The present study examined the association between the frequency of home-cooked meal consumption and nutrient inadequacy among Japanese adults aged 18–64 years. We found that inadequate intake of dietary fiber and several minerals was associated with a higher frequency of eating out or take-away meal. To the best of our knowledge, this study is the first to examine the relationship between the frequency of home-cooked meal consumption and nutrient inadequacy, based on dietary reference values among Japanese adults.
In this study, participants were classified into three groups (Low group, Moderate group, and High group according to the frequency of the home-cooked meal consumption) based on the response to questions about the frequency of eating out and take-away meals. The results by the questionnaire on the frequency of habitual eating out and take-away meals assessed was comparable to results in the dietary survey, despite being based on one-day dietary record method.
Several factors increased the likelihood of eating food prepared away from home. Men and younger people had a lower frequency of consuming home-cooked meal. These results were consistent with those from previous studies that showed a higher frequency of eating out among men and younger adults when compared with that among older adults[13, 41], or higher proportion of eating out in men than women[13]. Similar to a previous study among Japanese university students [36], living alone was associated with a higher frequency of eating meals prepared away from home. In addition, women living in urban areas had less home-cooked meal consumption, which is consistent with the result of a Vietnamese study.[42] Thus, the current results may indicate that younger adults, especially men, are more likely to consume meals prepared away from home in Japan as well as other countries.
The association between the frequency of consuming home-cooked meals and nutrients intake has been reported in several studies. An Australian study that used a 24-hour dietary recall reported that adults with a higher frequency of consuming foods prepared outside the home has lower iron and calcium intakes[2], which is similar to the results of iron and calcium intakes in the present study. However, inadequate intake of these nutrients based on dietary reference values was not observed except for iron among men in the present study. According to a previous Japanese study, approximately more than 50% of Japanese adults had inadequate intake of calcium.[43] Also, another study showed that the proportion of Japanese women who met the standard value of iron intake was low, whereas a large percentage of Japanese men met the standard.[44] Japanese usual insufficient intake status may reflect to the present results, regardless of the frequency of home-cooked meal consumption. In contrast, Japanese people rarely lack copper and protein[43], which may explain the no difference in inadequacy of these nutrients according to the frequency of home-cooked meal consumption. Of note, EAR is set by the perspective of avoiding insufficient intake, whereas DG is set for the prevention of non-communicable diseases, which may have determined differently the definition of inadequacy of each nutrient intake.
Dietary fiber was the only nutrient that significant differed depending on the frequency of home-cooked meal consumption. This finding was largely consistent with the previous studies that reported the association the frequency of eating out with dietary fiber intake.[2, 8] Lower dietary fiber intake was observed in the Low group in men, and the Middle group in women; this trend was similar for other nutrients. It has been reported that women in Japan and elsewhere cook more often than men.[27, 45] Higher income is associated with a higher frequency of eating out and take-away meals.[41] Additionally, better diets are seen in women compared with men [46] and high educated individuals have greater dietary fiber and healthy food intake despite more frequent eating out and take-away.[8] These reports may partly explain our present finding that lower frequency of home-cooked meals is associated with lower nutrients intakes and inadequacy of nutrient intake compared with that in the High group. Socioeconomic factor may be one of the important factors associated to home-cooked meals consumption Unfortunately, other than occupation, we did not consider other socioeconomic indicators. While the proportion of professional, manager, sales, service, and clerics differed among men and women, and was higher in the Low group, there was no significant different in occupation among the groups. Thus, future studies should consider socioeconomic factors such as educational background and income level.
The frequency of eating out and take-away meals has been associated with a lower intake of vegetable and a higher intake of fat and oils. [8, 16] These results are consistent with the present study. Low intake of vegetables may partly explain the inadequate intake of potassium among men, and inadequate intake of magnesium and vitamin C among women. There has been no report on potassium and magnesium intake and inadequacy based on frequency of home-cooked meals consumption. These results highlights the need for health promotion for people with a higher frequency of eating out or take-away meals, as well as for food industry.
In this study, approximately 45% of men and 30% of women regularly ate out or had take-away meal consumption. In Japan, the government has called for voluntary efforts among food industry to improve the food environment so that people can eat well-balanced meals, whether they eat out or prepare for themselves. Example of such effort include “Increase in number of corporation in food industry that supply food products low in salt and fat.”[47] However, the current recommendation focuses on preventing excess intake. Further efforts by the government is needed to increase the population intake of dietary fiber and minerals.
The study had some limitations. First, the participants were randomly selected from nationally representative households in Japan; therefore, the individual level response rate was unknown. This might have introduced some bias in the estimation of average intake in Japanese adults. Second, a dietary intake derived from one-day weighed dietary record is unlikely to represent the usual intake. Therefore, the variability in the dietary intake of individuals over a period of several days might have influenced the findings. It is noted that the one-day household-based dietary record method used in NHNS has been compared with individual dietary records among Japanese participants, and the correlation coefficients of the intakes of total energy and macronutrients, such as protein, fat, and carbohydrates were high (0.89 to 0.91). Thus, this method is fairly valid for the estimation of individual intake[48]. Third, it could have been difficult for participants to accurately weigh food consumption in the case of eating out, take-away, or ready-meal use, unlike when they consumed home-cooked meals and could weigh all the foods and beverages, including the amounts of food waste and leftovers. Therefore, nutrient and food intakes may not have been accurately assessed. Fourth, we limited the participants to those who had three meals a day in the present analysis, because we wanted to assess nutrient intake and nutrition adequacy by the difference in the frequency of home-cooked meals consumption. This might have induced some bias in the nutrient intakes. Finally, factors other than the frequency of home-cooked meals consumption may also affect the adequacy of nutrient intake. Future studies should examine the causes of nutrient intake inadequacy.