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Obesity is one of the leading preventable diseases in the world, such that over a third of the global population is classified as overweight or obese1. For adults, the World Health Organisation defines overweight as having a BMI greater than or equal to 25, while obesity as having a BMI greater than or equal to 301. As a significant public health threat, obesity increases the risk of diseases such as diabetes mellitus, cardiovascular diseases, musculoskeletal disorders, and a multitude of cancers1. It is also highly likely that there is a positive correlation between the rise in worldwide obesity prevalence with the increase in mental health disorders over the past few decades. Several studies have found that a significant proportion of patients with mental illness are obese compared to the general population, with one specific study based in Maryland, United States showing that their BMI was almost twice that of a control group2. It is important to investigate the nature of this relationship as current research has found that it is likely to be bidirectional. This means that obesity may increase an individual’s likelihood of developing a mental disorder while a history of mental illness may increase the risk of obesity. Therefore, this review plans to identify the potential bidirectional relationship between obesity and mental disorders, which would lead to improved diagnosis and better treatment methods for patients.

Obesity: Consequence of mental disorders

This first section examines obesity as a consequence of mental disorders and identifies the potential causal pathways between the two. One of the clearest examples of this inter-relationship would be patients with binge-eating disorder gaining weight due to their unhealthy coping mechanisms3. A binge episode is often followed by shame, guilt, and depression, followed by the need to soothe these feelings therefore triggering further binges. On another note, children with ADHD (Attention Hyper-Deficit Disorder) and other attention-deficit disorders are found to be heavier than their peers without behavioral disorders and are likely to remain overweight into adulthood4. Several studies have hypothesized about this link, an example being the impulsive behavior demonstrated by ADHD patients, leading to uncontrolled eating. A more in-depth explanation would be that food enables the increase of dopamine levels in the brain, therefore eating would be perceived as highly appealing to ADHD patients since they have lower dopamine levels (particularly in the prefrontal cortex)5. Interestingly, methylphenidate, the most widely used drug for the treatment of ADHD, is known to reduce appetite and potentially cause weight loss. This is shown by a study revealing that medication-naive ADHD patients are 1.5 times more likely to be overweight than those with ADHD who do use medication6. Children with ADHD also have difficulties sleeping, which can lead to obesity. It was recently demonstrated that with every hour of lost sleep, the BMI of a child is raised by 0.5 kg/m2 7. Sleep deprivation releases hormones that increase the appetite and cause fatigue during the day, leading to reduced physical activity. This symptom can hence be related to insomnia and other sleep disorders such as sleep eating.

Patients with anxiety disorders can also have symptoms of increased appetite and cravings for high-sugar and high-fat foods due to the dysregulation of the hypothalamic–pituitary–adrenal (HPA) axis, causing subsequent weight gain in stressed individuals8. Hormone imbalances such as increased cortisol levels due to anxiety are also hypothesized to cause a build-up of fat in the stomach9. Some anxiety-associated chronic conditions such as asthma can also affect patients’ abilities to engage in physical activities, resulting in excess weight8. Certain anxiety medications such as Xanax are reported to cause fatigue and consequently lead to adverse long-term health effects9. Since some symptoms of anxiety and depression overlap – namely, fatigue, sleeping problems, and difficulty concentrating – it is unsurprising that patients with major depressive disorders are also prone to weight gain. A study in 2010 has shown that depressed persons had a 58% increased risk of becoming obese, whereas obese persons had a 55% risk of developing depression over time. The main difference between anxiety and depression is that the former is characterized by heightened arousal whilst the latter is signified by feelings of hopelessness. Therefore, the lack of energy and physical activity among depressed individuals plays a key part in causing obesity. Research also identifies tricyclic antidepressant medications (e.g. amitriptyline) as being linked to weight gain, however, other studies have reported unclear effects of antidepressants on subsequent weight change10.

Epidemiological studies have found that there is an inconsistent relationship between substance abuse disorder and obesity. However, limited evidence from clinical samples appears to support an inverse relationship between current substance use disorders and obesity. A study has found inverse relationships between BMI and past-year alcohol and marijuana use among women seeking weight loss treatment11. It seems that compulsive eating habits and addictions to alcohol and other drugs share similar psychological and physiological underpinnings. Both intakes of food and drugs activate reward circuits in the brain, causing the release of dopamine. However, another study by Wang and his colleagues suggested that overeating and substance abuse may initially stimulate dopamine activity but eventually lead to a reduction of dopamine receptors and hence impede dopamine activity12.

