Consumption of meals prepared away from home is related to the inadequacy of dietary fibre and minerals intake among Japanese adults aged 18-64 years: analysis from the 2015 National Health and Nutrition Survey


 Background: Consumption of home-prepared meals may lead to better nutritional intake. Few studies have examined the association between the frequency of consuming meals prepared away from home and the adequacy of nutritional intake based on nutritional standards. We therefore aimed to investigate this issue 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, conducted as a cross-sectional survey. The frequency of consuming meals prepared away from home was defined using two questions inquiring 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 stratified into three groups based on the frequency of consumption of home-prepared meals. Group differences in nutrients inadequacy were assessed using the covariate and logistic regression analysis, respectively.Results: The proportion of participants who consumed home-prepared meals almost every day were 34.9% among men and 46.8% among women, and the proportion of those rarely consuming were 14.7% and 6.3%, respectively.A higher frequency of consuming meals prepared away from home was associated with lower intake and adequacy of dietary fiber and minerals (iron, calcium, potassium etc.), and with lower intake of vegetables and lower intake of oils.Conclusions: High frequency of consuming meals prepared away from home was associated with insufficient intake of dietary fiber and multiple mineral intakes among Japanese adults. Dietary fiber and mineral intake may be the potential target of interventions aimed at improving nutrient intake in individuals who predominantly eat food prepared away from home.

included households and family members (aged >1 year as of November 1, 2015) in 300 areas, were strati ed and randomly extracted from the general census areas in the Comprehensive Survey of Living Conditions in 2015. The 2015 NHNS consisted of physical examination, dietary survey, and lifestyle questionnaire. A total of 3,507 out of 5,327 eligible households (65.8%) and 8,583 people participated in the survey. This current study included 5,048 adults aged 18-64 years. We excluded participants with missing data required for analysis in the present study, such as dietary information (n=1,127), body weight (n=592), smoking status or/and habitual alcohol consumption (n=270). Moreover, we excluded those with missing data on the frequency of eating out and take-away meals (n=5). We also excluded those who skipped breakfast, lunch, or/and dinner (n=749), because meal skipping may affect nutrient and food intakes [31,32], and lactating or pregnant women who may have changed their usual dietary habits (n=84) . [33] Thus, the nal participants consisted of 2,221 Japanese adults aged 18-64 years (921 men and 1,300 women).
The permission to use the 2015 NHNS data was obtained from the Ministry of Health, Labour, and Welfare, and only anonymised information was availed for this study. As this survey was conducted according to the Health Promotion Act, all participants gave informed consent to the local government, and approval from Institutional Review Board was not required.

Dietary assessment
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. Also, participants reported the type of meals consumed at breakfast, lunch, dinner, and snacks on the recording day according to the following categories: prepared at home, take-away meals (dishes prepared outside home, but eaten at home), eating out, or prepared meals (food served at nursery school, kindergarten, elementary school, junior high school, high school, or workplace). This selection was identi ed by the main dishes (staple food in case there were no main side dishes).
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) [35] 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 macronutrient 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 consuming meals prepared away from home
The frequency of consuming meals prepared away from home was assessed by the combination of two questions in the lifestyle questionnaire 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). Figure 1 shows the classi cation of participants into three groups according to the frequency of consuming meals prepared away from home, based on the previous reports [3,15,18]. 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 High group (high frequency of consuming meals prepared away from home). Participants who responded to both questions "once a week," "less than once a week," "seldom" were classi ed in the Low group (low frequency of consuming meals prepared away from home). 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. [36][37][38] 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 (DG) to prevent lifestyle-related diseases is set to prevent noncommunicable diseases.
Nutrient intake 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%)[39] for women aged 20-49 years because the cut-point method is less applicable to these populations. [40,41] 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 salt-equivalent), 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 self-reported. 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 consuming meals prepared away from home were assessed by ANOVA in the crude model and a covariate analysis (ANCOVA) in the adjusted model. Dunnett test, with the Low 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 High and Middle groups according to the frequency of consuming meals prepared away from home compared with the Low 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 consuming meals prepared away from home [8,42]. 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. Table 1 shows the basic characteristics of participants according to their frequency of consuming meals prepared away from home. The proportion of participants classi ed into the High, Middle, and Low groups were14.7%, 50.5%, and 34.9% for men, and6.3%, 46.9%, and 46.8% for women, respectively. The mean age in the Low group was signi cantly higher than other groups (p=0.001 in men and p=0.004 in women). Additionally, there were more men and women living alone in the High group (p <0.001).

