Fast food may be a prime culprit in teenage depression

By Dr. Liji Thomas, MDAug 30 2019

Why is depression such a growing problem among teenagers in America? One answer is the kind of food they eat, according to a new study by researchers at University of Alabama at Birmingham.

Teenage depression is more than 50% higher in 2017 reports compared to 2005. Moreover, a shocking 63% older teens and young adults show signs of depression, mental distress, and have thoughts of suicide. This has been attributed to poor sleep, overuse of social media – and now, unhealthy food.

Fast food and cola - Image Credit: Alones / Shutterstock

Many large studies have shown that diets majoring on fast foods, frozen meals and unhealthy snacks, which contain a great deal of salt and not much fruit or vegetables, is associated with almost an 50% higher incidence of depression over time in. Randomized trials have also found that eating healthy foods reduces the symptoms of depression in adults.

Now, a study shows that the greater the amount of urinary sodium (which reflects sodium intake), the more severe are the features of depression, for both men and women. The risk is inverse for potassium in urine. Avoiding salty foods and ensuring sufficient potassium intake could help avoid depression in teenagers.

While cross-sectional studies have shown that adolescents who eat poor-quality food have a higher rate of depression-linked features, they cannot prove a cause-effect relationship. Some other researchers have followed up people who eat unhealthy food over a longer period to examine the link with depression but the results are not clear. One key issue with all these is the use of self-reported food intake as the basis for calculating the amount of nutrients ingested. This is known to produce a biased measurement.  

The current study captured the association of sodium and potassium intake with mental health in teenagers, especially looking at African-American youth in cities, who have been recognized to be at greater risk for an unhealthy diet and for depression. Instead of the snapshot-style cross-sectional study design, this was a longitudinal study which tracked the levels of sodium and potassium in the urine over time. These were reflective of the overall intake of these salts, and provided an objective marker rather than the subjective estimate obtained by food frequency questionnaires and other self-reported dietary intake measures.

How was the study done?

The study looked at 84 teenagers, half of them male, mostly from poor families. The average age of the group was 13 years. Most of them had access to free or subsidized lunches. The study was straightforward in design, and included a first interview. Using a validated model, the researchers looked for signs of depression over the previous two weeks. After this, a scheduled baseline urine sodium and potassium measurement was performed one week later. The final step was a reassessment of depression based on self-reported symptoms after 1.5 years.

There was a low level of depression, which did not fluctuate over time. However, the sodium levels in urine were in direct proportion to the prevalence and severity of depressive symptoms after 1.5 years, while the potassium levels were inversely related. Potassium intake was thus related to lower levels of depression at follow-up, with a stronger effect in girls than in boys. Sodium intake is associated with higher levels of depression in girls, but not as strongly as the potassium levels. A high ratio of sodium to potassium intake was able to predict the occurrence of depression in teenage girls.

What did the study show?

Thus the study does show that especially in girls there is a strong link between low potassium intake and the reduced incidence of depression, with a weaker but significant link between high sodium in urine and an increased incidence of depression. These results cannot be explained by the differences in body weight, the presence of depressive symptoms at baseline, stage of development, or blood pressure.

Many recent reviews of experimental diets in animal models have shown the harmful effects of high levels of sodium, saturated fat and added sugar on the brain areas that deal with thinking, emotion, memory, judgment and behavior. Many of these harmful effects occur specifically when the adolescent brain is exposed to these toxins. This could be one way in which foods containing high sodium and low potassium cause depression.

Another mechanism could be that these unhealthy levels of key nutrients in fast and highly processed foods affect the gut microbiome, which is known to affect brain function. In adolescence, the brain is developing rapidly in many areas, which may increase the risk of diet-modified abnormalities in the way the individual processes and experiences emotions, leading to depression.

The increased effect in females suggests that these changes may be more severe in this group. Females do have a higher prevalence of adolescent depression, experience puberty onset earlier, and have earlier maturation of brain function, which could explain this gender-related difference.

The current study therefore suggests, “Consumption of foods high in sodium and low in potassium is a modifiable risk factor for adolescent depression that can be targeted with interventions to improve youth mental health.”

Another valuable output is the use of urinary sodium and potassium levels as a reliable marker for the dietary intake of these salts in adolescents. A recent crossover study demonstrated that 90% of ingested sodium is mirrored in the urine excretion, and dietary nitrogen and sugar intake can also be precisely measured using urinary levels of these nutrients.

This should help predict the occurrence of depression in adolescence better than the presence of these symptoms at the time of first evaluation, and better than many other factors like poverty, blood pressure and body weight. Though these findings are very early and need to be replicated with larger studies, they do strongly suggest that reducing foods high in sodium and eating more potassium-containing foods like whole grains, vegetables and fruits can help prevent depression in this age group.

At the same time, cautions researcher Sylvie Mrug, “A poor diet could be linked to other risk factors for depression, such as social isolation, lack of support, lack of resources and access to healthcare and substance abuse.” In this case the low dietary quality could be just a marker for these other factors which are the actual triggers for depression.

The study was published in the journal The Physiological Societyon August 23, 2019.