Researchers have been investigating the connection between diet and mental health for about fifteen years. Studies suggest that a diet high in ultra-processed foods — such as packaged snacks, fast food, and sugary drinks — may be linked to a greater risk of experiencing depressive symptoms. However, it is not yet fully understood whether poor diet causes depression or if the relationship is influenced by other factors. Tasnime Akbaraly, a research director at Inserm and a specialist in nutritional epidemiology, explains that "junk food" is a commonly used term but lacks scientific precision. Instead, researchers use the term "unhealthy diet," which includes high levels of salt, added sugars, saturated fats, trans fatty acids, and ultra-processed foods, while being low in fruits, vegetables, fish, and other unprocessed foods. Ultra-processed foods are not always high in unhealthy ingredients, and some less processed foods can still be nutritionally poor. Depression is the mental health condition that has been most thoroughly studied in relation to diet. Researchers are focusing on how overall eating patterns, rather than individual nutrients, may influence the onset and recurrence of depressive symptoms. According to Akbaraly, the evidence is clear: diets that align with nutritional guidelines are associated with a lower risk of depressive symptoms. Conversely, diets high in ultra-processed foods, added sugars, salt, and fats are linked to a higher risk. A meta-analysis of 41 studies found that better-quality diets are associated with a reduced risk of depression. Akbaraly suggests these findings are strong enough to consider diet as a potential prevention strategy for mental health issues. A notable study, the British Whitehall II cohort study, followed 4,554 British civil servants and found that those who consumed the most ultra-processed foods — roughly a third of their diet — had a 30% higher risk of recurrent depressive symptoms compared to those who consumed less than a fifth. This link remained even after considering other factors like lifestyle and health status. However, Akbaraly cautions that these results indicate an association, not a direct cause-and-effect relationship. Researchers have proposed several possible explanations for the link between poor diet and mental health. One theory is that an unbalanced diet may promote inflammation, a factor that has been studied in relation to depression. Another possibility is that junk food could alter the gut microbiota — the community of microorganisms in the digestive system that play a role in digestion and communication with the brain. A diet low in fiber and high in refined sugars and fats might disrupt the balance of the microbiota, potentially affecting mental health. Researchers are also studying additives in industrial foods, such as emulsifiers, preservatives, sweeteners, and colorings. Some studies suggest these additives may influence the microbiota or inflammatory processes, but it is still too early to establish a direct link to depression. Researchers are also examining the potential effects of chemicals in food packaging, such as phthalates or bisphenol-related compounds, which can transfer into food. These substances are being studied for their potential impact on the body, but there is currently no conclusive evidence that they cause depression. Their role in mental health remains an area of ongoing research. An essential question in understanding the link between diet and mental health is whether one influences the other, or if the relationship works both ways. On one hand, depression may lead to changes in eating habits, such as a preference for fatty or sugary foods that are easier to consume. On the other hand, an unbalanced diet could promote biological changes that increase the risk of depression. This bidirectional relationship could create a cycle that is difficult to break. However, researchers suggest that improving one's diet may help break this cycle. While the link between depression and "junk food" is the most well-documented, knowledge about other mental health conditions, such as schizophrenia, bipolar disorder, and post-traumatic stress disorder, is still limited. This is partly because large studies can follow only a few thousand people, and those with certain psychiatric conditions are relatively rare, making it difficult to compare their diets and risks. Additionally, different mental health disorders have distinct symptoms, biological mechanisms, and risk factors, meaning that diet may play a different role in each condition. At this stage, it is important to remain cautious and not automatically apply findings from depression research to other psychiatric conditions. Akbaraly emphasizes that it is not necessary to completely eliminate "junk food" to protect mental health. Studies do not support the need to ban all ultra-processed foods, as this category includes a wide range of products with varying nutritional profiles. Instead, the focus should be on choosing more homemade and minimally processed foods, such as fruits, vegetables, legumes, whole grains, fish, nuts, and seeds. Foods high in sugars, salt, or fats, as well as ultra-processed foods, should be consumed in moderation. Occasional indulgences, such as an industrial pizza, cookie, or chocolate bar, are unlikely to harm mental health. What matters most is the regular and significant consumption of unhealthy foods within the context of an overall unbalanced diet and lifestyle.