European Review for Medical and Pharmacological Sciences

2021; 25: 6623-6632

Validity of a selective recall method forassessing water intake and its relationshipwith hydration status
T. UCHIDA1, Y. NAKAMURA1, H. TANAKA2, S. NAKAMURA2,3,
T. OKAMURA4, H. WATANABE1, N. MURAYAMA1
Suntory Global Innovation Center Limited, Research Institute, Seika-cho, Soraku-gun, Kyoto, Japan
Graduate School of Science and Technology, Nara Institute of Science and Technology, Ikoma, Japan
3
Data Science Center, Nara Institute of Science and Technology, Ikoma, Nara, Japan
4
Department of Preventive Medicine and Public Health, Keio University School of Medicine,
Shinjuku-ku, Tokyo, Japan1
2

Tomoki Uchida and Yumi Nakamura contributed equally to this work

Abstract. – OBJECTIVE: We previously es-

tablished a descriptive dietary record method that accurately quantifies habitual water intake from food and beverages, to ascertain therelationship between water intake and health.
Here, we verified the validity of a selective recall method, which is easy for users to answer
and analyze.
PATIENTS AND METHODS: Japanese men
and women aged 20-44 years (n = 16) and 4564 years (n = 16) participated over three working
days and one non-working day. The day following each of the surveyed days, participants collected their first morning urine for urinalysis andcompleted a selective recall and descriptive dietary record questionnaire.
RESULTS: The two methods of determining
water intake were positively correlated (r = 0.94,
p < 0.0001). Water intake volumes from non-alcoholic beverages (r = 0.94, p < 0.0001), alcoholic beverages (r = 1.00, p < 0.0001), and food (r =0.72, p < 0.0001), calculated using the two methods, exhibited strong correlation. No correlation
was observed between urinalysis parameters
and total water intake. A significant, negative
correlation was observed between urine osmolarity and total water intake in men (r = -0.55, p= 0.0011) and women (r = -0.51, p = 0.0032) aged
20-44 years.
CONCLUSIONS: Selective recall is a valid
method for assessing water intake from foodand beverages.
Key Words:
Water intake, Hydration, Water intake assessment,
Selective recall method, Urinalysis.

Introduction
Water constitutes 60% of the human body and
it is a vital component of life1. It is essential for
maintaining homeostasis; it acts as a medium forthe delivery of oxygen, nutrients, hormones, andother substances throughout the body. It is also
necessary for the removal of waste products andexcess substances2. Water is the major componentof intracellular and extracellular fluids, such as
blood plasma and interstitial fluid. It takes partin biochemical reactions inside the cells and contributes to temperature regulation3. There is anassociation between water intake and health. Inadequate water intake increases the risk of renal
and cardiovascular diseases and contributes tometabolic diseases4-6; in addition, it affects moodand cognitive function7-9. Therefore, maintaining an adequate intake of water is important for
reducing the risk of disease and for maintaining
daily mental health. In a previous randomized
trial, we examined the effect of increasing water
uptake to 1.1 L/day for 12 weeks, in healthy Japanese adults. This resulted in decreased systolicblood pressure, increased basal body temperature, reduced blood urea nitrogen concentration,
suppressed reduction of the glomerular filtration
rate, and changes in the intestinal microbiome10.
Some countries and organizations in Europe
and the USA have defined water as a nutrient
and, they have established adequate intake (AI)levels11. The European Food Safety Authority

