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1 From the Laboratory of Food Science, Kyushu University School of Agriculture, Fukuoka, Japan, and the Division of Food Science, National Institute of Health and Nutrition, Tokyo.
2 Address reprint requests to M Sugano, Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto, Kumamoto 862-8502, Japan. E-mail: suganomi{at}pu-kumamoto.ac.jp.
ABSTRACT
The amount of polyunsaturated fatty acids (PUFAs) in the food chain in Japan is reviewed on the basis of the newest nutrition survey data. The Japanese are currently consuming, on average, 26% of energy as fats with ratios of polyunsaturated to saturated fats and n–6 to n–3 fatty acids of 1.2:1 and 4:1, respectively. The significant contributors to this relatively high n–3 PUFA intake are not only fish and shellfish but also edible vegetable oils, almost exclusively rapeseed and soybean oils. Thus, the dietary habits of the Japanese have made possible a high n–3 PUFA intake within a low-fat regimen. In this context, the gradual decline, particularly in younger persons, in fish consumption habits weighs on our minds. Analyses of health indexes, including the increased average life span, support the superiority of the current Japanese eating pattern that harmonizes with the Western regimens. However, at present it cannot be disregarded that food intake varies considerably in all age groups, and only a limited number of people are consuming the recommended allowance for dietary fats.
Key Words: Polyunsaturated fatty acids • PUFAs • n–6 PUFAs • n–3 PUFAs • national nutrition survey • recommended dietary fat allowance • food chain • Japan
INTRODUCTION
Japan has one of the lowest intakes of fat in the world (1). The average current fat intake in Japan, just below 60 g/d or 26% of energy, approximates the upper limit of the recommended dietary allowance (RDA) for fat, which is 25% of energy (2). These intake levels are obviously lower than those of most Western countries. In addition, because the National Nutrition Survey is conducted in Japan every year (3), current information is always available on the nutrient intake at the national level. The RDAs are revised every 5 y (2), enabling up-to-date comparisons of fat intake with RDAs.
Although the physiologic significance of the current nutritional intake of the Japanese is difficult to evaluate, it seems likely that the nutrient intake pattern contributes at least in part to the life expectancy of the Japanese, which is the longest in the world (4). It is in turn true that the rapid westernization of Japanese eating habits after World War II also provided a health benefit. Thus, this review will start with an explanation of the current trend in health patterns and fat intake in Japan.
TRENDS IN FAT INTAKE AND CAUSES OF DEATH
During the 4 decades from 1950 to 1990 there were drastic changes in the major causes of death in Japan (Figure 1) (5). The most dramatic change was the reduction in the rate of death from cerebrovascular disease, which was once overwhelmingly the leading cause of death in Japan; Cerebrovascular disease peaked during the 1960s, has decreased rapidly since, and is now the third leading cause of death. The rate of death from tuberculosis also decreased to a low level. In contrast, 2 Western-type diseases, neoplasms and heart disease, increased gradually, and these are now the first and second causes of death, respectively.
FIGURE 1.. Trends in the major causes of death in Japan, 1950–1990 (5).
Rates of death from different types of cancer changed gradually over the same period (Figure 2), and the rate of death from stomach cancer, which had been the most common cancer in Japan, decreased, whereas rates of death from cancers of the lung, liver, large intestine, and breast increased, possibly reflecting changes in nutrient intake (5). Thus, the types of cancers in the Japanese are becoming more Western. The type of cerebrovascular disease is also changing, from cerebral hemorrhage to cerebral infarction. About two-thirds of Japanese deaths at present are attributed to 3 diseases: cancer, heart disease, and cerebrovascular disease. Although the pattern became westernized, the standardized death rates from cancer and heart disease are low compared with those in other major developed countries (5). The rate of death from cerebrovascular disease in Japan ranks at the midpoint compared with other developed countries, but the difference among the countries is relatively small compared with the difference in rates of death from cancer and heart disease.
FIGURE 2.. Trends in deaths from major cancers in Japan, 1955–1994 (5).
The change in nutrient intakes (Figure 3) is considered to be closely related to the observed change in the pattern of death rates (Figure 1). Analyses of nutrient intakes during 1950–1990 showed that there was a significant increase in fat intake during this period, from <20 to nearly 60 g/d. This dietary change appears to have been at the expense of carbohydrates, mainly rice, consumption of which fell from >400 to <300 g/d. However, fat consumption reached a plateau between 1980 and 1990.
FIGURE 3.. Trends in nutrient intakes in Japan, 1950–1994 (2).
With the increase in fat consumption came a significant change in its composition. The consumption of animal fat increased from 30% to 50% of total fats consumed, whereas that of vegetable fat decreased from 70% to 50% between 1950 and 1970; since then, the proportion of animal to vegetable fats has remained almost constant at 1:1. From these data, it is apparent that fat intake by Japanese has been relatively stable, at least during these 20 y. Therefore, it seems appropriate to focus on fat intake in these periods to understand polyunsaturated fatty acids (PUFAs) in the food chain in Japan.
