1. Introduction
Dairy products, such as milk, cheese, and yoghurt provide an abundant source of nutrients, including protein, vitamins, and minerals, and their nutritional value has been well recognised [
1,
2,
3]. Their beneficial roles have been examined in association with a variety of chronic diseases, including hypertension, metabolic syndrome, type 2 diabetes, and cardiovascular diseases, as well as in maintaining bone health [
3,
4,
5,
6,
7,
8,
9].
Several studies have shown that the intake of dairy products may improve nutrient intake [
10,
11,
12,
13,
14,
15,
16]. For example, in a study of Australian children, drinking milk was related to higher intake of micronutrients [
10]. Other studies targeting children also reported the positive influence of dairy consumption on the intake of nutrients [
11,
12,
13]. In an Irish study, it also has been suggested that dairy products contribute to the intake of micronutrients such as calcium and retinol [
14], while another study has reported that yogurt intake was associated with better diet quality in American adults [
15]. Moreover, some researchers have indicated that it is challenging to achieve adequate intake of essential nutrients, such as calcium, potassium, and magnesium, without dairy products in the diet [
17]. Studies on dairy intake in relation to nutritional quality have been conducted among Western populations. On the other hand, few comparable studies in Asian countries have been reported. A traditional, typical Japanese diet is a combination of rice, soybean products, fish, seaweed, and green tea [
18], which is different from Western countries. The intake of dairy product (milk, cheese, yogurt, and cream) in Japan has been reported to be approximately 110 g/day/capita, including only 63 g/day/capita of milk [
18], which is relatively low compared with Western countries [
19]. Thus, the influence of the amount/pattern of dairy product consumption on other dietary intakes may differ between Asian and Western populations. Hence, determining the influence of dairy products on adequate nutrient intake among Japanese people using a nationally representative sample is important to Asian populations.
The objective of the present study was to compare nutrient intake adequacy, as well as other food intake, between Japanese adult non-dairy-product consumers, dairy product consumers, and milk consumers, using the 1-day dietary record in the 2016 National Health and Nutrition Survey in Japan (NHNS).
3. Results
The basic characteristics of the participants according to dairy product consumption are shown in
Table 2. The mean ages were 57.2 years (standard deviation (SD), 17.4) old for men and 58.8 years old for women (SD, 17.5). The mean milk intake was 60.9 g/day (SD, 109.6) and 69.5 g/day (SD, 107.2) and that of other dairy products was 32.8 g/day (SD, 60.5) and 43.7 g/day (SD, 64.8) in men and women, respectively. A total of 3990 men (40.0%) and 5345 women (46.0%) reported consuming liquid milk and 2364 men (23.7%) and 3102 women (26.7%) reported consuming other dairy products on the dietary recording day, while the others did not consume dairy products. There were significant differences in the mean age, BMI, proportions of each BMI category, residential block, and the number of current smokers and habitual alcohol drinkers across the groups in both men and women (
p < 0.05).
Table 3 shows food group intake across the dairy consumption status. Men and women showed similar differences in food intakes across the groups. In both sexes, the intake of “grains”, “sugars”, “sesame and nuts”, “vegetables”, “fruits”, “mushrooms”, “fish and shellfish”, “oils”, “confectionaries”, “beverages”, and “seasonings” were significantly different (
p < 0.05 for each food group). In men, “pulses” and “egg” intake were also significantly different among the groups (
p = 0.037,
p = 0.002, respectively), while “seaweeds” intake was significantly different among the groups in women (
p = 0.029). Non-dairy consumers had the highest intakes of “grains” and “seasonings”, while they also had the lowest intakes of “sugars”, “sesame and nuts”, “fruits”, “mushrooms”, “oils”, and “confectionaries” (
p < 0.05). Among men, non-dairy consumers also had the lowest intake of “vegetables” (
p < 0.05). In milk consumers, the intake of the “oil” food group was higher than in other dairy consumers and non-dairy consumers in both men and women, while the intake of “fish and shellfish” was lower than in the other two groups (
p < 0.05). Other dairy consumers had a higher intake of “beverages” than the other two groups (
p < 0.05).