Corresponding Author: Tomoki Uchida, MD; e-mail: Tomoki_Uchida@suntory.co.jp6623

T. Uchida, Y. Nakamura, H. Tanaka, S. Nakamura, T. Okamura, H. Watanabe, N. Murayama

defines AI as 2.0 L/day for women and 2.5 L/day
for men12. The US Institute of Medicine defines
AI as 3.7 L/day for men and 2.7 L/day for women11. In contrast, Japan does not define water as anutrient, and it has not yet determined AI levels13.
One of the major reasons for this difference is theinsufficient amount of research on water intake inJapanese people, which is required for accurately
calculating the AI levels. There is only one reportaddressing water intake, including food-derived
water, in Japanese people14. The data on water
intake from other countries cannot be applied to
the Japanese population, because of the unique
dietary habits in Japan compared with that in theother countries. Traditional Japanese food consists of cooked rice, soup, and three side dishes,
including a main dish of meat or fish and two other dishes of fresh vegetables or simmered food.
Compared to the people in Europe and the USA,
Japanese people are believed to obtain morewater from food, such as cooked rice, soup, andvegetables. Food sources account for 50% of thewater intake in Japan14, while beverages accountfor the other half. Comparatively, only 20% ofwater intake in Europe and the USA is derived
from food, while 80% is obtained from beverages12. Additionally, the differences in the physicalcomposition of Japanese people, compared withthat of European or American individuals, prevent the direct application of AI data from othercountries to the Japanese population.
Dietary surveys can determine nutrient consumption; however, there is no established method for ascertaining water intake, in particular,water intake from food. In our previous study15,
we established the water conversion factor forcooking (WCFC), which accounts for the effectof food preparation on food moisture content. We
developed an approach for calculating the total
water intake from all foods and beverages; thisapplies the WCFC to information obtained via
the descriptive dietary record method. We used
this approach to obtain an accurate understanding of water intake in a clinical study in Japanesepeople10. However, the descriptive dietary record
method is not well suited to large-scale epidemiological studies, because collecting participant
responses and interpreting data are time-consuming processes16,17. Therefore, we have developed
the selective recall method as a less time-intensive strategy for calculating water intake. This
study analyzed the correlation between the water
intake calculated using descriptive diet recording
and the selective recall methods in Japanese peo6624

ple. We performed general urinalysis to detect the
representative indicators of hydration status. The
indicators were subsequently used to explore therelationship between water intake and hydration
status.

Patients and Methods
Study Design
The schedule for this observational cross-sectional study was as follows: participants wererecruited from July 3 to July 10, 2020. An
online briefing session was held, and free andvoluntary consent was obtained from each participant. The study was conducted over fourdays, including three working days and one
non-working day within the study period, from
July 27 to August 2.
ParticipantsParticipant recruitment was limited to employees working at the Suntory World ResearchCenter (Kyoto, Japan). Those who met the selection criteria, based on a self-report, were enrolled. The selection criteria were as follows: (1)men and women aged ≥20 to <65 years and (2)those who read, understood, agreed with, andconsented to the explanation of the study in advance. Thirty-two participants, comprising eight
men aged ≥ 20 to < 45 years (young men), eight
women aged ≥ 20 to < 45 years (young women),eight men aged ≥ 46 to < 65 years (middle-agedmen), and eight women aged ≥ 46 to < 65 years(middle-aged women), were included (Figure 1).
Exclusion criteria were not applied in this study.
The rationales for the sample size are as follows18:
the number of cases required to achieve a correlation coefficient of 0.7 at power = 0.85 and α= 0.01 is 21.
Survey Implementation
Participants were assigned any four days within the study period, including three working days
and one non-working day, as the survey days. On
the day after each survey date, they were asked to(1) collect first morning urine, (2) answer the selective recall method questionnaire, (3) fill in thedescriptive dietary record method questionnaire,and (4) answer the other questionnaires on daily
life and background information. Answers to thequestionnaire were conducted in the order listed
above. The selective recall method questionnaire
was submitted in Google forms, and once submit-

Validity of a new water intake asessing method

Figure 1. Flowchart of participant recruitment and study.

