Why sugar is so much worse for teenagers' brains

The rate of obesity is increasing worldwide and the increase has been particularly dramatic in young people. Young people are the greatest consumers of high-energy, sugary and fat-laden “junk” foods and sweetened drinks.

The heightened metabolism and rapid growth during puberty can protect against obesity. However, easy access to cheap junk foods and increasingly sedentary lifestyles outweighs the protection from growth spurts.

Diets high in refined sugar and saturated fat not only contribute to weight gain and associated health issues, but also have a profoundly detrimental impact on brain function.

It is known excessive consumption of junk foods damage areas of the brain essential for learning and memory processes. Neurons in brain regions, including the hippocampus, that encodes memories, no longer work efficiently, leading to poorer learning.

This is of great concern as adolescence is a critical formative period for learning about the world. Adolescence is also a time of newly found independence, including food choices.

Recent research in rodents has shown the adolescent brain is at an increased risk of developing diet-induced cognitive dysfunction. Adolescent, but not adult, mice develop memory problems after consuming high-fat diets.

Teenage rats that drank sugary beverages were less able to remember a specific location leading to an escape hatch. This was compared to adult rats drinking sugary beverages, and teenage rats that had low-sugar diets.

The brains of the adolescent sugar-diet rats also showed increased levels of inflammation in the hippocampus, disrupting learning and memory function. Inflammation in the brain can contribute to cognitive decline and dementia.

The negative effects of obesity on the brain have been observed in young people too. Obese adolescents performed worse at maths, spelling and mental flexibility than healthy-weight adolescents. Structural brain scans revealed that obese teenagers had smaller hippocampi. This provides evidence that excessive body fat impacts the brain’s learning centre.

Teenage brains are a work in progress

The teenage brain undergoes major developmental changes in terms of structure and function. Adolescence is a period of increased neuroplasticity due to the dramatic changes in connectivity within brain regions.

Brain-imaging studies show that the prefrontal cortex doesn’t fully mature until the early 20s. A major role of the prefrontal cortex is performing executive functions. This term encapsulates behavioural control, attention and decision-making.

Poor regulation of the prefrontal cortex during adolescence can explain the increased risk taking behaviours in teenagers, including dangerous driving, drug use and binge drinking.

Educational efforts to provide teens with information about unsafe behaviours tends to fall on deaf ears. The prefrontal cortex helps us to resist performing behaviours triggered by events in the environment. Resisting these behaviours in the face of immediate reward can be difficult, particularly for teenagers.

Teenage brains love rewards

The risky behaviours teenagers engage in are often immediately rewarding. The brain’s reward system releases the neurotransmitter dopamine when stimulated by pleasurable events, increasing the drive to carry out these activities.

Teenagers are particularly drawn to rewards, including eating tasty foods high in fat and sugar. The adolescent reward system is sensitive to stimulation and may be permanently altered by overactivation during this period.

Combined with the reduced ability to resist rewarding behaviours, it is not surprising that teenagers prefer to eat foods that are easy to obtain and immediately gratifying, even in the face of health advice to the contrary.

Changes in the brain caused by overconsumption of sugary foods during adolescence can manifest in later life as difficulties in experiencing reward. Research has shown male rats that drank sugar water during adolescence showed reduced motivation and enjoyment of rewards when they were adults.

These behaviours are core features of mood disorders including depression. Importantly, this shows that how we eat during adolescence can impact brain function as adults, leading to long-lasting changes in food preference and learning about rewards.

Teenage brains are more plastic

Excessive consumption of junk foods during adolescence could derail normal brain maturation processes. This may alter normal development trajectories, leading to enduring behavioural predispositions – in this case, the habit of consuming fatty and sugar foods, leading to obesity.

Fortunately, the increased plasticity of the adolescent brain means that young people may be more responsive to change. Opportunities to identify and intervene in high-risk youths may avert destructive negative behavioural spirals that may originate in adolescence. This can encourage life-long healthy habits.

Amy Reichelt receives funding from the Australian Research Council.