This study clarified the relationship between poverty and the frequency of breakfast intake and each food group among Japanese high school students. The strengths of this study are the large number of subjects than that for in previous studies, the precise definition of poverty, and confounding adjustments. High school students from poverty group consumed "SSB," "instant noodles," and "fast food" more frequently than those from non-poverty group. This association remained significant even after adjusting for confounders. High school students in the poverty group also tended to consume breakfast, fish and meat, fish and meat products, vegetables, fruits, and dairy products less frequently than those in the non-poverty group. However, after adjusting for confounders, significant associations disappeared. The trend shown in this study of higher consumption of SSBs, instant noodles, and fast food among the poverty group is generally consistent with trends in systematic reviews of diet and socioeconomic disparities among adolescents and young adults . A previous study reported that the consumption of SSB and energy-dense foods is associated with socioeconomic status worldwide. Consumption of SSBs and energy-dense foods tended to be higher when socioeconomic status indicators were unfavorable. Other foods, in addition to socioeconomic status-related indicators, were reported to be very specific to each country or region and influenced by ethnic and immigrant disparities. One mechanism for the link between poverty and diet is that energy density and costs are inversely related . People living in poverty try to reduce food costs and choose cheaper and more energy dense foods to maintain their dietary energy. SSBs, instant noodles, and fast food are classified as ultra-processed foods (UPFs) . UPF is an inexpensive, energy-dense, and nutrient-dense food with lower nutrient density than fresh foods. The UPF was designed to produce a low-cost, profitable product. It is very tasty, easy to prepare, and convenient for food. However, they are high in energy intake, refined carbohydrates, saturated fatty acids, salt, and food additives, and low in micronutrients and dietary fiber . In recent years, it has been shown that high purchase and consumption of UPF are associated with overweight and obesity. In a cross-sectional study in Brazil , the price of UPFs was inversely associated with the prevalence of overweight and obesity, mainly among those with the lowest socioeconomic status. A systematic review of childhood and adolescence  found a positive correlation between UPF consumption and body fat. In this study, the poverty group consumed UPF more frequently than the non-poverty group, and had a higher prevalence of obesity. Although the association between UPF consumption and obesity was not examined, the association between poverty and obesity among high school students in this study may be mediated by UPFs, such as SSB, fast food, and instant noodles. Breakfasts, fish and meat, fish and meat products, vegetables, and milk/dairy products were consumed less frequently in the poverty group than in the non-poverty group.
A cross-sectional study of Japanese adults  found that households with lower incomes consumed more staple foods and fewer vegetables, fruits, and fish. A cross-sectional study  of Japanese elementary school children reported that children from low-income households had less frequently of eating out, consumed less breakfast and vegetables and more fish and meat products and instant noodles than children whose households were high-income households. The results of this study were similar for breakfast, fish, vegetables, and instant noodles. However, fish and meat products showed different trends in previous studies. In this study, the non-poverty group consumed fish and meat products more frequently than the poverty group. One possible reason for this is that the dietary culture of Okinawa Prefecture, the study area where fish and meat products are often eaten, outweighed the effects of poverty. People of Okinawa frequently eat fish and meat products, such as pork luncheon, corned beef, and canned tuna. After World War II, these canned goods were introduced to the United States, where they are still popular . The results of the national household survey show that the purchase of processed meats other than ham, sausage, bacon, and canned seafood is the highest in Japan .
This study has several limitations. First, because of the cross-sectional research design, it was not possible to establish a causal relationship between poverty and diet among high school students. However, the reverse causality and eating habits of high school students are unlikely to affect the poverty status of households. Second, the survey was conducted in a specific region, limiting the generalizability of the results. Okinawa Prefecture, the target of the survey, is a remote island located at the southernmost tip of Japan. It is also characterized by the extremely poor socioeconomic conditions of residents. The per capita income of the prefecture is the lowest in the country  and the divorce rate  is the highest in the country . However, we have the advantage of revealing the reality of the most impoverished areas in Japan. The third point is the possible bias due to missing values (n = 1,467). The results may be underestimated because the sample that dropped out had a higher percentage of poverty. Fourth, the frequency of food intake was based on a self-administered questionnaire rather than on an actual food survey. Recall bias, underestimation, and overestimation bias are inevitable. Finally, the food preferences of high school students were not adjusted. Fruits and vegetables have been found to increase in intake with increasing preferences .