ted, it could not be corrected by the participants.
The two methods were performed on the same
day consecutively to minimize recall bias andcompare water intake calculated by each method.
Study Outcomes
Water Intake Calculated from a Selective
Recall Method
On the day after each survey day, participants
were asked to recall their intake of beverages,
alcoholic drinks, and food consumption on theprevious day. They submitted their responses
through Google forms.
The questionnaire for beverages accounted forthe following items: water, tea, coffee, milk beverage, and other beverages. The intake from (1)waking up to breakfast, (2) at breakfast, (3) frombreakfast to lunch, (4) at lunch, (5) from lunch todinner, (6) at dinner, (7) from dinner to 30 min
before bedtime, (8) 30 min before bedtime, and(9) from bedtime to waking up, was recorded.
The amount consumed was selected from five options: approximately 100 mL, 200 mL, 350 mL,
500 mL, and more than 700 mL. Participants did
not provide an answer to this question if they did
not drink any beverages.
For alcoholic drinks, the participants chose
between the same five volume options for beer,whiskey, sake, shochu, wine, chuhai (a fruit-flavored carbonated alcoholic drink) or sour, andother alcoholic drinks. Participants did not provide an answer to this question if they did not
drink any alcoholic beverages. The amounts wereset at levels that are easy to understand, considering the volume of commercial beverage products
and cups. The classification and approximate
volume of each beverage was indicated at the beginning of the questionnaire to make it easier forthe participants to track their intake.
For meals, the amount of 40 frequently consumed items from approximately 2.3 million
dishes, reported by ~28,000 people in the Japanese application FoodLog (foo.log Inc., Tokyo,
Japan), was selected for (1) breakfast, (2) lunch,(3) dinner, and (4) nighttime/snacking. Intake
options were: 0.5, 1, 1.5, and more than 2 servings. The 40 items are shown in Supplementary
Table I.
The data were multiplied by the WCFC15 foreach beverage, alcoholic beverage, and meal
(Supplementary Table I) to calculate the dailywater intake. Water content per 100 g non-alcoholic beverages and alcoholic beverages was
taken obtained the Standard Tables of Food Composition in Japan 2010 (Sixth Revision). The
water content of each cooked dish was calculated
by multiplying the water content of the uncooked
ingredients by a cooking factor19. Water metabolism was included in the water content of food. It
was calculated using a standard method, based on
the amount of carbohydrates (g), protein (g), andfat (g) in the food20. Based on the water content
of 521 dishes, 40 food categories were definedin terms of water content, cooking method, andserving size. The median water content of thedishes in a given food category was used as thewater content for that category.
Water Intake Calculated from a
Questionnaire Using a Descriptive
Dietary Record Method
The day after each survey day, participantsfilled out a paper-based descriptive dietary record method questionnaire for food and drink
6625

T. Uchida, Y. Nakamura, H. Tanaka, S. Nakamura, T. Okamura, H. Watanabe, N. Murayama

consumed on the previous day (Supplementary
Figure 1). A dietitian inputted the data fromthe completed form into the original nutritional
calculation software (foo.log.Inc.). The data were
multiplied by the WCFC15 for each food and drink
to calculate the daily water intake.

the difference between two dependent correlations
that share one variable. Data aggregation and statistical analysis were performed using Microsoft
Excel 2013, JMP ver. 14.0.0 (SAS Institute Inc.,
Cary, NC, USA), and R ver. 4.0.1 (R Foundationfor Statistical Computing, Vienna, Austria).

Urinalysis
Osmolarity, and the concentration of aquaporin 2 (AQP2), creatinine (Cre), sodium, andpotassium, in the first morning urine collected on the day following each survey day wasmeasured. Osmolarity was measured using thefreezing point depression method; AQP2, using
the enzyme-linked immunosorbent assay (Otsuka Pharmaceutical Co., Ltd., Tokyo, Japan); Cre,
using the enzymatic method; and sodium andpotassium, using the electrode method. The measurements were carried out by LSI Medience Co.
(Tokyo, Japan). Based on the measured values,
AQP2 was evaluated following Cre correction(AQP2/Cre). Sodium excretion was calculatedusing the following Tanaka method formula21.
24 h urinary sodium excretion (g/day) = 21.98
× [urine sodium concentration (mEq/L)/urine creatinine concentration (mg/dL)/10 × predicted 24 hurine creatinine excretion*1]0.392*1
Predicted 24 h urinary creatinine excretion(mg/day): Weight (kg) × 14.89 + height (cm) ×
16.14 - age × 2.04 - 2244.45

Ethical Standards DisclosureThis study was conducted according to theguidelines laid down in the Helsinki (revised by
the Fortaleza General Meeting of the World Medical Association), and all procedures involving
human participants were approved by the ethics
committee of Suntory Holdings Limited (IRB
No.15000139). This study is also in compliancewith the Ethical Guidelines for Medical Research
Involving Human Subjects (2014 Ministry of Education, Culture, Sports, Science and Technology
and the Ministry of Health, Labour and Welfare
Ministerial notification No.3). Written informed
consent was obtained from all participants.

