An analysis of data from the Twins Early Development Study and the UK Adult Twin Registry has uncovered that the relationship between body mass index (BMI) and depressive symptoms is bidirectional between the ages of 12 and 16. In this age group, depressive symptoms contribute to subsequent weight gain, and an increased BMI leads to the emergence of depressive symptoms. After the age of 16, however, only depressive symptoms contribute to later weight gain. The new research was published in Psychological Medicine.
Obesity and depression are two of the most frequent adverse medical conditions. The share of individuals suffering from them has been increasing strongly in the last few decades. Obesity is a medical condition characterized by excessive body fat accumulation that presents a risk to health, often defined by a body mass index of 30 or higher. Body mass index is calculated by dividing a person’s weight in kilograms by the square of their height in meters.
Depression, on the other hand, is a common and serious mental health disorder characterized by persistent feelings of sadness, loss of interest, and a range of emotional and physical problems, significantly impacting daily functioning. Both obesity and depression carry an increased risk of socio-economic hardship, cardiovascular disease, death from different causes, and other adverse consequences. They also tend to appear together, with obese individuals suffering from depression and vice versa.
Study author Ellen J. Thompson and her colleagues wanted to investigate whether there is a causal relationship between depression and obesity and, if there is such a relationship, which of these two conditions causes the other. They note that it is possible that obesity and depressive symptoms do not cause one another, but that they are both caused by the same environmental processes. There could also be common genetic factors influencing both disorders.
To try to answer this question, the researchers analyzed data from two large, longitudinal twin studies in the UK – the Twins Early Development Study (TEDS) and the UK Adult Twin Registry (TwinsUK). TEDS followed twin pairs born in England and Wales between 1994 and 1996, encompassing about 10,000 pairs, and collected data at ages 12, 16, and 21, including body mass index and depressive symptoms. TwinsUK, established in 1992, includes 14,575 adult twins aged between 18 and 101, born in the UK, initially aimed at studying osteoporosis and osteoarthritis.
In the TEDS study, participants completed the Short Moods and Feelings Questionnaire (SMFQ) for depression assessment and self-reported their weight and height, which researchers used to calculate BMI. The TwinsUK study employed the Hospital Anxiety and Depression Scale for assessing depressive symptoms and used whole-body dual-energy X-ray absorptiometry scans to measure participants’ height, weight, and body fat percentage.
Because these databases contained data from different timepoints, it was possible to make inferences about their cause-and-effect relationships. Namely, if one of these conditions causes the other one (if one is the cause and the other is the consequence), then the association between past cause and future consequence should be stronger than between past consequence and future cause. Since causes cannot work backwards in time, this reasoning allows researchers to make cause-and-effect inferences even though there was no experimental manipulation of factors involved in the study.
Results showed that depressive symptoms increased with age. Depressive symptoms were associated with body mass index values measured at the same time point, but this association was weak at ages 12, 16, and 21. However, there was a strong correlation between depressive symptoms at one time point and body mass index in the future. Depression scores at different time points were also associated.
Further analysis showed that the relationship between body mass index and depression is bidirectional between ages 12 and 16. Higher body mass index at 12 led to higher depressive symptoms at 16. However, higher depressive symptoms at 12 also led to higher body mass index at 16. Between 16 and 21 years of age, this link was no longer bidirectional. It was only depressive symptoms at age 16 that led to higher body mass index at age 21, but not vice versa.
Analysis of genetic and environmental influences showed that body mass index was highly heritable at ages 12, 16, and 21, while environmental influences were low. Depression was moderately heritable at these ages, but there was also a high influence of environmental factors not shared between twins (those that affect each twin differently, their unique experiences).
“The current study demonstrated a bidirectional association between BMI [body mass index] and depression from ages 12 to 16 with stronger directional effect from BMI to depression,” the study authors concluded.
The study sheds light on the nature of links between depression and body mass index during adolescence. However, it also has limitations that need to be taken into account. Notably, the study was conducted on twins. Results on singletons might not be the same. The study design also assumes that there are no interactions between genes and environment and this might not be the case.
The paper, “The relationship between weight-related indicators and depressive symptoms during adolescence and adulthood: results from two twin studies”, was authored by Ellen J. Thompson, Georgina Krebs, Helena M.S. Zavos, Claire J. Steves, and Thalia C. Eley.
