Infrequent home-cooked meal consumption is related to the inadequacy of dietary bre and minerals intake among Japanese adults aged 18-64 years: analysis from the 2015 National Health and Nutrition Survey

Background: Consumption of home-cooked meals may lead to better nutritional intake. Few studies have examined the effect of frequency of home-cooked meal consumption on inadequacy of nutritional intake based on nutritional standards. We therefore aimed to examine the association between the frequency of home-cooked meal consumption and nutrient intake inadequacy among Japanese adults. Methods: This study was a secondary analysis of the 2015 National Health and Nutrition survey in Japan, involving 921 men and 1300 women aged 18–64 years, a cross-sectional survey. The frequency of home-cooked meal consumption was determined using two questions enquiring about the frequency of eating out and take-away meals. Data on dietary intake were collected using a one-day semi-weighed household dietary record. The inadequacy of each nutrient intake was assessed by comparing estimated average requirement (EAR) level for 14 nutrients and the range of the dietary goal (DG) for seven nutrients according to the 2015 version of the Dietary Reference Intake for Japanese. Participants were stratied into three groups based on the frequency of consuming home-cooked meals. Group differences in EAR and DG were assessed using the covariate and logistic regression analysis, respectively. Results: Among men and women, the proportion of participants who consumed home-cooked meals almost every day and meals prepared out of home at least once a day were 34.9% and 46.8%, and 14.7% and 6.3%, respectively. A higher frequency of consumption of home-cooked meals was associated with higher intake and adequacy of dietary ber and minerals (iron, calcium, potassium etc.), and with higher intake of vegetables and lower intake of oils. Conclusions: Low frequency of home-cooked meal consumption was associated with insucient intake dietary ber multiple mineral Japanese Dietary and intake may This cross-sectional study indicated that Japanese adults aged 18–64 years with lower frequency of home-cooked meal consumption were less likely to meet the standard values of dietary ber and mineral intake. Our ndings suggest that dietary ber and mineral intake may be the focus of an interventional approach to improve the nutrient intake status of those with low home-cooked meal consumption among Japanese adults. Further studies targeting food environment, including the food industry, are needed to improve nutritional adequacy for those with higher frequency of eating out or consuming take-away meals.

Dietary intake data was collected using a one-day semi-weighed household dietary record administered in November 2015, excluding Sundays and public holidays. Prior to completing the survey, trained eldworkers (mainly registered dieticians) provided an outline of the survey and explained to the participants how to complete the dietary record. The main record-keepers in the household (members who are usually responsible for preparing meals) were instructed to weigh all foods and beverages consumed by the household members and the amount of food waste and leftovers and record their names and weights on recording forms. Additionally, the main record-keepers recorded the approximate proportions of the food consumed by each household member when members shared foods from the same dish to enable estimation of individual intake. If weighing was not possible because of eating out, the portion size consumed, or quantity of foods and details of any leftovers was recorded. Trained eldworkers visited each household and checked for any missing information and errors. In accordance with the survey manual of the NHNS, the trained eldworkers converted these estimates of portion sizes or quantity of foods into weights of foods and coded each food item, according to the NHNS food number lists based on the Standard Tables of Food Composition in Japan [34] to calculate the intake of energy and nutrients. The trained eldworkers inputted collected dietary intake data using software speci cally developed for the NHNS.
Energy and nutrients were calculated based on the 2010 Standard Tables of Food Composition in Japan, and foods were classi ed into 17 groups based on its food group table. [34] In this study, we adjusted the reported dietary intake based on the assumption that each participant reported the estimated energy requirement (EER) when their physical activity level was at the second level, to render the comparison between the reported nutrient intake and the Dietary Reference Intake for Japanese (DRIs) [28] values practically possible. The following calculation was used: dietary intake (unit/day) = reported dietary intake (unit/day)/reported energy intake (kcal/day) × EER (kcal/day). For protein, total fat, saturated fat, and carbohydrate, %energy for each nutrient was also calculated. Additionally, food intake values were energy-adjusted using the density method (i.e. their amounts per EER for food groups) to minimise the in uence of dietary misreporting.