Other Questionnaires
Daily activities, such as the number of toilet
visits on the day of the study, were assessed
based on the answers provided in the daily life
questionnaire, completed on the day following
each day of the survey. Participants also self-reported their gender, age, height, and weight on a
background information form. Body mass index(BMI) was calculated using the formula BMI =weight (kg)/(height (m))2.
Statistical Analysis
The values obtained are presented as mean ±standard deviation (SD). Correlation analysis was
performed using simple linear regression analysis,for the water intake values calculated from theselective recall method and the descriptive dietary
record method. Water intake for validation was
averaged over 4 days. Daily values of water intake
from both methods were used for correlation analysis with the urine test values. The Pearson’s correlation test was used to calculate the relationships
between the data. Williams’s test was applied for
6626

Results
Study Population
All participants completed the study. The baseline characteristics and survey results are shownin Table I. The mean number of night-time toilet
visits was 0.1 times per night (Table I).
Correlation between Water Intake
Calculated using the Descriptive
Dietary Record Method and the
Selective Recall Method
A strong positive correlation was observed between the total water intake calculated using thedescriptive dietary record method and that calculated by the selective recall method (r = 0.94; p <0.0001; linear regression, y = 1.20× − 568; Figure2a). In addition, the water intake from non-alcoholic beverages (r = 0.94; p < 0.0001; linear
regression, y = 1.20× – 407; Figure 2b), alcoholic
beverages (r = 1.00; p < 0.0001; linear regression,y = 1.05× – 7.66; Figure 2c), and food (r = 0.72;
p < 0.0001; linear regression, y = 0.90× + 166;
Figure 2d) recorded using the descriptive dietary
record method and the selective recall method
exhibited a positive correlation.
Correlation between Urine Osmolarity
and Water Intake
It was confirmed whether the correlation between urinary osmolality and water intake could

Validity of a new water intake asessing method
Table I. Baseline characteristics for all participants.
							All aged
Young men Young women		≥ 20 toaged ≥ 20 toaged ≥ 20 toParameter< 65 years< 45 years< 45 years

