Denmark has health registries with data spanning more than 50 years that provide unique research opportunities to benefit patients. Researchers used these registries and found that people with an eating disorder have four times the risk of developing another mental disorder. Conversely, people with a mental disorder have three times the risk of developing an eating disorder. Although the study cannot determine the causes, it clearly shows that healthcare professionals and relatives should be attentive if a person is diagnosed with an eating disorder or other mental disorder. Now the researchers want to examine the data in greater detail to determine whether these associations are linked to genetic factors.
About 1 in 3 people globally have one or more mental disorders during their lives, and research unfortunately shows that one type of mental disorder rarely comes alone. For example, many people have both anxiety and depression. Recent research suggests that eating disorders are also closely linked to other mental disorders, but there is no consensus on exactly how close this link is. One reason is that studies have often been based on people who already have mental disorders – without having a sufficiently good control group for comparison.
“We carried out a comprehensive study of everyone born in Denmark between 1963 and 2010 to find any bidirectional associations between the disorders. More than one quarter of the people diagnosed with an eating disorder are diagnosed with anxiety over the next 15 years, four times higher than for the rest of the population. Conversely, for people with another mental disorder, the risk of developing an eating disorder is almost tripled. So relatives and doctors should pay extra attention to symptoms,” explains Liselotte Vogdrup Petersen, Senior Researcher, National Centre for Register-based Research, Department of Economics and Business Economics, Aarhus University.
Difficult differentiating between cause and effect
The researchers used data from three registries, including Denmark’s Civil Registration System, on all 3 million people born in Denmark between 1963 and 2010. The data on people with eating disorders and other mental disorders were obtained from the Danish Psychiatric Central Research Registry and the Danish National Patient Registry.
“The types of comorbid mental disorders can vary according to the age of onset of each mental disorder. Certain types of mental disorders tend to co-occur. Previous studies often focused on eating disorders as one group and selected mental disorders as another. We investigated whether we could observe any specific associations between, for example, anorexia and other specific mental disorders,” says Liselotte Vogdrup Petersen.
Although the study found various patterns of comorbidity for certain types of eating disorders, including anorexia nervosa and bulimia nervosa, the variation was generally small. Nevertheless, the study found clear associations between eating disorders and other mental disorders. People diagnosed with an eating disorder have almost a 7% risk of developing a diagnosed anxiety disorder within 1 year, almost 10 times higher than in the control group.
“Nevertheless, some people probably have undiagnosed disorders that are only discovered when another mental disorder is diagnosed, so which of the disorders occurred first can be difficult to determine with certainty. However, although the results suggest that the effect is bidirectional, both disorders could also be caused by a common third factor,” explains Liselotte Vogdrup Petersen.
Patterns will provide some answers
The new study covers more than 3 million individuals and is thus the largest of its kind to date. In contrast to previous studies, it shows both the relative and absolute risks between specific types of eating disorders and many other mental disorders. It thus confirms previous assumptions that the risk of eating disorders is significantly increased among people with mental disorders – and vice versa.
“About 40% of the people diagnosed with an eating disorder are diagnosed with another mental disorder over the following 10–12 years. Although we cannot yet pinpoint exactly which eating disorders tend to lead to which subsequent diagnoses, this should indicate the need for ever greater vigilance in treating a possible new disorder early or consider measures to prevent the disorder from developing,” explains Liselotte Vogdrup Petersen.
Over the next few years, the researchers hope to examine the data on eating disorders and other mental disorders in greater detail to discover specific patterns, which they hope can determine how the associations emerge. A major study published in Nature Genetics in 2019 indicated that anorexia nervosa not only shares genetic causes with other mental disorders but also the genetic variants associated with metabolism.
“People who have anorexia nervosa more often have genetic variants that are associated with metabolism and increased levels of physically activity. People have these genetic variants long before developing the disorder, and it may be one cause. This suggests that an individual’s genes – in interaction with environmental factors – contribute to an increased risk of developing anorexia nervosa and possibly also other mental disorders,” concludes Liselotte Vogdrup Petersen.