Table 4 and
Table 5 show the energy and nutrient intake and prevalence of not meeting DG and EAR across the groups in men and women, respectively. Significant differences in mean energy and nutrient intake values between the groups were observed for all nutrients in men and women (
p < 0.05), except for vitamin B12 in men. In men, carbohydrate and sodium intakes were higher among non-dairy consumers, while all the other nutrient intakes, except for niacin and iron, were higher in milk and/or other dairy consumers. In women, carbohydrate, sodium, and copper intakes were higher among non-dairy consumers, while dietary fibre, folate, and vitamin C intakes were the highest in other dairy consumers (
p < 0.05). Iron intake was lower among milk consumers compared to other groups (
p < 0.05). Intake of all other nutrients, except for niacin, in milk and/or other dairy consumers compared to non-dairy consumers was higher in women. Among men, the prevalence of not meeting DRIs was only significantly higher for saturated fat in milk consumers and dairy consumers than in non-dairy consumers. The prevalence of not meeting DRIs for all remaining nutrients, except for niacin and copper, was significantly higher among non-dairy consumers than other groups (
p < 0.05). Among women, saturated fat was the only nutrient that showed a higher prevalence of not meeting DRIs in milk or other dairy consumers than in non-dairy consumers (
p < 0.001). The prevalence of not meeting DRIs for the other nutrients, except for total fat, dietary fibre and copper, was higher in non-dairy consumers (
p < 0.05).
The mean numbers of nutrients with not meeting DG and EAR are shown in
Table 6. Among men, the mean number of nutrients with not meeting DG was 3.8 for milk consumers, 3.8 for other dairy consumers, and 4.0 for non-dairy consumers. For not meeting EAR, the mean number was 3.3 for milk consumers, 3.9 for other dairy consumers, and 4.7 for non-dairy consumers. After age-adjustment, non-dairy consumers had a significantly higher number of nutrients with not meeting DG and EAR than other groups (
p < 0.001). Among women, the mean number of nutrients with not meeting DG also showed a significant difference across the groups, with the numbers of nutrients calculated at 3.8 in the milk consumers, 3.7 for other dairy consumers, and 3.9 for non-dairy consumers (
p = 0.02). The number of nutrients with not meeting EAR was 3.3, 3.9, and 4.7 in milk consumers, other dairy consumers, and non-dairy consumers, respectively (
p < 0.001).
4. Discussion
In the present study, we compared the nutrient intake adequacy of individuals who reported consuming milk or other dairy products to that of those who reported not consuming any dairy products using a 1-day dietary record. In a comparison of the number of nutrients not meeting DRIs, dairy consumers (both milk consumers and other dairy consumers) were more likely to have adequate nutrient intake than non-dairy consumers. However, for saturated fat, dairy consumers had higher intake (above the DG level) than non-dairy consumers. To the best of our knowledge, this is the first report that studied nutritional adequacy according to milk and dairy products intake among Japanese adults. It is important to examine this issue in a population with lower dairy intake to understand the influence of dairy products intake on nutritional intake in people with different characteristics.
Even among the dairy consumers in the current study, the mean daily intake of dairy products was approximately 160 g in milk consumers and 70 g in other dairy consumers; this was the same in both men and women. In an Irish study, the mean dairy product intake in dairy consumers was reported as 291 g [
14], and the average liquid milk intake in the US was reported to be around 180 mL (186 g, using 103.2 g/100 mL for conversion factor [
32]) [
33], while in Japan the reported amount is about 60 mL (62 g) [
18]. In the current study, dairy consumers had less intake of “grains” and “seasoning” than non-dairy consumers. In addition, milk consumers had less intakes of “fish and shellfish” than non-dairy consumers. A previous study has indicated that dairy products are consumed with bread rather than rice or noodles as dietary staples among Japanese population [
34]. Together with the present findings, this suggests that a non-dairy diet may be a result of rice-based meals with greater amounts of fish, which is regarded as a traditional Japanese diet. In turn, this might also suggest that a diet with dairy products is a Westernised Japanese diet. Another Japanese study using NHNS data described that a dietary pattern mainly consisting of bread and dairy products has been increasing in Japanese adults [
35], which may point to a shift in the Japanese diet, despite much lower dairy consumption compared to that in Western countries.