Middle-aged
men aged≥ 46 to< 65 years

Middle-aged
women aged
≥ 46 to< 65 years

Baseline characteristic				   Age42.1 (12.6)
33.3 (6.1)
28.1 (2.9)
55.4 (5.5)
51.6 (4.0)   Height (cm)167.1 (8.1)
172.4 (3.8)
160.5 (6.1)
174.3 (5.4)
161.3 (5.0)   Weight (kg)59.3 (9.8)
63.1 (8.9)
50.0 (4.9)
69.4 (6.3)
54.8 (4.7)   BMI
21.1 (2.2)
21.2 (2.7)
19.4 (1.3)
22.8 (1.4)
21.1 (1.7)
Water intake (descriptive type)					   Total (mL/day)
2970 (1059)
3534 (1488)
2513 (798)3312 (968)2520 (459)   From non-alcoholic1775 (912)
2501 (1375)
1581 (667)1617 (557)1401 (429)   beverages (mL/day)   From food (mL/day)
1032 (329)989 (183)
830 (216)
1251 (420)1060 (305)   From alcoholic beverages
162 (289)
44 (92)103 (225)
444 (442)
59 (103)(mL/day)
Water intake (selective type)					   Total (mL/day)
2949 (828)3474 (1022)
2647 (772)
3102 (824)2576 (334)   from non-alcoholic beverages(mL/day)
1825 (720)2413 (936)1670 (703)
1667 (510)1549 (360)   from food (mL/day)
963 (256)
1010 (252)870 (237)
1006 (240)968 (312)   from alcoholic beverages
161 (274)
51 (93)107 (198)
428 (403)
60 (106)(mL/day)
Urinalysis					   Urine Osmolarity (mOsm/kg)
604 (247)
610 (236)
649 (270)
663 (226)
493 (227)   Urine Na (mEq/L)
102.4 (54.2)
105.4 (59.7)
110.3 (48.5)
118.3 (56.1)
75.4 (43.6)   Urine K (mEq/L)
28.9 (16.4)
28.8 (16.7)
27.8 (18.2)
30.4 (13.6)
28.4 (17.3)   Urine Creatinine (mg/dL)
133.6 (64.9)
153.2 (67.3)
143.7 (74.6)
141.0 (50.0)
96.7 (52.1)   Urine AQP (ng/mL)
5.01 (5.45)
4.24 (4.29)
6.14 (6.82)
6.74 (6.36)
2.92 (2.65)   Urine AQP2/Cre
0.03 (0.02)
0.02 (0.02)
0.04 (0.02)
0.04 (0.03)
0.03 (0.01)   Urine Na/K
4.1 (2.1)
4.5 (2.4)
4.9 (2.3)
4.1 (1.8)
2.9 (1.1)   Na excretion (mEq/day)
131.1 (26.0)
132.3 (28.1)
125.4 (25.0)
142.7 (25.0)
124.1 (22.5)
Toilet visits					   Day time (times/day)
6.6 (1.6)
7.1 (1.5)
6.5 (1.3)
6.5 (1.3)
6.5 (2.0)   Nighttime (times/day)
0.1 (0.3)
0.1 (0.3)
0.0 (0.0)
0.2 (0.4)
0.2 (0.4)
Data are given as mean (±SD).

be evaluated in the same way using either of thetwo methods. The correlation coefficient between
urine osmolarity and water intake calculated using the selective recall method was -0.27 (Figure3a), while that calculated using the descriptive dietary record method was -0.31 (Figure 3b). There
was no significant difference in these correlation
coefficients (r = 0.70). In addition, there was no
significant difference in the correlation coefficients based on sex or age group. Analysis of allparticipants revealed a weak correlation between
the total water intake and urine osmolarity (Figure 3a, b). Analysis based on sex and age group
showed a significant correlation in the values
from young men (Figure 3c, d) and young women(Figure 3e, f); however, there was no significant
correlation in the values from middle-aged men(Figure 3g, h) and women (Figure 3i, j).

Relationship between Urine Osmolarity
and Urine AQP2/Cre
The correlation between AQP2/Cre, an indicator of reabsorption, and urine osmolality was
examined to determine the body’s responses
to hydration status. Analysis of all participantsrevealed a positive correlation between urine
osmolarity and urine AQP2/Cre (r = 0.67; p <0.0001; Figure 4a). In addition, a positive correlation was observed in each of the four groups(Figure 4b-e).

Discussion
This study verified the validity of using theselective recall method to calculate total water intake. Specifically, the total water intakes,
6627

T. Uchida, Y. Nakamura, H. Tanaka, S. Nakamura, T. Okamura, H. Watanabe, N. Murayama
Figure 2. Correlation between total water intake calculated using the descriptive
dietary record method andthe selective recall method.
Correlation for (a) total water
intake; (b) water intake from
non-alcoholic beverages; (c)water intake from alcoholic beverages; (d) and water
intake from food. Data represents mean total water intake over 4 days.

