The “1975 Diet” and the Secret of Japanese Longevity

Japan boasts one of the longest life expectancies on earth, and it also a world leader in “healthy life expectancy”—the number of years of good health people can expect on average. Since diet is believed to play a key role in a population’s health and longevity, researchers around the world have been studying the benefits of the Japanese diet for some time now. 

But what exactly is the Japanese diet? The people of Japan do not dine primarily on sushi, tempura, or other well-known Japanese specialties. Moreover, their eating habits have changed over the years. For our research, we used national surveys to compile weekly menus representative of the Japanese diet at various points in time over the past half century. In the following, we will take a look at the comparative health effects of these menus.

Japanese Versus American

First, we conducted experiments on lab rats to compare the health impact of a typical Japanese diet of the late 1990s with an American diet of roughly the same period. Drawing on the results of the 1999 Japanese National Health and Nutrition Survey and a comparable study conducted in the United States in 1996, we compiled two weekly meal plans,  Japanese and American (each consisting of 21 meals), under the supervision of a registered dietician. Then we prepared the dishes on the menus, freeze-dried them, and fed them to two separate groups of rats. Finally, we compared the outcomes with respect to various health indicators.

After three weeks, the group fed the Japanese diet showed increased expression of genes associated with energy, sugar, and lipid metabolism and decreased expression of stress-response genes. The Japanese-diet group also had less visceral fat—fat stored  within the abdominal cavity—and lower levels of blood lipids. Surprisingly, the balance between carbohydrates, fats, and proteins was very similar in both diets. The disparities in the test results seem to relate to qualitative differences, such as whether the carbohydrates come from wheat or rice, and whether the protein is primarily from fish and soybeans or beef and pork. 

Decline of the Japanese Diet 

The Japanese diet has changed with the times, and many believe that the spread of Western foods and eating habits among the Japanese is contributing to a marked increase in such  lifestyle diseases as atherosclerosis (a condition where plaque builds up inside arteries) and diabetes. But very little effort has been made to scientifically quantify the health impact of changes in the Japanese diet. Can we use food science to identify the era in which the Japanese diet was at its healthiest? 

Applying much the same methodology described above, we produced weekly menus representative of diets from 2005, 1990, 1975, and 1960. We freeze-dried the meals and fed them to separate groups of mice over a period of eight months to determine the diets’ relative impact on lifestyle diseases, aging, and health maintenance. 

Compared with the 2005 group, the 1975 and 1990 groups had less visceral fat and were less inclined to obesity. The 1975 group had the lowest risk for diabetes and fatty liver. Our analysis of gene expression in the liver found relatively active breakdown of fats and suppressed synthesis of fatty acids in the 1975 group, which most likely accounts for the lower accumulation of visceral fat and fat in the liver. 

In terms of content, the 1975 diet was higher in legumes, fruit, seaweed, seafood, seasonings, and spices than the other three. It also contained a greater variety of ingredients. At the same time, consumption of juice and sweetened soft drinks was relatively low.

Prevention of Aging-Associated Diseases 

We looked at the comparative impact of these diets on longevity and the aging process with the aid of senescence-accelerated mice, widely used as a model for the study of aging-associated diseases in humans. 

At 24 weeks, there was little to distinguish the groups in terms of physical aging, but by 48 weeks senescence was more pronounced in the 2005 group than in the 1990 and 1975 groups. Aging progressed most slowly in the 1975 group.

Similarly, at 24 weeks the mouse groups displayed little difference with respect to learning and memory, but by 48 weeks, the 1975 group showed less loss of memory and learning capacity than the 2005 group. The 1975 and 1990 groups also had longer lifespans on average than the 2005 group, and the 1975 group performed especially well in terms of longevity.

The results of these experiments strongly suggest that the Japanese diets of 1990 and 1975 are healthier for mice than that of 2005. But do the outcomes apply to human beings as well? After securing the authorization of the Tōhoku University Research Ethics Committee, we embarked on a study using human subjects.

Gut Flora and Aging

For our first experiment, we assembled a group of moderately obese subjects aged 20 to 70 and divided them into two groups, then had those two groups adhere to distinct diets for 28 days (three meals a day). The first group ate meals consistent with a typical Japanese diet of today, while the second group adhered to the 1975 diet. By the end of the experiment, the average body mass index and weight of the 1975 group members had declined significantly, along with waist circumference. Levels of LDL (or “bad”) cholesterol fell, while HDL (“good”) cholesterol rose. Hemoglobin A1C (used to diagnose and monitor diabetes and pre-diabetes) declined as well.

Next, we performed a similar dietary experiment on non-obese subjects aged 20 to 30. In this case, we also had the subjects engage in an hour or more of moderately intense exercise three days a week for the duration of the experiment. Pre- and post-experiment measurements showed reduced stress and increased fitness among the 1975 group. Changes found in the 1975 group’s gut microbiota include a smaller population of certain bacteria believed to be associated with a higher risk of lifestyle disease, suggesting that the health benefits of the 1975 Japanese diet may relate to gut flora and their metabolites.

In short, our research to date supports the hypothesis that, when it comes to health and longevity, the 1975 Japanese diet is superior to the typical Japanese diet of today. 

Secrets of the 1975 Diet 

What distinguishes the Japanese diet of 1975 to today’s westernized diet? I believe that it strengths of the former can be boiled down to the following attributes. 

Variety: The daily menu featured a relatively large number of small dishes (typically at least three, in addition to soup and rice).(*1)

Cooking methods: The top three modes of preparation were simmered, steamed, and raw, followed by boiled and grilled. Frying and sauteeing were somewhat less common. Cooking at high heat, as when frying in oil, can cause nutrients to break down. For example, oily fish like horse mackerel (aji) are rich in the healthy omega-3 fatty acids EPA and DHA. However, the fried version provides only about one-tenth the EPA and DHA of sashimi. 

Ingredients: The 1975 diet was rich in soy products, seafood, tubers, green and yellow vegetables (including pickles), fruit, seaweed, mushrooms, and green tea. Eggs, dairy products, and meat were consumed as well, but in moderation.

Seasoning: The skillful use of fermented seasonings (soy sauce, miso, vinegar, mirin, and sake) along with dashi broth helped impart a satisfying flavor to foods without the heavy use of salt and sugar. 

For our experiments, we incorporated all of the above features into weekly menus to scientifically verify and measure the health benefits of the “1975 Japanese diet.”

Uncertain Outlook for Life Expectancy

How are recent changes in the Japanese diet likely to affect life expectancy in Japan?

The segment of the population that is pushing the average Japanese lifespan to record levels is the 60-and-up age group, which was still largely adhering to the 1975 diet in middle age—the time when adults become vulnerable to lifestyle diseases. One may question how long the trend toward healthy longevity will continue, given the westernized eating habits of the population’s 20–40 age group. In fact, the incidence of diabetes and other lifestyle diseases among the Japanese has been rising year by year, raising concerns that life expectancy in Japan is approaching its peak. On the other hand, if we can make a timely course correction and recapture the health benefits of the 1975 Japanese diet, the day may come when almost anyone can reasonably expect to live to 100. 

Sample Menu from the 1975 Japanese Diet

(Originally published in Japanese on April 15, 2019. Banner photo: Pixta）