Results
The residing region signi cantly differed in women, with more women in the High group living in the urban Kanto area (p=0.002). In the High group, approximately 90% of men and 70% of women reported eating out or having take-away meals in their one-day dietary records as well, which showed a signi cantly higher proportion compared to other groups. There were no differences in mean BMI, type of occupation, current smoking and consumption of snacks across the groups in both men and women. Table 2 shows the nutrient intakes on the dietary recording day according to the frequency of consuming meals prepared away from home. Among men, the intake of protein, calcium, iron, copper, dietary ber and potassium was signi cantly lower in the High group than in the Low group (p=0.020, 0.044, 0.008, 0.027, 0.002 and 0.004, respectively). In women, the intake of folate, calcium, magnesium, iron and potassium in the Middle group was signi cantly lower than in the Low group (p=0.046, 0.036, 0.014, 0.001 and 0.026, respectively). Dietary ber intake was higher in the Low group compared the Middle and High groups (p=0.005).
The multivariate-adjusted odds ratios (ORs) for nutrient intake adequacy according to the frequency of consuming meals prepared away from home are shown in Table 3. Most men and women had adequate intake of protein, niacin and copper in comparison to EAR. Among men, the proportion of those having adequate intake of iron, protein %energy, dietary ber, and potassium in the High group was signi cantly lower than in the Low group.  Table 4 shows food group intakes according to the frequency of consuming meals prepared away from home. For both men and women, vegetable intake in the Low groups was higher than that of the High group (p=0.004 and p=0.012 in men and women, respectively). Fat and oil intake in the Low group was lower than the Middle and High groups among men (p=0.002); signi cant difference was observed only between the Low and Middle groups in women. Among men, a higher intake of mushrooms was observed in the Low group than in the High group (p=0.015). Among women, a higher intake of potatoes and lower intake of meat and poultry were observed in the Low group than in the Middle group (p=0.002 and p=0.032, respectively).
The relationship between the frequency of eating out and take-away meals assessed through the one-day dietary record and their nutrient intake adequacy status is shown in the Supplementary table. Subjects in the High group were less likely to have adequate vitamin and mineral intake, except vitamin B 1 , vitamin B 12, calcium and copper (both men and women), and vitamin B 2 and vitamin C (only women) compared to women in the Low group.

