Recently, obese subjects have increased because of the change of lifestyles such as westernization of the eating habits or the lack of exercise in Japan. Prevention of obesity is important to prevent metabolic syndrome such as dyslipidemia, hyperglycemia, hypertention and arteriosclerotic disease. Obesity, which is induced by multiple causes, is mainly determined by the interaction between genetic and environmental factors. To search the gene associated with obesity, candidate gene analysis and whole genome analysis are carried out using the single nucleotide polymorphisms (SNPs). As the result, several candidate genes have been reported to associate with obesity. It is necessary to diagnose multiple SNPs associated with obesity in many subjects to prevent obesity as the personalized medicine for the future. The SmartAmp (Smart Amplification Process) method is expected to be applied clinically for personalized medicine because of the simple, rapid and accurate diagnosis of SNPs.

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Metabolic syndrome is today attributed to obesity-related (induced) metabolic defects. However, the essence of metabolic syndrome is ascribed to lipotoxicity caused by ectopic accumulation of lipids that damage and destroy nonadipose organs. Here, I would like to reconsider the lipotoxicity (steatosis) and the metabolic syndrome.

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Recent epidemiologic studies reported that the intake of saturated fatty acid and trans isomers of fatty acids associates the risk of the coronary heart disease (CHD). The free fatty acid including the saturated fatty acid deteriorates insulin resistance, glucose and lipid metabolism and obesity by its lipotoxicity and promotes the risks of cardiovascular disease, including inflammation and atherosclerosis. On the other hand, the unsaturated fatty acid can attenuate the activation of inflammation by the saturated fatty acid. A recent clinical trial revealed that highly purified eicosapentaenoic acid (EPA), an n-3 polyunsaturated fatty acid, reduces the incidence of cardiovascular diseases. Therefore, it is important and effective to guide the lipid management based on not only quantity of fat intake but also quality of it and the ratio of saturated fatty acid/unsaturated fatty acid in dietary therapy, to prevent the onset of CHD. In addition to the research of the effects of individual fatty acids, the lipidomics analysis of hundreds of fatty acids and eicosanoids, including their numerous metabolites, has lately attracted considerable attention.

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Dietary fat plays an important role in the human body. The prevalence of obesity has risen dramatically over the two decades in Japan. There is a difference opinion about whether the percentage of dietary fat plays an important role in the rising prevalence of obesity and in its treatment. Overall, it is necessary for us to understand the nutritional role of fat and to take proper measures with bodyweight gain.

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Fat is the most energy dense macronutrient and attractive for animals. They have special brain system which actively consumes fat. This system consists of the hypothalamus, the limbic system and the basal ganglia. The hypothalamus acts as center of general feeding regulation and maintain homeostatic energy balance of organism. The hypothalamic feeding regulation highly depends on the energy balance and less leads to excessive energy intake. On the other hand, the limbic system and the basal ganglia control feeding less dependently on the energy balance. Limbic system mainly promotes palatability-induced feeding which is mediated by endogenous opioids and cannabinoids. Basal ganglia system manages reinforcement learning of feeding behavior which may cause development of habit-based eating. High-fat diet is highly palatable and highly reinforcing, and therefore, is highly influenced by the limbic and basal ganglia systems-mediated feeding control. These energy balance free feeding may promote feeding high-fat diet and, therefore, cause increasing of obesity in our modern food abundant society.

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Compared with other countries, Japan has a low incidence of obesity. However, obesity has been defined as the main cause of metabolic syndrome, and in Japan a national system of special annual health examinations has been initiated to counter and prevent metabolic syndrome.
Following WWII, in order to improve the quality of the Japanese diet, it was recommended that Japanese move from a rice centered diet, to a diet higher in protein and fats through increased dairy and meat consumption. It has been thought that this change in diet, especially the higher intake of fats, resulted in higher obesity rates. This paper surveys fat intake and obesity rates, but the close relation between the two suggested by epidemiological surveys and papers from researchers in this field were not found. For example, over the past 10 years, males in their 30s and 40s have decreased their fat consumption by 10 percent; nevertheless, the incidence of obesity in these age groups increased by 10 to 20 percent. Despite this, the “healthy oils” and “zero fat” products are widely consumed, reflecting the persistent idea that dieting means decreasing fat intake. Fats are important structural and physiological components of food; therefore restricting fat intake necessarily has a large effect on health. On the other hand, with the development of food technology, many foods now contain fat, and since the energy content of fat is high, it has been easy to assume a connection between fat intake and obesity. Therefore along with the increased interest in health and recognizing fat as an essential nutrient, it is important to develop guidance through dissemination of information, development of new products and education in schools so that consumers can use fats wisely.

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