Compared with other countries worldwide, Japan has one of the longest life expectancies [1
]. We previously investigated the Japanese diet, which is a lifestyle component unique to Japan, as a factor favoring longevity. The Japanese diet has changed since the end of World War II; accordingly, we evaluated the effects of the diets of each era on the health of mice [2
]. Specifically, the diets of 2005, 1990, 1975, and 1960, according to food consumption surveys, were recreated by mixing 21 foods typically consumed during 1 week and freeze-drying and pulverizing the mixture. Subsequently, the diets were provided to mice. The 1975 diet, with a high miso content, had more pronounced anti-obesity effects than the current Japanese diet [2
Consumption of miso by Japanese people has decreased continuously since 1975, with the consumption rate in 2010 approximately half that in 1975 [3
]. Miso is prepared by mixing soybeans, malted rice, wheat or barley, and salt and allowing the mixture to ferment and mature. The principal ingredient, soybeans, contains large quantities of soybean-specific proteins (glycinin and β-conglycinin), lipids rich in polyunsaturated fatty acids, vitamin E, lecithin, saponin, and isoflavones (genistein, daidzein, daidzin, and glycitein) [5
]. Therefore, soybeans are a functional food, and fermentation thereof results in a high nutrient content that includes amino acids and isoflavones. Miso has previously been reported to effectively prevent lifestyle-related diseases such as cancer, hypertension, and hypercholesterolemia [5
]. With its antioxidant activities, miso may play a role in preventing aging due to oxidative stress [8
]. However, the anti-obesity effects of miso have been rarely investigated.
Obesity leads to metabolic disorders such as dyslipidemia and type 2 diabetes and is linked to life-threatening conditions such as arteriosclerosis; thus, prevention of obesity is critical [9
]. Most previous research has focused on single food components in miso and not the functionality of miso itself in the diet. Moreover, most research has focused on the efficacy of miso components at a very high level, with almost no investigation of their efficacy at concentrations present in a real diet.
In addition to diet, exercise is effective for the prevention of obesity. Exercise increases the energy demand of skeletal muscles, which leads to energy generation by mitochondria, with sugars and lipids as substrates [11
]. Along with substrates present in tissues, muscles generate energy using glucose and fatty acids extracted from the bloodstream [12
]. Furthermore, lipolysis is promoted in adipose tissues, forming fatty acids, whereas in the liver, glucose is formed by gluconeogenesis; these substances are subsequently transferred to the blood and then transported to the muscles [12
]. Exercise thus promotes energy consumption. Food components such as catechin and chlorogenic acid are known to affect this pathway and thus potentiate the effects of exercise [14
]. Therefore, foods containing such components are considered to be highly beneficial for maintaining health.
In this context, the present study focused on miso and the effects of its consumption on the development of obesity. Two test diets containing miso at levels similar to those in ordinary Japanese diets in 1975 and 2010 based on food consumption surveys were prepared. The effects of these test diets on obesity in mice were then investigated. Subsequently, we investigated whether the combination of exercise and miso consumption potentiates the anti-obesity effects of exercise.
In Experiment 1, we investigated the effects of consumption of miso at quantities similar to those in typical Japanese diets in the years 1975 and 2010 on obesity in mice. When mice were provided the 2010 and 1975 test diets with miso contents of 1.6% and 2.6%, respectively, the mice provided the 1975 miso diet had significantly lower white adipose tissue weight than those provided the 2010 miso diet. In addition, adipocyte size was significantly lower in the 1975 group than the control group. These findings revealed that high miso consumption suppresses fat accumulation in white adipose tissue and thus has anti-obesity effects.
Soybeans contain large quantities of soybean-specific proteins (glycinin and β-conglycinin), lipids rich in polyunsaturated fatty acids, vitamin E, lecithin, saponin, and isoflavones (genistein, daidzein, daidzin, and glycitein). In particular, soybean proteins and isoflavones have been shown to reduce visceral fat accumulation in rats and mice with genetic or diet-induced obesity [27
]. The suppressive effect of miso on visceral fat accumulation observed in this study suggested that these miso components function in an additive or synergistic manner. In addition, this effect was achieved at a level common to an actual human diet, rather than at artificially high levels.