Frequency of home-cooked meals
The frequency of home-cooked meal consumption was assessed by the combination of two questions asking about the frequency of eating out and take-away meals. Participants reported the frequency of eating out and take-away meals (twice a day or more, once a day, 4-6 times per week, 2-3 times per week, once a week, less than once a week, seldom) in the lifestyle questionnaire. The classi cation of participants into three groups according to the frequency of home-cooked meals consumption is shown in Figure 1. Participants who answered, "twice a day or more" to either question and those who answered, "once a day," "4-6 times a week" or "2-3 times a week" to both questions were classi ed into the Low group (low frequency of home-cooked meal consumption). Participants who responded to both questions "once a week," "less than once a week," "seldom" were classi ed in the High group (high frequency of home-cooked meal consumption). If none of the above applies to those, participants were classi ed into the Middle group.

Determination of inadequate nutrient intake
Inadequate intake of each nutrient was determined by comparing consumed nutrient levels with the relevant dietary reference value according to the Japanese DRIs, using a previously reported method. [35][36][37] In the Japanese DRIs, different types of dietary reference values were established according to their purpose. The estimated average requirement (EAR) is set to prevent insu cient intake of nutrients, whereas the tentative dietary goal to prevent lifestyle-related diseases (DG) is set to prevent noncommunicable diseases.
Nutrient inadequacy was de ned as follows: intake level below EAR was considered as inadequate using the cut-point method for the following 14 nutrients with known EARs: protein, vitamin A (as retinol activity equivalents), vitamin B1, vitamin B2, niacin (as niacin equivalent), vitamin B6, vitamin B12, folate, vitamin C, calcium, magnesium, iron, zinc, and copper. Regarding iron intake in menstruating women, we applied the value <9.3 mg/day as recommended by the World Health Organisation (WHO) (bioavailability of iron as 15%, probability of inadequacy as 50%) [38] for women aged 20-49 years because the cut-point method is less applicable to these populations. [39,40] For the following seven nutrients, the intake level outside the range of DG values was considered as inadequate: protein (as % energy), total fat (as % energy), saturated fat (as % energy), carbohydrate (as % energy), total dietary ber, sodium (as saltequivalent), and potassium.

Other variables
Body height (to the nearest 0.1 cm) and weight (to the nearest 0.1 kg) were measured for approximately 90% of the participants by trained eld workers according to standardised procedures. For the remaining participants, height and weight were measured either by other household members at home or were selfreported. BMI was calculated as weight (kg) divided by height (m) squared. Smoking status and alcohol drinking habits during the preceding month were assessed by a self-administered questionnaire.

Statistical analysis
All statistical analyses were strati ed by sex. The differences in characteristics among three groups according to the frequency of the home-cooked meal consumption were compared using the chi-square test for categorical variables and analysis of variance (ANOVA) for continuous variables. Differences in daily nutrients and food intake among the three groups according to the frequency of home-cooked meal consumption were assessed by ANOVA in the crude model and a covariate analysis (ANCOVA) in the adjusted model. Dunnett test, with the high group as reference, was performed in the post-test. The nutritional inadequacy of each nutrient intake was represented as the proportion of participants whose intake was above the EAR or in the range of the DG in each group. Logistic regression analysis was used to examine the difference in the prevalence of meeting DRIs based on the Low and Middle groups according to the frequency of the home-cooked meal consumption compared with the High group in the crude and adjusted model. Confounding factors considered in the adjusted model were age category (18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50), and 51-64 years), occupation (professional/manager, sales/service/clerical, security/transportation/labour, student, housekeeper, and not in paid employment), living alone or not (yes or no), region (Hokkaido/Tohoku, Kanto, Hokuriku/Tokai, Kinki, Shikoku/Chugoku, Kyusyu), current smoker (yes or no) and habitual alcohol drinker (yes or no), which was reported as a factor affecting the frequency of consumption of meals prepared away from home [8,27]. All statistical analyses were performed with SAS statistical software, version 9.4 (SAS Institute Inc., Cary, NC, USA). All reported P values were two-tailed, with a P-value <0.05 considered statistically signi cant.

