While the traditional Japanese diet (or Washoku) is generally said to be nutritious and balanced, its actual benefits — particularly with regards to mental health — have not been properly detailed.
In order to ascertain the health benefits of Washoku, researchers from Hokkaido University and Tenshi College conducted a survey to assess the dietary habits and mental and physical health of the Japanese population.
They recruited 278 physically and mentally healthy participants and assessed their daily intake of staple foods such as rice, rice-based foods, bread, noodles, cereal for their three main meals (breakfast, lunch and dinner).
Have you bean eating your rice?
Subsequently, they observed "associations between rice consumption and improvements in quality of life (vitality) and sleep quality", whereas bread and noodle consumption did not have the same associations.
They said this was consistent with previous research, and added that consuming a combination of rice and another Japanese dietary staple, miso (soybean paste) soup, could also have a positive impact on both physical and mental health.
They referred to a recent longitudinal study that had assessed changes in the Japanese diet every 15 years — from 1960 to 2005 — and found that the 1975 Japanese diet, which contained more rice and miso soup, contributed the most to preventing cognitive decline.
This was attributed to the flavonoids in miso, which are believed to help promote brain function and learning ability, as well as enhance memory. They are also said to aid in preventing cancer, heart disease and osteoporosis.
In addition, the researchers wrote that "a rice-centred diet may have negative effects on impulsiveness".
Other factors and mediators
They noted that how long an individual has been regularly consuming Washoku might also affect its impact on one's health.
The present study's subjects were all at least 40 years old, meaning they had been consuming the recommended traditional diet since 1975. As such, "the foods identified as associated with improvements in health might correspond to representative foods in the participants' diets throughout their lifespan".
The study also made mention of other health mediators central to Washoku, namely, green tea and natto (fermented soybean), which have been major components of the diet since 1975.
The researchers concluded: "The present study suggests an association between rice intake and physical and mental health, with indirect contributions from intakes of other foods as mediators. The foods that were associated with improvements in health were components of the traditional Japanese diet.
"The contribution of the rice-centred Japanese diet to health may be explained by the effects of mediators, which are foods that have been familiar to the Japanese for a long time.
"Further research is required to assess the associations between modern foods and health and the biological mechanisms underlying these associations."
Source: PLOS ONE
"Mediators of the effects of rice intake on health in individuals consuming a traditional Japanese diet centered on rice"
Authors: Minori Koga, et al.