The following article review published in Psychological Medicine by Ikeda, Tanaka, Saito, Ozaki, Kamatani, and Iwata (2018) proposes a theory on how the genetics of different body compositions may play a role in psychiatric disorders.
Introduction
There have been different theories on how body composition may play a role in personality, most notably two theorists, Kretschmer and Sheldon, have proposed similar ideas (Ikeda et al., 2018). Kretschmer described four different body types consisting of a slender body prone to schizophrenia, a round body prone to bipolar disorder, an athletic body prone to epilepsy, and a category for all other body types (Ikeda et al., 2018). Sheldon described the more current classifications of ectomorph, mesomorph, and endomorph body types that may be correlated to different personalities (Ikeda et al., 2018). To challenge past psychological associations, more current genome-wide association studies have been conducted to relate phenotypes and genetics. These studies have gone to find that there are relationships between body mass index and schizophrenia and major depressive disorder (Ikeda et al., 2018).
Method
A meta-analysis was conducted to understand research findings on multiple trait relationships for schizophrenia and bipolar disorder in European databases as well as results published on the Japanese studies conducted by the presented authors and the Psychiatric Genomics Consortium (Ikeda et al., 2018). The relationships must have had a correlation in Major Histocompatibility Complex single nucleotide polymorphism (MHC SNP) >0.7 (Ikeda et al., 2018). Genetic correlations were also crossed between BMI in Eastern Asia and European findings (Ikeda et al., 2018).
Results
It was found when mapping genetic correlations with BMI that schizophrenia had a negative correlation of -0.094, meaning that when BMI goes down, chances of developing schizophrenia increase (Ikeda et al., 2018). Bipolar disorder had a negative correlation of -0.069 (Ikeda et al., 2018). These results showed a low variation among studies used in the meta-analysis (p<0.05), whereas the results for major depressive disorder showed a high population variation between European and Japanese (p=2.0x10-6) (Ikeda et al., 2018). When comparing BMI to bipolar disorder, the results were deemed non-significant, but results may have been affected by the low sample size and cross examination factors of ethnicity and phenotype (Ikeda et al., 2018).
Discussion and Application
The findings on low BMI and schizophrenia add sustenance to the theories proposed by Kretschmer and Sheldon in 1925 and 1940, respectively (Ikeda et al., 2018). These theories did not hold true for bipolar disorder, however. It is uncertain if low body weight truly is a cause of schizophrenia or that people who develop schizophrenia typically tend to have lower BMIs, seeing as there are many reported clinical cases of people suffering from these psychoses who are obese (Ikeda et al., 2018). Therefore, patients may have developed schizophrenia or bipolar disorder as a result of medication or lifestyle/living conditions of being obese. Findings on major depressive disorder affecting Europeans and Japanese inversely may be attributed to high BMI in Europeans and low BMI in Japanese. Europeans with major depressive disorder tended to eat more and gain weight during an episode, where Japanese were the opposite, tending to eat less and lose weight (Ikeda et al., 2018). These determinations are limited by the fact that only females were used in the studies of the meta-analysis criteria. In conclusion, this meta-analysis showed support for schizophrenia associations with a low BMI, but not for bipolar disorder and BMI. Further research must be completed in determinations for major depressive disorder and genetic correlations to BMI. Research on the current topic is important for finding new links between genetics and phenotypes in individuals and in populations (Ikeda et al., 2018).
References
Ikeda, M., Tanaka, S., Saito, T., Ozaki, N., Kamatani, Y., & Iwata, N. (2018). Re-evaluating classical body type theories: genetic correlation between psychiatric disorders and body mass index. Psychological Medicine, 48, 1745-1748. Retrieved from https://www.cambridge.org/core/services/aop-cambridge-core/content/view/0B4E6C8FC74B0B3A82E79DA9F4809D3A/S0033291718000685a.pdf/reevaluating_classical_body_type_theories_genetic_correlation_between_psychiatric_disorders_and_body_mass_index.pdf
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