Discussion
The present study examined the association between the frequency of consuming meals prepared away from home and nutrient intake adequacy 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 consuming meals prepared away from home and nutritional adequacy, based on dietary reference values among Japanese adults.
In this study, participants were classi ed into three groups (Low, Middle, and High group according to the frequency of consuming meals prepared away from home) based on the response to questions about the frequency of eating out and take-away meals.
This categorization based on the questionnaire on the frequency of habitual eating out and take-away meals was comparable to that based on the dietary records, as shown in Table 1, despite being based on one-day dietary record method. Moreover, the relationship between the frequency of eating out and take-away meals assessed through the one-day dietary record and nutritional adequacy (Supplementary table) showed similar results. When the frequency of home-prepared meals were less, the less adequate their dietary intake tended to be. Although the number of vitamins and minerals considered inadequate was larger when the classi cation using the dietary records was applied compared to that from the lifestyle questionnaire, the two different methods consistently supported our present ndings.
Several factors increased the likelihood of eating food prepared away from home. Men and younger people had higher frequency of consuming meals prepared away from home than others. This is consistent with previous studies that showed a higher frequency of eating out among men and younger adults when compared with that among older adults [15,43], or higher proportion of eating out in men than women [15]. Similarly to a previous study among Japanese university students [37], living alone was associated with a higher frequency of eating meals prepared away from home. In addition, women living in urban areas had more eating out and take-away meals, which is consistent with the result of a previous study in Vietnam.
[44] Thus, the current results may indicate that younger adults, especially men, are more likely to consume meals prepared away from home in Japan as observed in other countries.
The association between the frequency of consuming meals prepared away from home and nutrients intake has been reported in several studies. Studies from Australia and Europe that used 24-hour dietary recalls reported that adults with a higher frequency of consumption of foods prepared outside the home had lower intake of iron and calcium, and vitamin C and calcium, respectively. [2,13] Additionally, a review article reported that people with higher frequency of eating out had lower vitamin C, iron and calcium intakes [12]. These reports showed similar results to the current results about iron and calcium intakes. 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.
[45] 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.
[46] Japanese usual insu cient intake status may re ect to the present results, regardless of the frequency of consuming meals prepared away from home. In contrast, Japanese people rarely lack copper and protein [45], which may explain the current results where no difference was observed in the proportion of inadequate intake of these nutrients according to the frequency of consumption meals prepared away from home. Of note, EAR is set for the purpose of avoiding insu cient intake, whereas DG is set for the prevention of non-communicable diseases. It is possible that the de nition of inadequacy of each nutrient intake may have determined differently, which suggests a caution of interpretation of the results.
Dietary ber was the only nutrient that signi cantly differed depending on the frequency of consuming meals prepared away from home. This nding is largely consistent with the previous studies that reported the association the frequency of eating out with dietary ber intake. [2,8] In this study, lower dietary ber intake was observed in the High group in men, and the Middle group in women; this trend was similar for other nutrients. It has been reported that women cook more often than men in Japan and other countries.[42, 47] The dietary ber intake has been reported to be association with frequency of cooking and cooking skill [48].
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 highly 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 ndings that higher frequency of consuming meals prepared away from home is associated with lower nutrients intakes and inadequacy of nutrient intake compared with that in the Low group. Also, socioeconomic factor may be one of the important factors associated with consumption of meals prepared away from home. Unfortunately, other than occupation, we could not consider other socioeconomic indicators. While the proportion of occupation (professional, manager, sales, service, and clerics) differed among men and women, and was higher in the High group, there was no signi cant difference among the groups. Thus, future studies are needed with consideration of socioeconomic factors such as educational background and income level.
The frequency of eating out and take-away meals has been reported to be associated with a lower intake of vegetable and a higher intake of fat and oils. [8,18] 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.
Especially, as there has been no report about inadequate intake of potassium and magnesium according to the frequency of consuming meals prepared away from home, our results may highlight the need for health promotion interventions for people with a higher frequency of eating out or take-away meals, as well as for the food industry.
In this study, approximately 45% of men and 30% of women regularly ate out or had take-away meal. In Japan, the government has called for voluntary efforts among the 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 efforts include "Increase in number of corporations in the food industry that supply food products low in salt and fat."[50] However, the current recommendation hugely focuses on preventing excess intake, and 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; however, the individual-level response rate was unknown. This might have introduced some bias in the estimation of average intake in Japanese adults. Also, the sample was essentially selected through a cluster of the households; however, it is di cult to treat them with the household cluster in our analysis. 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 may be valid for the estimation of individual intake[51]. 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 the purpose of this study was to assess nutrient intake and nutrition adequacy by the difference in the frequency of consuming meals prepared away from home. This might have induced some bias in the nutrient intakes. Finally, factors other than the frequency of consuming meals prepared away from home 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 a higher frequency of consuming meals prepared away from home 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 a high frequency of eating out and take away meals 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.
Abbreviations ANCOVA, covariate analysis; ANOVA, analysis of variance; BMI, body mass index; DG, tentative dietary goal to prevent lifestylerelated diseases; DRIs, Dietary Reference Intake for Japanese; EAR, estimated average requirements; EER, estimated energy requirements; NHNS, National Health and Nutrition Survey in Japan; OR, odd ratio; 95% CI, 95% con dence interval.

Declarations
Ethics approval and consent to participate Not applicable.

Consent for publication
Not applicable.

Availability of data and materials
This study was a secondary analysis of the 2015 National Health and Nutrition Survey in Japan and was conducted with the permission of the Ministry of Health, Labour and Welfare, in Japan. 37. Kobayashi S, Asakura K, Suga H, Sasaki S: Living status and frequency of eating out-of-home foods in relation to nutritional adequacy in 4,017 Japanese female dietetic students aged 18-20 years: A multicenter cross-sectional study. J Epidemiol 2017.   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 (18-34, 35-50, 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 significant difference by Dunnett test compared with High group in the adjusted model. 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.

Kohri T, Kaba N, Itoh T, Sasaki S: Effects of the National School Lunch Program on Bone Growth in Japanese
The probability of inadequacy >50% for menstruating women whose bioavailability of iron is 15% (<9.3 mg/d) was considered inadequate for women aged 20-49 y.