A meta-analytic review of previous studies indicates that obese individuals have an increased risk of depression and that this relationship isn’t just correlational — obesity might be one of the causes of depression. The researchers argue that the increased occurrence of depression in the population in recent decades might be at least partly due to the increase in obesity. The study was published in the Journal of Psychiatric Research.
Obesity is a medical condition where a person has too much body fat. It is determined by calculating a person’s body mass index (BMI) using their height and weight. Obesity is associated with several health problems like heart disease, diabetes, certain cancers, and musculoskeletal disorders. The number of obese individuals in the population has been increasing rapidly in recent years.
A similar trend of increase in the same period has been observed for depression, a mental health disorder characterized by persistent feelings of sadness, hopelessness, and a loss of interest or pleasure in activities. Studies have shown that individuals with obesity also have an increased risk of depression. Across a number of studies, obese individuals had a 55% higher risk of depression compared to the non-obese. The question that remains is whether obesity predisposes one to develop depression.
To answer this question, Markus Jokela and Michael Laakasuo from the University of Helsinki in Finland examined potential effects of increasing obesity rates on how common depression is in the population. To this end, they reviewed the results of studies that used a method called Mendelian randomization. They also analyzed data from two ongoing national surveys conducted in the United States and the United Kingdom.
In a Mendelian Randomization study, researchers use genetic information to understand if a certain behavior or a trait (body mass index in this case) actually causes a specific outcome (depression). They look for genetic markers that are related to the behavior or trait studied and see if those markers are also related to the outcome. This helps them determine if there is a real cause-and-effect relationship between the exposure and the outcome.
After inspecting 539 different studies, the researchers found eight Mendelian randomization studies that contained the data they were looking for. The results from these studies showed that obese individuals had a higher risk of depression compared to non-obese individuals. Across these samples, individuals with higher BMI values, regardless of whether its value indicated obesity or not, had a higher risk of depression.
The researchers then compared the findings from these studies to data collected from the Health Survey for England, which began in 1991, and the U.S. National Health Interview Survey, conducted since 1957. These surveys collected information on psychological distress levels and asked participants about their height and weight. (Psychological distress is a broad measure of mental health that includes symptoms of depression but also anxiety and somatic complaints.)
The researchers looked at the changes in psychological distress and obesity levels reported by these surveys between 1991 and 2016 and predicted the changes in psychological distress that would be expected if obesity caused depression, given the changes in population obesity rates in this period. The predictions were based on the findings about the relationship between obesity and depression reported by the Mendelian randomization studies they analyzed.
The researchers found that, as the percentage of obese individuals has doubled since the 1990s, that could have contributed to a half percentage increase in the prevalence of psychological distress. In England, it would explain an increase in the share of people reporting psychological distress from 15.7% to 16.3% between 1991 and 2016 and from 19.4% to 20% in the U.S. in the period between 1997 and 2016.
“In sum, obesity seems to be a causal risk factor for depression, increasing its odds by 33%. Between 15% and 20% of the general population are estimated to suffer from at least moderate psychological distress. The doubling of obesity prevalence from the 1990s–2010s would have increased this prevalence [of psychological distress] by one-half percentage points,” the study authors concluded.
The study makes a valuable contribution to the scientific systematization of knowledge about the relationship between obesity and depression. However, it also has limitations that need to be taken into account. Notably, the study only focused on depression, while it relied on a broader mental health indicator from national surveys – psychological distress. It also did not consider other mental health consequences of obesity, such as reduced risk of suicide.
The study, “Obesity as a causal risk factor for depression: Systematic review and meta-analysis of Mendelian Randomization studies and implications for population mental health”, was authored by Markus Jokela and Michael Laakasuo.