calculated using the descriptive dietary record
method and the selective recall method, showeda strong correlation. This demonstrated the validity of the selective recall method. The water
intake from non-alcoholic beverages, food, andalcoholic beverages showed a strong correlation
between the two survey methods. Therefore,
the selective recall method provides a highly
accurate understanding of total water intake,for each source of water intake. The descriptive
dietary record method documents all food andbeverages consumed and it is one of the mostaccurate diet survey methods16,17. We developed
an extremely rigorous technique for verifying
water content that involves taking dietary information, collected using the descriptive dietary
record method, and multiplying each item by
the WCFC15. However, this method is time andlabor-intensive, because the registered dietitian
manually enters the paper-based description into
the nutritional calculation software. The selective recall method saves time and effort, because
the data from the participant is collected online
and processed directly. The strong correlation
between the values obtained using the descriptive dietary record method and selective recall
method indicated that the selective recall method provides a highly accurate understandingof water intake. The correlation between urine
6628

osmolality and water intake was not significantly different between the two methods, and thevariability of the scatter plot showed a similarcorrelation.
Considering that the daily dietary intake ishighly variable, validating a method for assessing
water intake must involve collecting informationon the different foods consumed over several
days. Food intake is commonly monitored for
3-7 days22. This study accounted for variationin the diet by surveying food intake over fourdays consisting of three working days and one
non-working day.
The proportion of water intake based on origin
was determined using the selective recall method;
the ratio of water intake from beverages to thatfrom food was 2.1:1, among all participants. Earlier surveys suggested that the Japanese people
derive a greater proportion of their water intake
from food than that by people from other countries and that they derive an equivalent proportionof water intake from beverages as from food14.
However, the participants in this study derived agreater proportion of water intake from beverages
than from food. The beverage consumption could
be higher in this study than expected because thesurvey was conducted in the summer. This could
also be attributed to the characteristic feature ofthis sample group.

Figure 3. Correlation between urine osmolarity and water intake calculated using (a, c, e, g, i) the selective recall method and (b, d, f, h, j) the descriptive dietary record method.
Correlation for (a, b) all participants, (c, d) young men, (e, f) young women, (g, h) middle-aged men and, (i, j) middle-aged women. Data represent a single day for each participantgroup.

6629

T. Uchida, Y. Nakamura, H. Tanaka, S. Nakamura, T. Okamura, H. Watanabe, N. Murayama

Figure 4. Correlation between urine osmolarity and urine aquaporin (AQP2)/creatinine (Cre). Correlation for (a) all
participants; (b) young men; (c) young women; (d) middle-aged men; and (e) middle-aged women. Data represent a single day
for each participant group.

The 95% confidence limit of osmolality in thefirst morning urine of young men was 377-1194
mOsm/kg23; our results were similar. Urine osmolarity with euhydration is 818-924 mOsm/kg, andthe water intake for that range is 2049-2453 mL/
day23; this is similar to the regression line valueof our results. Urine osmolality is higher (924mOsm/kg) with dehydration23; however, our results showed that few people were dehydrated, andmany were euhydrated to hyperhydrated. Additionally, the correlation between total water intake
and urinalysis parameters was analyzed taking
into account the sex and age group. Among theyoung participants, the total water intake correlated with the urinalysis parameters, urine osmolarity, and urine AQP2/Cre (Supplementary Figure2). Urine osmolarity is an established indicator ofwater intake and dehydration status24,25. Amongthe middle-aged participants, no correlation wasobserved between the total water intake and urinalysis parameters. Therefore, the body’s response
to water intake could differ between young andmiddle-aged adults. This difference could be attributed to the different requirements in terms ofwater volume and functional differences in renal
water regulation between these groups26,27.
Elevated plasma osmolarity induces the pituitary gland to secrete vasopressin. This triggers
AQP2 translocation to the plasma membrane
6630

on the luminal surface of the kidney collecting
duct28,29. Water is reabsorbed via AQP2, thereby
regulating the body fluid volume29,30. The quantityof AQP2 in urine is correlated with the quantity of
AQP2 translocated to the plasma membrane; it isused as an indicator of resorption29,30. Among middle-aged men, high urine AQP2/Cre levels wereobserved in some participants with high water
intake (Supplementary Figure 2). This is potentially due to the greater water intake from alcoholic
beverages. This results in a larger overall water
intake, owing to the diuretic properties of alcohol,
and increased urine AQP2/Cre levels that promotewater resorption. Meanwhile, among middle-aged
women, urine AQP2/Cre levels remained low evenamong those with low water intake (Supplementary Figure 2). This could be because the bodymeets the required water intake or, alternatively,
it does not meet its required water intake, but theresorption response is not properly activated.
Comparing urine osmolarity and urine AQP2/
Cre revealed a positive correlation among bothyoung and middle-aged participants. Translocation of large amounts of AQP2 to the plasma
membrane enhances water resorption volumes,
and lower urine output promotes higher urine
osmolarities29. This response was evident regardless of the participant sex or age group. Urine wascollected immediately upon rising; and therefore,