TRENDS IN POLYUNSATURATED FATTY ACID INTAKE
The major food sources of fat in Japan are shown in Figure 4. Reflecting the trend of the fat intake pattern, the contribution of individual food groups was apparently constant from 1975 to 1994. Among dietary fat sources, edible fats and oils are the largest, contributing >25% of total dietary fats. Meats are also important fat sources, providing 20% of total fats. It is quite characteristic that in the Japanese diet fish and shellfish contribute 10% of the total fat intake. Intake of fats from milk, dairy products, and eggs is relatively low compared with that in Western countries.
FIGURE 4. . Sources of dietary fat by food groups in Japan, 1955–1994 (2).
Consequently, for the past 2 decades the proportion of fats in the Japanese diet from animals, vegetables, and fish has been 4:5:1. This proportion was the basis of the recommended balance of n–6 and n–3 PUFA intake in the recent RDA for fat in Japan (3). The proportion of dietary PUFAs showed a marked change before 1975. Since then, intake from each fatty acid group, ie, saturated, monounsaturated, and polyunsaturated, has remained constant (Figure 5). The analyses based on the National Nutrition Survey data indicate the constancy of PUFA intake from 1975 to 1994 (6). The ratio of n–6 to n–3 PUFAs was essentially constant at 4:1 throughout this period and the ratio polyunsaturated to saturated fatty acids was 1:1.2. A similar trend in the PUFA intake pattern was reported earlier (1).
FIGURE 5. . Trends in polyunsaturated fatty acid (PUFA) intakes in Japan, 1971–1994. Reproduced with permission from reference 6.
Advances in knowledge concerning physiologic functions of dietary PUFAs, in particular n–3 PUFAs, have led to deep concern about the decrease in fish consumption in Japan, although its significance is not necessarily well understood. Fish is not the "almighty" food, but it appears to provide a variety of health benefits compared with meats. For example, fish may reduce the risk of so-called adult diseases in a dose-dependent manner (7–9).
Food sources of n–3 PUFAs are limited, even in Japan. Fish, shellfish, and edible fats and oils are the most important sources of n–3 PUFAs (Figure 6). Pulses and cereals also contribute a significant portion of n–3 PUFAs. In contrast, the sources of n–6 PUFAs are more diverse. Although edible fats and oils contribute far more n–6 PUFAs than does any other food source, pulses, cereals, fish and shellfish, meats, and eggs also provide meaningful amounts of these fatty acids.
FIGURE 6. . Contributions of dietary polyunsaturated fatty acids (PUFAs) by food groups. Values are means of 1971–1990 data. Reproduced with permission from reference 6.
Fish and shellfish now provide 5–6 g fat and 1–2 g n–3 PUFA per person per day in Japan. About 60% of these fats are from fresh fish: horse mackerels, sardines, and tuna are quantitatively the most important sources. These fish are usually rich in n–3 PUFAs, in particular docosahexaenoic acid (DHA; 22:6n–3) and eicosapentaenoic acid (EPA; 20:5n–3), which constitute 15–35% of total fatty acids by weight. Not all fish are consumed raw in Japan; a considerable portion is eaten after cooking. In addition to salted and dried fish, shellfish are also good sources of n–3 PUFAs.
As mentioned previously, another aspect of the Japanese diet that contributes to the relatively high level of consumption of n–3 PUFAs is that, almost exclusively, only 2 edible vegetable oils, soybean and rapeseed oils, are consumed. These oils represent a total of 65–70% of the vegetable oils consumed (10) and contain -linolenic acid.
Although the Japanese are currently eating 80 g fish and shellfish/d, consumption is gradually decreasing, particularly in younger persons in keeping with the westernization of their diets. The fish intake of the young is less than half that of the adult or aged population (2). As a result, serum lipids of young people have a higher ratio of n–6 to n–3 PUFAs, and this ratio decreases with age (Figure 7; 11). Consequently, the ratio of EPA to arachidonic acid (AA; 20:4n–6) is the reverse of the ratio of n–6 to n–3 PUFAs. A similar pattern also was reported by Nakamura et al (12). The ratio of n–6 to n–3 PUFAs in dietary fat also differs depending on location within Japan and, in some cases, with one's occupation (10).
FIGURE 7. . Age-related differences in serum lipid polyunsaturated fatty acid (PUFA) concentrations of Japanese during 1985–1987. Means of data from 394 subjects (n = 233 M, 161 F). EPA, eicosapentaenoic acid; AA, arachidonic acid. Reproduced with permission from reference 11.
In the 1994 National Nutrition Survey (2), a question about fish intake was included. The survey found that >50% of young adults ate fish regularly. However, >60% were satisfied with their current low level of fish consumption (less than once a week).
