Neuroscience, 125, 943-955.
Kendig, M.D. (2014). Cognitive and behavioural effects of sugar consumption in rodents. A review. Appetite, 80, 41-54.
Kenny, P. J. (2011). Reward mechanisms in obesity: new insights and future directions. Neuron, 69(4), 664-679.
Schultz, W. (1998). Predictive reward signal of dopamine neurons. Journal of Neurophysiology, 80, 1-27.