Results
The basic characteristics of participants according to frequency of home-cooked meal consumption are shown in Table 1. The proportion of participants classi ed into the High, Middle, and Low groups were 34.9%, 50.5%, and 14.7% for men, and 46.8%, 46.9%, and 6.3% for women, respectively. There were signi cantly fewer young men and women in the high frequency of home-cooked meal consumption group. Additionally, men and women living alone had signi cantly less home-cooked meal consumption (p <0.001). The region signi cantly differed in women, with more women in the Low group living in the urban Kanto area (p = 0.002). In the Low group, the proportion of people eating out or having take-away meals were approximately 90% of men and 70% of women, which was signi cantly higher both among men and women than in other groups. There were no differences in mean BMI, type of occupation, current smoking and consumption of snacks both among men and women.
SD, standard deviation * Means for continuous values were compared by an analysis of variance and proportions for categorical values were compared by the chi-square test between High, Middle and Low groups. Table 2 shows the nutrient intakes on the dietary recording day according to the frequency of homecooked meal consumption. Among men, the intake of protein, calcium, iron, copper, dietary ber and potassium was signi cantly lower in the Low group than in the High group (p = 0.020, 0.044, 0.008, 0.027, 0.002 and 0.004, respectively). While in women, the intake of folate, calcium, magnesium, iron and potassium in the Middle group was signi cantly lower than in the High group (p = 0.046, 0.036, 0.014, 0.001 and 0.026, respectively). Dietary ber intake was higher in the High group compared the Middle and Low groups (p = 0.005).  The p values are shown for an analysis of variance to analyze differences of nutrient intake between three groups. ¶ The p values are shown for covariate analysis to analyze difference of nutrient intake between three groups adjusted for confounding variables of age category (18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50), and 51-64 years), occupation (professional / manager, sales / service / clerical, security / transportation / labour, student, housekeeper, and not in paid employment), living alone (yes or no), region (Hokkaido and Tohoku, Kanto, Hokuriku and Tokai, Kinki, Shikoku and Chugoku and Kyusyu), current smoker (yes or no) and habitual alcohol drinker (yes or no). * There is signi cant difference by Dunnett test compared with High group in the adjusted model.
The multivariate-adjusted odds ratios (ORs) for nutrient intake adequacy according to the frequency of home-cooked meal consumption are shown in Table 3. Only few men and women had inadequate intake of protein, niacin and copper compared with EAR. Among men, the proportion of those having inadequate intake of iron, protein %energy, dietary ber, and potassium in the Low group was signi cantly higher than in the High group.  Percentage of subjects whose intake was in the range of DG or above the EAR. Each energy-adjusted nutrient intake (unit/d) was compared with each DRI value (unit/d), using the cut-point method.

Discussion
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 ber 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 rst 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 classi ed 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 insu cient intake status may re ect 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 insu cient intake, whereas DG is set for the prevention of non-communicable diseases, which may have determined differently the de nition of inadequacy of each nutrient intake.
Dietary ber was the only nutrient that signi cant differed depending on the frequency of home-cooked meal consumption. This nding was largely consistent with the previous studies that reported the association the frequency of eating out with dietary ber intake. [2,8] Lower dietary ber 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 ber and healthy food intake despite more frequent eating out and take-away. [8] These reports may partly explain our present nding 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 signi cant 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 ber 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 in uenced the ndings. 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 coe cients 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 di cult 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.

Conclusions
This cross-sectional study indicated that Japanese adults aged 18-64 years with lower frequency of home-cooked meal consumption were less likely to meet the standard values of dietary ber and mineral intake. Our ndings suggest that dietary ber and mineral intake may be the focus of an interventional approach to improve the nutrient intake status of those with low home-cooked meal consumption among Japanese adults. Further studies targeting food environment, including the food industry, are needed to improve nutritional adequacy for those with higher frequency of eating out or consuming take-away meals. Figure 1 Classi cation of study participants based on frequency of consuming home cooked meals

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