Validity of a new water intake asessing method

urinalysis parameters could have been affectedby the toilet frequency during bedtime. However,
the mean toilet frequency during bedtime was
0.1 ± 0.3 times/night; therefore, the effect of thisfactor on urinalysis parameters is negligible.
This study has the following strengths. First,
it is the first study to evaluate the validity of a
selective recall method for assessing water intake,from both beverages and food. This makes assessing water intake easy and accurate for future
investigations. Second, we measured not only thefluid intake but also the hydration status throughurinalysis. However, general verification only
compares the numbers between the questionnaires. We verified the validity of the selective
recall method by comparing it with the existing
method, and by evaluating the relationship between hydration status and water intake. Third,
we conducted the survey on four days, including
working and non-working days. Therefore, habitual water intake was calculated taking into account the daily dietary variations in one answer.
This study had some limitations. First, thesurveys were conducted only in summer using
a relatively small participant group. Second, theparticipants recruited were employees from Suntory World Research Center; this might haveintroduced selection bias for attributes pertaining
to working for a food company. Individuals withdifferent attributes and from a different region
were subjected to the same correlation analysis,
considering the water intake from beverages over
3 days. The correlation between the values fromthe descriptive questionnaire method and selective questionnaire was r = 0.93, with a linear regression line y = 0.96x + 5.88 (where x represents
water intake based on the selective questionnaire,and y represents water intake based on the descriptive questionnaire and general linear model,
p < 0.001)10. Therefore, a strong correlation between the descriptive dietary record method andselective recall method was observed in differing populations. Third, the study was conductedduring July and August of 2020, when many people refrained from going outdoors, because of the
spread of COVID-19 in Japan. This could haveresulted in changes to the living circumstancesthat affected the diet. However, the water intakes
based on the two methods were surveyed at thesame time; and therefore, they are comparable.
Fourth, we collected first morning urine to assess hydration status. 24 h urine collection is thepreferred method for assessing hydration status;
however, there is a strong correlation between

the osmolality of the first morning urine and thatof 24 h urine23. Therefore, we validated the questionnaire using the first morning urine. Fifth, thestudy was conducted on Japanese participants,
which limits the generalizability of the results.
A large-scale study including participants frommultiple regions and conducted over all four seasons is needed to examine the potential impact ofparticipant attributes and seasonality.

Conclusion
In this clinical study, there was a strong correlation between the water intake calculated using the descriptive dietary record method andthe selective recall method. This is the first study
to ascertain the validity of the selective recall
method for assessing water intake that includes
water intake from food. We are in the process ofdeveloping another method of assessing habitualwater intake that uses an even simpler questionnaire. This will provide a simple method for epidemiological studies for examining water intake
habits and the relationship between these habits
and health. These data have the potential to reveal
findings that lead to disease prevention and improved disease outcomes, while maintaining andimproving overall community health.
Conflict of Interest

The Authors declare that they have no conflict of interests.

Acknowledgements

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. This study was funded by Suntory Holdings Limited.
However, the funders had no role in the design of the study;in the collection, analyses, or interpretation of data; in thewriting of the manuscript, or in the decision to publish theresults. T.U., Y.N., H.W., and N.M. are employees of Suntory Holdings subsidiary Suntory Global Innovation Center
Limited. Conceptualization, T.U., Y.N., H.W., and N.M.; investigation, Y.N. and H.T.; formal analysis, T.U. and Y.N.;
writing and original draft preparation, T.U. and Y.N.; supervision, T.O. All authors have read and agreed to the published version of the manuscript.

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