From the findings above, it can be concluded that general symptoms of individuals with mood disorders such as indulging in “comfort eating”, eating impulsively, and lack of exercise compound together to cause weight gain. Hormone imbalances and treatment medications also play a part in exacerbating the effects of obesity.

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Obesity: Cause of mental disorders

The second section discusses obesity as the cause of mental disorders, be it directly or indirectly. Disorders such as anorexia nervosa and bulimia nervosa most likely appear due to a fear of being overweight or obese. The social stigma regarding body weight and image is deeply entrenched in our society, causing weight-based discrimination to be widespread. This target-based discrimination can lead to anxiety and depression, especially since individuals would tend to fear judgment and scrutinization from society13. Obese individuals have poorer social support and social networks as well as lower self-esteem compared with normal-weight individuals. Many studies have reported an increased incidence of bullying amongst overweight/obese adolescents, which is in line with the narrative that being overweight is a deviation from the community’s social norms14. Obese individuals might feel pressured to gain control over their weight, which can be distressing, particularly when repeated failed attempts to lose weight are the norm. It is therefore unsurprising that dieting was found to be closely related to anxiety. On the other hand, disturbed eating patterns, lethargy associated with weight gain, and general body dissatisfaction are known to increase the risk of depression. Despite not being able to clearly establish the direction of the causal relationship between obesity and depression, this study in 2008 suggests that the relationship possibly involves multiple mechanisms including decreased physical activity, increased caloric intake, and negative body image15.

High-risk groups

Multiple risk factors increase the likelihood of obesity, namely genetics (race and gender), socioeconomic factors (income and education), individual behaviors and lifestyle as well as environmental factors. Specific to the causal relationship discussed in this review, a study in South Africa found that young women were most at risk for mental illness if obese17. This might be a result of women being more distressed about weight perception, which makes them more susceptible to mental illness. Women also tend to resort to food as a coping mechanism, and women with mood disorders are more likely than men to report increased appetite as a symptom of depression. Ethnicity also plays a part in this link between obesity and mental illness. A study conducted at University College London found that African girls were less affected by being overweight as compared to Caucasian girls as body image is less stigmatized in their society18. Worryingly, a meta-analysis by Bak and colleagues has reported that almost all psychiatric medications cause weight gain19.

Effective treatment methods for obese patients with psychotic disorders

In this day and age, psychiatrists need to have adequate training to counsel psychiatric patients who are obese or overweight or are experiencing weight gain due to medication intake. Conversely, health experts must also be aware of obesity manifesting as a symptom of mental disorders. At the moment, behavioral treatments for patients with obesity generally have 3 components: dietary change, increased physical activity, and behavior therapy techniques. However, the effect of psychiatric disorders on obesity treatment must be taken into account. Patients with depression or anxiety would likely struggle to adopt routines such as regular exercise and preparing healthier meals as this requires constant effort. Poor impulse control associated with ADHD presents a challenge in adopting a healthier lifestyle. Additionally, patients may be reluctant to proceed with treatment, especially if the drugs cause subsequent weight gain. Anticipating these challenges would enable psychiatrists to be more prepared when suggesting treatment methods for these individuals.

For treatment to be effective, anxiety and depression patients require constant encouragement and oftentimes gradual exposure to prevent the patients from being overwhelmed20. ADHD patients must also develop routines and undergo self-monitoring to practice their self-control20. It is hoped that once clear links are established between obesity and a specific mental health disorder, treatment methods can be tailored to ensure better results.


The reasons cited for the link between obesity and mental disorders can be narrowed down to four main reasons: behavioral issues, negative body image stemming from social stigma, medication effects, and hormone imbalances. Behavioral issues include physical inactivity and uncontrolled eating habits. From the brief review conducted, it is evident that obesity can mainly cause anxiety and depression, whereas multiple mental disorders can have side effects related to unhealthy weight gain. This is an interesting finding as it presents obesity as a long-term symptom of these disorders rather than the root cause. However, alternative research designs including longitudinal and experimental studies are required to ultimately clarify the direction of this causal relationship.


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