It is possible that the fish-eating habits of the Japanese will change to an appreciable extent in the near future as a result of small but cumulative changes. Because of this, and for other reasons, we must develop a new national fish menu that appeals to the younger generation and prevents further loss of n–3 PUFAs. In this context, the report of Vatten et al (13) is of interest. In an epidemiologic analysis in Norwegian women, they showed no association between the overall frequency of fish for dinner and breast cancer incidence, although fish was more effective than meat in reducing breast cancer risk. However, and interestingly, they observed a borderline negative association between breast cancer incidence and the frequency of main meals containing "fish in a poached form." In contrast, no association was observed with fried fish or minced fish combined with milk and starch, ie, balls and puddings. This observation may indicate the importance of the way fish is prepared and served. The Japanese have a custom of eating raw fish, but boiled fish may be more preferable to affect certain aspects of human physiologic functions. The elucidation of the mechanisms underlying this interesting phenomenon deserves future study.
However, irrespective of this situation, the average net n–3 PUFA intake has not changed demonstrably, as described above. This is mainly due to the current preference for cultured fish over wild fish. At present, more of the fish consumed is from cultured than from wild sources; cultured fish are in general more fatty and less expensive, whereas the content of n–3 PUFAs in cultured fish is generally comparable with those of wild fish (Table 1) (14). Consequently, cultured fish as a whole can provide more n–3 PUFAs than wild fish per gram consumed. Although the variety of cultured fish is increasing, the fat content of cultured fish is unfortunately gradually being reduced to the amounts contained in wild fish to give the cultured fish natural taste and texture.
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TABLE 1.. Polyunsaturated fatty acid composition of wild and cultured fish in Japan1
DIETARY PUFA AND BLOOD FATTY ACID COMPOSITION
Reflecting these fat consumption patterns, the Japanese have higher concentrations of plasma lipid n–3 PUFAs than do the inhabitants of Western countries. An example is shown in Table 2, in which the comparisons were made between Japanese, Japanese Americans, and white Americans (15); n–3 PUFA concentrations were >3 times higher in the Japanese living in Japan than in the Americans. It is interesting that the proportion of n–6 PUFAs is higher in Americans than in Japanese, reflecting the difference in the ratio of n–6 to n–3 dietary fats. Similarly, breast milk from Japanese mothers contains more n–3 PUFAs, in particular DHA, than that from mothers in Western countries (16, 17).
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TABLE 2.. A comparison of fatty acid composition of human plasma total lipids1
RECOMMENDED DIETARY RATIO OF n–3 TO n–6 PUFAS
Several countries have recommend allowances not only for the absolute amount of PUFAs, but also the balanced intake of n–6 and n–3 PUFA. These recommendations are summarized in Table 3 (3, 18, 19). Although fish is an excellent source of n–3 PUFAs, fish consumption is limited in the world, and only a few countries eat fish regularly by custom. The ratio of n–6 to n–3 PUFAs of 4–10:1, which is now recommended for some Western countries, however, is without a firm theoretical basis. It may be difficult to reduce the ratio below 10:1 in these countries.
View this table:
TABLE 3.. Recommended dietary allowance of fat in the world1
In contrast, it is not difficult to develop a menu with a ratio of n–6 to n–3 PUFAs of 3–4:1 in Japanese homes. By using a model menu, diets that contain reasonable amounts of total PUFAs (15–17 g/d) and have a ratio of n–6 to n–3 PUFAs of 3.5–4.2:1 can be prepared, even in areas in which dietary habits are considerably different (20).
However, this model menu cannot be generalized and the deviations from this goal are numerous. It is curious, for example, that not all students in women's nutrition colleges in Japan understand what the RDA means. Thus, as shown in Table 4, many undergraduate students in these colleges are eating more n–6 PUFAs and less n–3 PUFAs than is recommended (21–25). This finding suggests a sizable diversity of PUFA intake in Japan, particularly among younger Japanese. The variation is also observed in total fat consumption. It is surprising that only 45% of Japanese families are consuming the recommended amount of fat, which is 20–25% of energy. In this sense, only a limited number of Japanese are following a healthy way of eating. Hence, a more exhaustive nutritional education effort is necessary.
View this table:
TABLE 4.. Fat intake of students in women's dietician colleges in Japan
The classic international comparative study by Keys (26) that was designed to relate customary diets to serum cholesterol concentrations and, hence, potential heart risk, revealed sizable differences in these indicators within Japan as a result of the geographic differences in eating habits. However, the difference is gradually disappearing.
Finally, we would like to comment on the role of fish as a source of calcium and vitamin D. Calcium intake in Japan has been 10% lower than the recommended amount for >20 y (2). In addition, >30% of Japanese have low calcium intakes. In this situation, the contribution that fish makes to calcium and vitamin D intake, 15% and 80% of total intake, respectively, is of great value (2).
In conclusion, we stress the importance of eating moderately, including fat. In this context, the Japanese way of eating, "eat more kinds of foods, but in smaller amounts," appears to be the most important instruction for our health.
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