Two studies reinforce the previously reported association between obesity and depression. The studies were published in the March and May 2010 issues of Archives of General Psychiatry and Psychiatry Research, respectively.
Both obesity and depression are widely prevalent health problems that cause huge economic burden. Obesity frequency has more than doubled in the past 2 decades in the US, while 120 million people in the world are suffering from depression at any given time.
The May 2010 study  was a meta-analysis of 17 previous cross-sectional studies on the obesity-depression association. A meta-analysis is a method used in statistics that synthesizes research results from several studies in order to increase the sample size and provide stronger conclusions. Cross-sectional study is a research method that involves observation of certain groups of people, all at the same time.
This cross-sectional meta-analysis used data from 200,000 people and found a positive association between depression and obesity, which appeared to be more prominent in women than in men. This finding confirms the results of previous studies conducted throughout the last decade, which used smaller population samples.
The March 2010  study was a meta-analysis of 15 previous longitudinal studies on the relationship between obesity and depression. A longitudinal study, unlike a cross-sectional study, is the research method of collecting data from a population at repeated intervals over an extended span of time, rather than at a single moment, in an attempt to establish a relationship between two conditions.
The goal of this study was to provide information on the direction of the obesity depression association. Is it obesity that causes depression, or vise versa? Obesity was defined as having a Body Mass Index (BMI) higher than 30. BMI is an index of obesity based on an individual’s height and weight.
In order to find if obesity and overweight are predictors of depression, about 55,000 people were followed for 10 years. Those who were obese at the beginning of the study had a 55% higher risk of developing depression within 10 years than the subjects who maintained a normal BMI.
In order to evaluate depression as a predictor of obesity and overweight, about 7,100 people were interviewed and followed up after 10 years. Those diagnosed with depression had a 58% higher risk of becoming obese than normal individuals.
Therefore, the study found that obesity and depression are linked bi-directionally. The one is a predictor of the other. What is the reasoning behind this association?
How Can Obesity Cause Depression?
First, obesity has been recognized as an inflammatory disease. Weight gain stimulates inflammatory mechanisms in the body, and inflammation has been linked to depression
in a number of studies.
Second, obesity has been suggested to dysregulate the part of the neuroendocrine system known as hypothalamic-pituitary-adrenal axis. Impairment of this system has been shown to be involved in depression.
Third, being overweight is socially viewed as being unattractive. Culturally, thin is beautiful. Obese individuals are prone to feelings of dissatisfaction and lower self-esteem, which increase psychological stress.
How Can Depression Cause Obesity?
According to Dr. Jeffery A.N.’s recent finding, there is an association between low mood in children and poor metabolic health .
Other studies have found that depressed people have higher than normal levels of cortisol. Elevated cortisol promotes fat storage in the intra-abdominal area, as it inhibits the enzymes that breakdown stored lipids in visceral adipose tissue.
Further, depressed people are usually not motivated or simply lack the energy to exercise. They also tend to adapt unhealthy dietary patterns. This makes them susceptible to becoming obese.
Finally, anti-depressant medication causes weight gain in at least 25% of people. Commonly prescribed drugs can cause patients to gain 10 pounds or more.
These two population studies establish a reciprocal interaction between obesity and depression. The one health condition can be a consequence of the other. Obesity is a predictor of depression and visa versa. Adapting healthy diet and exercise habits or following a proven weight loss program will help overweight and depressed people improve their emotional health and develop a better psychological status.