That teenager in your kitchen feasting on fast food, candy bars and pop might not be able to help themselves – all the more reason for adults to help them before they cause long-term damage to their developing brains.
In a new study, Western researchers Cassandra Lowe, J. Bruce Morton and Amy Reichelt highlighted adolescence as a period of “dual susceptibility.”
While teen brains are still developing decision-making capabilities, their limited restraint and heightened reward system make them more prone to eating poorly, which in turn may lead to negative changes in the brain.
These findings, according to researchers, show the importance of changing behaviors and helping adolescents form healthy habits early on to minimize these changes.
The study, Adolescent obesity and dietary decision making – a brain-health perspective, was published today in The Lancet Child and Adolescent Health.
“Adolescents are more prone to eating calorie-dense, high-sugar foods because they lack the control to regulate it,” said Lowe, a BrainsCAN postdoctoral scholar.
“Their brain is still maturing so they’re more sensitive to the rewarding properties of these foods. But, at the same time, they lack the control mechanisms to prevent themselves from eating junk foods.”
During adolescence, the prefrontal cortex – involved in self-regulation, decision-making and reward-seeking – is developing, making it difficult for teenagers to resist unhealthy foods.
Until this area of the brain matures, adolescents are more likely to take part in impulsive and reward-seeking activities.
“The prefrontal cortex is the last area of the brain to develop. It’s the part of the brain that is critical for behavioral regulation; it’s the manager of the brain,” said Reichelt, a BrainsCAN postdoctoral scholar.
“The teenage brain has a triple-vulnerability – a heightened drive for rewards, reduced self-regulation abilities and susceptibility to be changed by environmental factors – including junk foods.”
Over time, the excessive consumption of calorie-dense foods can lead to changes in the structure and function of the prefrontal cortex, including altering dopamine signaling and inhibition.
The neurotransmitter dopamine is released when the brain’s reward system is activated. It can be activated by natural rewards, such as social interaction, as well as eating calorie-dense foods.
If a behavior is rewarding, dopamine makes us want to carry out that behavior again,” Reichelt added.
“Adolescents have increased numbers of dopamine receptors in the brain, so when they do experience something rewarding, that experience of reward and how the brain processes it is heightened compared to that of an adult.”
As adolescents overstimulate their reward systems, these unhealthy diets can result in poor cognitive control and heightened impulsivity as they move into adulthood.
This demonstrates the importance of changing behaviors and helping adolescents form healthy habits early on to minimize changes to the brain.
“One avenue we really need to look into is the use of exercise as a way of regulating changes in the brain that can help us make better dietary choices,” Lowe said.
“There’s evidence that exercise can help improve the brain in terms of cognitive control, but also reduce reward sensitivity to things like food items.”
“Teenagers don’t want to be told what to do – they want to be able to make their own informed choices,” Reichelt said.
“If you provide them with easily understandable information about how their diet is affecting their brain, while providing them with other alternative behaviors, that’s going to help them in maintaining healthy lifestyle practices long term.”
Fast food consumption has increased significantly worldwide. Fast food typically refers to food that is quickly prepared, rich in saturated fat, purchased from restaurants using precooked ingredients, and served in a packaged form .
The rise in obesity rates in American adults (68.8%) currently classified as overweight or obese  link to increased intake of sugar-containing beverages . The prevalence of DM2 in the Kingdom of Saudi Arabia is also increasing; among 6024 participants, diabetes mellitus was present in 1792 (30%) patients .
It is worth mentioning that fast food consumptions is increasing among Saudis in different age groups for both genders , as expected (Collison et al., 2010) found that the average fast food intake was 4.47 meal/week in different age groups, although girls consumed more fast food than boys .
Fast food has many unpleasant health consequences. It negatively affects brain health by damaging regions relevant to memory tasks and by diminishing brain-derived neurotrophic factor levels .
This amplifies the risk of developing dementia and Alzheimer’s disease later in life . A high intake of Western food, characterized by high levels of saturated fat, was associated with increased serum total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C), with an 8% increase in the likelihood of having sustained high LDL-C .