The association between dairy products intake and nutritional intake has been examined in several studies, mainly in Western countries. For example, a diet modelling study showed that increasing dairy food consumption to recommended amounts could improve the intake of essential vitamins and minerals that were inadequately consumed [
36]. Similar to our results, a US study reported that consumption of milk (plain or flavoured) was associated with better macro- and micronutrients intake among children and adolescents [
11], and other studies have shown similar results [
6,
10]. In the present study, the greatest difference in inadequate intake of nutrients between milk consumers and non-dairy consumers was found to be for calcium, which could be greatly responsible for the total number difference of those not meeting EAR. The contribution of dairy intake to calcium intake has been reported using nutrient profiling methods [
37]. Only 28% of the calcium intake of a Japanese adult is received from dairy products. Other foods, especially vegetables or pulses, also contribute to calcium intake in the Japanese diet [
18]. Although in this study the intake of “vegetables” was higher in dairy consuming men, a similar pattern was not observed in the intakes of “pulses” and “vegetables” in dairy consuming women. Therefore, dairy intake itself might be responsible for the relatively adequate calcium intake in this study. However, there still was a considerable number of non-dairy consumers (approximately 20%) who achieved adequate calcium intake without dairy products. This is not consistent with previous Western studies which suggested that milk and dairy intake particularly influence calcium intake because only those who consumed milk had an average dietary calcium intake close to their recommended levels [
10,
11,
17,
36]. Further studies are needed to explore the factors contributing to adequate calcium intake in the Japanese population for future public health strategies. On another note, more fruit and mushrooms were consumed by dairy consumers in this study. This result is also similar to that described in a previous study, which identified a combination of dairy products and fruit as one of the typical meals among Japanese people [
34]. These food intakes among dairy consumers could also explain the better intake of certain nutrients, such as vitamin C and dietary fibre, because milk and many other dairy products lack such nutrients [
22]. Our finding also suggests that older people are more likely to consume dairy products, which is consistent with the annual reports of NHNS showing higher dairy products intake among older people than younger people [
18]. The NHNS has shown that older people are more likely to have better dietary intake, such as more fruit and vegetables [
18]. Further assessments are needed to investigate the dietary intake patterns according to the age difference, and in relation to dairy products intake.
Despite adequate nutritional intake observed among dairy consumers for EAR, the difference in the number of those not meeting DG was small, even though statistically significant differences were observed. The DG values were set to achieve better nutritional intake from the standpoint of prevention of life-style related diseases such as diabetes, hypertension, and dyslipidaemia [
25]. Among nutrients with DG, the difference in the prevalence of not meeting DG was observed for several nutrients such as potassium and protein according to dairy intake status, with more favourable intake in dairy consumers. However, there was a higher prevalence of excess saturated fat intake in both male and female dairy consumers. These could substantially explain the small difference in the number of not meeting DG compared to not meeting EAR. Regarding excess saturated fat intake, similar results have been observed in previous studies [
10,
11,
13,
14,
17]. In NHNS 2016, in the same participants used as the sample for this study, 17% of saturated fat came from dairy products [
18]. Dairy product intake has been identified in meals with bread in Japanese adults [
34] and another Japanese study reported that breakfast bread intake was associated with higher saturated fat intake in young women [
38]. From these findings, not only dairy foods, but the combination with bread may play an important role in Japanese saturated fat intake. According to a Japanese dietary guideline (Japanese Food Guide Spinning Top), daily intake of one serving of milk (200 mL; 206 g) or its equivalent is recommended [
39], which contains a saturated fat level of 4.8 g (full-fat milk) [
22]. This milk amount exceeds the median milk intake of the participants of the current study, even that of milk consumers. A Japanese study has attempted to design optimal dietary intake to achieve a full set of nutritional recommendations in the Japanese DRIs, suggesting a reduction in consuming full-fat dairy and increasing the intake of low-fat dairy products [
40]. This might partly reflect the contribution of full-fat dairy to saturated fat intake among Japanese adults. In fact, most of the milk consumed in this study was full-fat milk, with the mean low-fat milk consumption being 21.1 g and 20.6 g in men and women, respectively (data not shown). Therefore, to develop an appropriate public health strategy, further studies using diet modelling methods are needed to set an adequate dairy intake in the Japanese diet. In contrast, a US study has recently reported that long-term intake of dairy fatty acids was not associated with all-cause mortality and cardiovascular disease mortality among older adults [
41]. Furthermore, there has been a report showing a negative association between risk of cardiovascular diseases and dairy saturated fat intake [
42]. The effectiveness of dairy product intake as a source of saturated fat intake also needs to be further examined.
Several limitations of this study need to be mentioned. First, the study samples were randomly selected from nationally representative households in Japan; however, only 44.4% of selected households participated in the survey. Furthermore, the individual-level response rate within the household is unknown. Additionally, the dietary survey was conducted on intentionally selected single weekdays in only two months from October to December. This might induce some bias in the estimation of average intake in Japanese adults. Second, a dietary intake derived from a 1-day weighed dietary record is unlikely to represent the usual intake of individual respondents. Therefore, day-to-day variability in the dietary intake of individuals might have influenced the findings. Moreover, while the validity of the household-based dietary record used for this survey has been examined among young women, the true utility of this methodology for assessing daily dietary intake among men or people in other age groups is unknown. Furthermore, measurement error could be caused by misreporting of self-reported food intake. Finally, we classified the participants according to their milk or dairy consumption only, regardless of the amount consumed. Further investigation is needed to estimate the adequate number of dairy products. Additionally, it should be noted that diet quality is defined by the totality of foods consumed rather than by the presence or absence of a single food [
43,
44,
45]. Although nutrition intake was better among dairy consumers in part, public health messages should focus on promoting healthier food choices.