Changes in the association between the traditional Japanese diet score and suicide rates over 26 years

Article in review: Sanada, M., Tomoko, I., Sezak, A., Miyamoto, K., Kawase, F., Shirai, Y., … & Shimokata, H. (2021). Changes in the association between the traditional Japanese diet score and suicide rates over 26 years: A global comparative study. Journal of Affective Disorders, https://doi.org/10.1016/j.jad.2021.07.020

Summary

Formed with a basis in previous research that has sought to determine the relationship between diet and mental health, a new assessment has been undertaken to determine the change in the association between traditional Japanese diets and suicide rates. It was hypothesised that the traditional Japanese diet, which is high in vegetables, fruits, grains, legumes, and fish but low in meat and dairy products, would be inversely associated with depression and suicide. Spanning a 26 year period, this study found that consuming a traditional Japanese diet may reduce suicide rates.

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Existing knowledge

Previous studies to assess the influence of diet on suicide or depression have found:
• An inverse correlation between depression, which is an important factor in the cause of suicide, and dietary patterns characterised by high intake of vegetables, fruits, and fish
• Higher levels of depression among those who have a diet high in sweet desserts, fried foods, processed meats, refined grains, and high-fat dairy products
• Less depression and anxiety among those with higher dietary diversity
• Lower rates of depression among those eating a traditional Japanese diet
• Lower rates of fibre intake amongst those with a history of suicide attempt
• The higher the quality of the diet, the lower the risk of developing depressive symptoms

Intent

The study spanned data from 1991 to 2017 and made consideration for controlling foraging rates, financial status, years of education, and lifestyle, using data on food supply, suicide rates, and other factors. Lifestyle factors such as diet, smoking, exercise and sleep are increasingly being recognised as important contributors to mental health. Dietary habits may relate to mental health through:
• Inflammation
• Oxidative stress
• Epigenetics
• Mitochondrial dysfunction
• Gut microbiota
• Tryptophan-kynurenine metabolism
• The hypothalamic-pituitary-adrenal axis
• Neurogenesis and the brain-derived neurotrophic factor
• Obesity

Methods

The study was completed through a linear mixed model analysis of a combination of four sets of data variables:
• Suicide Rates
• Foods
• Traditional Japanese Diet Score
• Socioeconomic and lifestyle indicators

The interactions between these variables and the association between each variable and the suicide rate were investigated. The extensive data set utilised vast population and demographic information from many countries over 26 years.

The traditional Japanese diet score (TJDS) was formulated using nine frequently eaten food components, including rice, fish, soybeans, vegetables, eggs, and seaweeds calculated at a rate per person, and foods not commonly eaten were scored in the opposite direction. The TJDS is similar to the Mediterranean diet, but varies in the amount of wine and olive (Mediterranean) and seaweed, soybeans and fermented foods in the Japanese diet.

Results

• Characteristics of variables in 1991, 2004, and 2017 – The overall population increased across the nations studied, as did the population density. There was a significant increase in the aging rate, GDP, and years of education, but no significant change in the percentage of Muslims or unemployment rates. For lifestyle variables, smoking rates decreased, but BMI and energy supply increased. The annual suicide rate significantly decreased.

• Changes in TJDS and suicide rate from 1991 to 2017 by region – in Japan, the TJDS began high but decreased. The TJDS has increased in South Asia and remained steady or increased slightly in other nations and regions.

• Mixed-effects models of TJDS and suicide rates – the TJDS was found to significantly reduce suicide rates; suicides became less frequent as the rate of TJDS became higher. A significant negative association was noted in the analysis of the relationship between the TJDS and suicide, accounting for all covariates.

Effects of socioeconomic and lifestyle factors on suicide rates

Suicide rates were lower in those countries with higher:
• GDP
• aging rates
• percentage of Muslims
• longer duration of education
• employment rates

Population density, smoking rate, and physical activity were not significantly associated with the suicide rate.

How diet can impact depression and suicide

The study suggests that the foods contained in the TJDS contain nutrients that can be negatively associated with depression and suicide. The Japanese diet contains many foods and nutrients that can be considered beneficial for physical and mental health. Fish specifically could be an important contributor. A study of about 10,000 Japanese men and women found those who died by suicide had significantly lower levels of fish in their diet. How fish in the diet works as a preventative for suicide remains unclear.

A diet high in vegetables, fruits, and grains will also help to maintain healthy intestinal microflora. The high number of fermented foods in the Japanese diet may also be beneficial to intestinal microbial health. Eating a balanced and healthy diet, such as the TJDS has benefits for physical and mental health. It is crucial to promote improved diets as a suicide prevention strategy.