In combination with a sedentary lifestyle, an increased prevalence has been noted of chronic non-communicable diseases, such as diabetes, heart disease, and cancer, which are estimated to account for 78% of all deaths (WHO, 2014).
Thus, this diet is detrimental to the health and will aggravate existing lifestyle diseases [26, 28]. The most common risk factor for developing coronary heart disease in Saudi patients is the consumption of a high-fat diet which contains high levels of LDL-C [29, 30].
Several cross-sectional studies have found significant associations between poor nutritional status and behavioral disturbances [31, 32, 33, 34, 35], worse cognitive status , and more impaired functioning in adult daily living activities [36, 37, 38, 39].
While these studies have demonstrated an association between nutritional status and aged population, very limited have examined the association of cognitive function with food addiction in young population and the majority of the studies have been conducted in clinical samples.
Considering the lack of studies conducted in Saudi Arabia or in the Middle East regarding fast food consumption and its effects on cognitive function, we hypothesized that increased consumption of fast food would impair cognitive function, metabolic functions, and quality of life in young adults.
The result showed significant effect of fast food on metabolic function but not cognitive function in healthy population.
One study from Ye et al  showed cognitive impairment (mostly memory function but other domains like attention and executive function were affected as well) in middle age group taking habitual sugar intake which included fruit drinks as well as soft drinks.
In that study cognitive function assessed by the MMSE significantly correlated with sugars components like sucrose, glucose, and added fructose. It showed that an increase in the consumption of added sugar was significantly associated with lower MMSE scores.
This differs from our study, in which fast food consumption was not associated with a number of specific cognitive domains, including attention, memory, working memory and executive function (Table 2).
Our study might be different in terms of age group, fast food consumption and cognitive assessment measured tools [2, 3, 30]. The mechanism by which cognitive function is affected by diet is still not fully understood (Molteni et al., 2002),  found that HFS diet (high fat sucrose diet) was negatively associated with a decrease in hippocampal BDNF mRNA and protein, animals with higher BDNF had a better cognitive performance.
Animals that were on HFS diet for a longer time exhibited lower levels of BDNF, which emphasizes on the importance of the duration of fast food consumption.
The longer the duration of fast food consumption the lower the BDNF levels, although they found that 2 months on HFS diet were sufficient enough to reduce hippocampal levels of BDNF and spatial learning performance.
Our result showed metabolic difference in two groups which are in line of Raben et al.  study used to compare two group consume fast food for 10 weeks and increase blood pressure.
Fast food consumption on regular basis are major player for cardio-metabolic disease, including obesity, DM2, metabolic syndrome, and cardiovascular disease [5, 7, 8, 11] and all have negative impact on cognition [14, 15], .
Previous study lasting 6 months showed metabolic changes (visceral, liver, and muscle fat, TAG, TC, and systolic blood pressure) .
In KSA, over the past two decades, rapid economic development, global trade, and cultural exchange have meant that the FF industry and young population’s FFC have been increasing rapidly. In this study, we defined Western FF as food sold in these fast food chains, e.g. KFC, McDonald’s and Pizza Hut.
This would make our estimate more conservative. So, we could not obtain information on the quantity of FFC, total daily energy intake, and FFC’s contribution to total daily energy intake among the children. It may affect the assessment of the relationship between FFC and health outcomes.
This study has limitations which must be acknowledged. Our data is cross-sectional, and the dietary questionnaire used has a number of limitations. The sizes of fast food were not specified. Self-reported nutritional intake can lead to underestimation or overestimation of true associations, and measurement at only one point may not reflect long-term consumption patterns.
Therefore, more studies, especially longitudinal studies based on large national representative samples with exact measures of quantity of fast food consume intake and its contribution to total daily energy intake, are needed to detect the association between fast food consume and health outcomes.
This is the first local cross-sectional study to examine fast food consumption and cognitive performance using a standardized battery of cognitive tests. It is very important in future to do longitudinal studies large, well-controlled, long-term interventional trials are needed locally.