Miso was a staple of the 1975 Japanese diet, which had high anti-obesity effects. This study showed that miso suppressed obesity induced by a high-fat diet and, therefore, may be partially responsible for the anti-obesity effects of the 1975 Japanese diet. The difference in miso content between the 1975 and 2010 test diets was only approximately 1 g per 100 g—equivalent to one to two bowls of miso soup in a human diet—yet different effects on the development of obesity were found. We recently showed that consumption of natto, another fermented soybean product, or the 1975 Japanese diet markedly changes the intestinal microflora [29
]. Changes in the intestinal microflora are known to affect the development of numerous diseases, and it is therefore possible that the results of the present study are attributable to changes in the intestinal microflora.
Experiment 2 was conducted to determine whether the known anti-obesity effects of exercise are potentiated by combination with miso consumption. We confirmed that miso consumption in combination with exercise potentiates suppression of fat accumulation in white adipose tissue and the liver, in comparison with exercise or miso intake alone. In addition, adipocyte size was decreased in this group. To elucidate the mechanism underlying these effects, the expression of genes associated with lipid metabolism was measured in the white adipose tissue, and it was found that Pparγ
mRNA was increased by exercise and further increased by miso consumption. Pparγ
is a transcription regulatory factor that is expressed at high levels in adipocytes and regulates adipocyte differentiation and proliferation [31
]. In a study in which rats fed high-fat diets were administered Pparγ
agonists, the number of hypertrophic adipocytes was reduced, and the number of miniaturized adipocytes was increased [32
]. Therefore, Pparγ
is considered to play important roles in suppressing excessive adipocyte hypertrophy and ensuring normal adipocyte size [33
]. Moreover, macrophage infiltration and inflammatory cytokine secretion increase in connection with adipocyte hypertrophy and, therefore, suppression of adipocyte hypertrophy is important for preventing obesity and insulin resistance [34
]. Furthermore, owing to its influence on metabolism in various organs, prevention of adipocyte hypertrophy is critical. In the present study, exercise and miso consumption suppressed adipocyte hypertrophy, conferring a positive effect on the whole body. Moreover, this combination significantly reduced blood glucose. Exercise-induced activation of Pparγ
in adipose tissue promotes lipolysis, mediated by Hsl
]. In the present study, high expression of Hsl
mRNA was observed as a result of exercise and miso consumption. Exercise-induced, Hsl
- and Atgl
-mediated lipolysis plays an important role in the systemic supply of fatty acids [37
] and is linked to suppression of fat accumulation in adipose tissues. This is not a transient effect occurring only at the time of exercise, as long-term effects have been confirmed when exercise is continued. Therefore, Pparγ
activation due to exercise and miso consumption was considered to promote Hsl
- and Atgl
-mediated lipolysis, thereby reducing white adipose tissue weight.
Long-term, habitual exercise results in muscle fiber hypertrophy [39
] and increased insulin sensitivity [40
]. In the present study, skeletal muscle weight and glucose metabolism were increased by exercise, and miso consumption potentiated these effects. The exercise regimen implemented in this study was of moderate intensity, at 50% to 75% of maximal oxygen consumption [41
], equivalent to gentle running by humans. At this exercise intensity, both carbohydrates and lipids are used as energy sources [43
], and the routine was, therefore, effective for suppression of visceral fat accumulation.
Exercise alone had no significant effects on hepatic fat accumulation due to a high-fat diet, but this accumulation was significantly reduced by miso consumption in combination with exercise. Expression of Acc
mRNA in the liver did not change with exercise alone, but it decreased significantly with miso consumption in combination with exercise. Acc
is the rate-limiting factor for fatty acid synthesis and, under the condition of a high-fat diet, Acc
suppression is known to suppress fatty liver development [44
]. Consequently, the combination of exercise and miso consumption was considered to suppress lipid synthesis in the liver. Soybean proteins, which are contained in miso, suppress hepatic fat accumulation in genetically obese rats, and suppression of hepatic fatty acid synthesis has been reported as one factor involved in this process [45
]. Similar effects have been reported with isoflavone [45
]. In addition, exercise resulted in decreased expression of genes associated with fatty acid β-oxidation in the liver, but this was considered a physiological response occurring in conditions of decreased lipid levels. Based on the aforementioned observations, the effects of various functional components of miso along with exercise synergistically suppress fat accumulation in the liver.