The prevalence of obesity continues to rise in the U.S., standing at a staggering 41.9% as of March 2020. The buildup of excess fat in your body — whether contributed by genetics, lifestyle choices, or medications — is not just a body characteristic, but rather a disease state that is fast becoming a leading cause of death. For one, obese patients face an increased risk of diverse diseases including type 2 diabetes (T2D), cardiovascular diseases (CVD), and cancer. On top of that, the obese population can experience lower self-esteem and worsened body image, contributing to deteriorating mental health. In this article, we will take a closer look at what depression is, how obesity and depression are linked, and explore the physiological connection between the two conditions.

What is depression?

Depression induces feelings of sadness, anxiety, and hopelessness that last for at least two weeks. Depression covers a wide range of disorders such as bipolar disorder and postpartum depression, but we will focus on major depressive disorder (MDD). In MDD, the following symptoms can arise:

• Angry outbursts and increased irritability or frustration
• Sleep disturbances (sleeping too much or too little)
• Fatigue and/or lack of energy
• Slowed thinking, speaking, or body movements
• Brain fog (trouble thinking, concentrating, making decisions, remembering things)
• Suicidal thoughts
• Physical symptoms not easily explained, such as headaches or muscle aches

What is the relationship between obesity and depression?

Much research discusses obesity and MDD as participating in a bidirectional relationship. This means that having either obesity or MDD increases the risk of getting the other. 

On the one hand, the literature provides support for obesity being a risk factor for MDD:

• While body-mass index (BMI) has limitations in identifying obese populations, people with BMIs indicating obesity (>30) are more likely to have currently diagnosed depression or lifetime-diagnosed depression and anxiety.
  
• Obesity is also a risk factor for being diagnosed with MDD among adolescents. Obese teens have a higher incidence of mental health problems, especially MDD, and poorer self-esteem. The effect may be worse for obese adolescent females than males, who are more likely to develop MDD-like symptoms than their non-obese peers.
  
• A systematic review of 46 eligible studies determined that multiple factors contribute to obesity increasing the risk of depression, such as body image, binge eating, interpersonal communication, and experiences of stigma. Each of these variables may impart themselves to the obese, driving greater depressive symptoms.

On the other hand, patients with depression are more likely to be obese. In these cases, the symptoms associated with MDD increase the risk of weight gain through various behavioral mechanisms.

• A cross-sectional, retrospective chart review of 102 adolescent patients showed that patients diagnosed with depression were 3.5 more times likely to experience severe obesity than those without depression. Interestingly, the study also noted that this link was not mediated by emotional eating. Another systematic review of 13 longitudinal studies determined that adolescents who were depressed had a 70% increased risk of being obese. 
  
• Similar results have been observed in adult patients. A meta-analysis of 16 studies showed that adults diagnosed with depression were at a significantly higher risk for developing obesity. Worryingly, the study also notes that the risk was particularly high for adolescent females.
  
• While some studies do not attribute emotional eating to depression-driven obesity, other research suggests that higher emotional eating, combined with shorter sleep durations, leaves people with depression more vulnerable to substantial weight gain.

Physiological contributions to the obesity-depression connection

The scientific literature provides a strong argument for the intricate link between depression and obesity. These mechanisms generate a vicious cycle with biochemical links. But what are the mechanisms that mediate this relationship?

Bodily inflammation may provide some answers to this question: 

• Adipose tissue expansion: Adipose tissue, also known as body fat, stores fat in your body and acts as an energy reservoir. When they expand, adipose tissues release molecules called proinflammatory cytokines that signal an enhanced inflammatory response. The increased abundance of these cytokines is linked with heightened anxiety and depression symptoms.
  
• Alterations to the gut microbiome: The gut microbiome is a key player in the gut-brain axis. The gut-brain axis represents the bidirectional relationship between the gut and the brain and is mediated by nerves connecting the gut to the central nervous system (CNS). While it is known that junk food consumption increases the risk of mood disorders through altered gut function, the role that the gut microbiome plays in mood disorders remains conflicted. Nevertheless, it has been established that changes to the gut microbiome can induce depressive-like behaviors in mouse models.
  
• Modifications to the nervous system: The enhanced inflammatory responses observed among obese patients can also have direct impacts on nervous system function. Elevated inflammation can disrupt the neurotransmitter systems that help your brain function. The nerves of animal models with increased mitogen-activated protein kinase-driven (MAPKs) activation are more likely to reuptake serotonin, a behavior implicated in depression. Oxidative stress driven by increased inflammation can also reduce the production of serotonin and similar neurotransmitters.

Changes in mood driven by the endocrine system provide an additional mechanism through which obesity and depression link together:

• Reduced insulin sensitivity: Obese patients are more likely to demonstrate insulin resistance, the cause of T2D. This, in turn, has been linked with worsened depressive symptoms. The link between reduced insulin sensitivity and depression may stem from the compound’s impacts on the brain. Disruptions to insulin signaling in the brain can reduce the brain’s ability to adapt over time, which can contribute to depression.
  
• Lowered blood leptin levels: Leptin is known as the satiety hormone and is produced by adipose tissue. Decreases in blood leptin levels have been linked with anxiety and depression for both men and women. On the other hand, leptin can increase the effects of antidepressant medications such as selective serotonin reuptake inhibitors (SSRIs).  

Ways to improve mental health through dietary changes

Many of the mechanisms mentioned previously are driven by excess nutrient intake, driving physiological changes that augment inflammation and magnify the vicious cycle. With that in mind, here are some dietary suggestions that can help reduce your weight and, in turn, improve your mental health:

• Consider adopting the Mediterranean diet: Adopting a Mediterranean-style diet can help reverse some of the inflammation driven by eating too many saturated fats. This may reduce the risk of developing depressive symptoms and cognitive impairment, as was observed in a meta-analysis of 22 studies. On top of that, the Mediterranean diet is rich in polyphenols, one of many classes of compounds that reduce oxidative stress. The ability to lower oxidative stress can also alleviate depressive symptoms.
  
• Integrate more plant-based foods into your regime: Like the Mediterranean diet, the plant-based diet is rich in fruits and vegetables, nuts, and legumes. The high fiber content in this diet helps maintain your gut microbiome and enhances short-chain fatty acid production (SCFA). Both mechanisms reduce the risk of obesity, which in turn can alleviate depressive symptoms.
  
• Introduce more omega-3 fatty acids in your meals: Omega-3 unsaturated fatty acids are an excellent replacement for saturated fatty acids. On top of providing intestinal immunity, omega-3 consumption can make adipose tissue cells smaller to reduce fat storage. Integrating omega-3s into your diet also has direct benefits to your brain function, enhancing brain development in adolescents and improving blood flow to the brain. These mechanisms may contribute to the reduced risk of depressive symptoms that come with omega-3 ingestion.

Key takeaways

Obesity and depression are two of the more challenging disorders to treat. Part of this difficulty stems from the fact that the two disorders are closely linked with each other. The research among adults and adolescents demonstrates a bidirectional relationship between the two conditions, both of which are heightened in females. The two conditions are linked together by diverse metabolic mechanisms that drive bodily inflammation and nervous system modifications. While changes to one’s diet can help mitigate obesity, which could in turn help alleviate depressive symptoms, treating depression may require more extensive psychiatric help. Talk to your doctor and psychiatrist if you are experiencing worsened mental or suicidal thoughts. There is help available for you and depression is treatable.