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An empirical method to determine inadequacy of dietary water
Nutrition xxx (2015) 1–4
Contents lists available at ScienceDirect
Nutrition journal homepage: www.nutritionjrnl.com
Applied nutritional investigation
An empirical method to determine inadequacy of dietary water Lawrence E. Armstrong Ph.D. a, b, *, Evan C. Johnson Ph.D. a, Amy L. McKenzie Ph.D. a, ~ oz Ph.D. a Colleen X. Mun a b
University of Connecticut, Department of Kinesiology, Human Performance Laboratory, Storrs, Connecticut, USA University of Connecticut, Department of Nutritional Sciences, Storrs, Connecticut, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2015 Accepted 28 July 2015
Objectives: The physiological regulation of total body water and fluid concentrations is complex and dynamic. The human daily water requirement varies because of differences in body size, dietary solute load, exercise, and activities. Although chronically concentrated urine increases the risk of renal diseases, an empirical method to determine inadequate daily water consumption has not been described for any demographic group; instead, statistical analyses are applied to estimate nutritional guidelines (i.e., adequate intake). This investigation describes a novel empirical method to determine the 24-h total fluid intake (TFI; TFI ¼ water þ beverages þ moisture in food) and 24-h urine volume, which correspond to inadequate 24-h water intake (defined as urine osmolality of 800 mOsm/kg; U800). Methods: Healthy young women (mean standard deviation; age, 20 2 y, mass, 60.8 11.7 kg; n ¼ 28) were observed for 7 consecutive days. A 24-h urine sample was analyzed for volume and osmolality. Diet records were analyzed to determine 24-h TFI. Results: For these 28 healthy young women, the U800 corresponded to a TFI 2.4 L/d (39 mL/kg/ d) and a urine volume 1.3 L/d. Conclusions: The U800 method could be employed to empirically determine 24-h TFI and 24-h urine volumes that correspond to inadequate water intake in diverse demographic groups, residents of specific geographic regions, and individuals who consume specialized diets or experience large daily water turnover. Because laboratory expertise and instrumentation are required, this technique provides greatest value in research and clinical settings. Ó 2015 Elsevier Inc. All rights reserved.
Keywords: Fluid intake Urine volume Urine osmolality Hydration assessment Renal
Introduction The human body cannot produce adequate metabolic water (250–350 mL/d) [1] to offset fluid losses from the kidneys, lungs, and skin (adult range, 1.1–3.1 L/d) [2]. The thirst drive does not stimulate drinking until water loss reaches 1% to 2% of body weight [3], and solid food provides only approximately 20% of daily total fluid intake [1]. Because of these diverse factors, as well as differences in body size and dietary intake, daily water requirement varies from person to person. Further complicating the matter, the 24-h fluid intake of adult women [1] ranges from 1.3 (lowest decile; n ¼ 3091) to 6.1 (highest decile) L/d, suggesting that drinking behavior involves a sizeable volitional component. Despite the advanced technological capabilities that science employs today, a universal daily adequate water intake has not been defined precisely [3] for any demographic group (i.e., adults, * Corresponding author. Tel.: þ011-860-486-2647; fax: þ011-860-486-1123. E-mail address: [email protected] (L. E. Armstrong).
children, pregnant women) because body water turnover is complex and dynamic [4–6]. The European Food Safety Authority (EFSA) [7] approached this problem by considering fundamental principles of urinary water and solute excretion. Specifically, EFSA determined adequate intakes (AI) for water (i.e., the AI meets or exceeds the needs of most healthy individuals in a specific life-stage and sex) by combining median water consumption data with a theoretical desirable urine osmolality [7]. This approach is justified, in part, by the fact that urine is the only avenue of fluid loss which is homeostatically regulated by the brain. Although numerous indices of hydration status are utilized today (e.g., body weight change; plasma osmolality; and urine volume, color, specific gravity, and osmolality) [8], no single biomarker unequivocally represents human hydration status in all settings and in all persons [6,9]. However, a number of physiologists and dietitians have proposed that urine osmolality (Uosm) distinguishes mild dehydration from euhydration when it exceeds 800 mOsm/kg [10–15]. For example, in a population study of German children that analyzed urine collections (n ¼ 718) excreted
http://dx.doi.org/10.1016/j.nut.2015.07.013 0899-9007/Ó 2015 Elsevier Inc. All rights reserved.
Please cite this article in press as: Armstrong LE, et al., An empirical method to determine inadequacy of dietary water, Nutrition (2015), http://dx.doi.org/10.1016/j.nut.2015.07.013
2
L. E. Armstrong et al. / Nutrition xxx (2015) 1–4
during a 24-h period, Manz and Wentz [16] statistically identified 830 mOsm/kg as the lower limit of dehydration. A complex statistical evaluation of hydration biomarkers [9], involving receiver operating curve analyses of sensitivity and specificity, identified 831 mOsm/kg as the criterion value for dehydration in adult men and women. Similarly, two recent publications involving healthy adults evaluated commonly used hydration biomarkers. In the first, a Uosm of 800 was categorized as mild dehydration [17]; in the second study, no man or woman (n ¼ 71) who consumed more than 2 L water/d (range, 2–4 L/d) experienced a Uosm >800 mOsm/kg during 4 d of observations [18]. These publications independently support using a 24-h urine osmolality of approximately 800 mOsm/kg as the threshold that determines dehydration. Additional support for using urine osmolality as an index of inadequate water intake arises from studies of water homeostasis, in that thirst is sensed when plasma osmolality reaches 290 to 295 mOsm/kg; this tonicity corresponds to a urine osmolality of approximately 800 mOsm/kg [12,19], suggesting that the brain initiates thirst, in part, to avoid concentrating urine excessively. Therefore, we propose that inadequate water consumption exists when urine osmolality exceeds 800 mOsm/kg in a 24-h sample (U800). Using U800 in the present investigation, we identify the 24-h total fluid intake (TFI; TFI ¼ water þ beverages þ moisture in solid foods) and 24-h urine volume that represent the threshold of inadequate daily water consumption. Although more cumbersome than a spontaneous urine sample, a 24-h urine collection is advantageous because 1) it represents the sum of all behavioral (i.e., diet and exercise) and neuroendocrine responses that influence renal concentration or dilution throughout a day [5], and 2) it represents whole-body hydration status more accurately than spontaneous measurements made at a single point in time [5,18,20]. Furthermore, the large natural variance of 24-h urine osmolality (i.e., interindividual and intraindividual) is acknowledged and incorporated into the U800 technique.
participant to occur on the first day that she consumed the first placebo pill of her contraceptive pill pack. This was done to reduce the variance of day-to-day fluctuations in total body water due to reproductive hormone changes across the menstrual cycle. On 7 consecutive days, 24-h urine output was collected and analyzed for four variables: total volume by mass (Ohaus, Ranger, Parsippany, NJ); urine osmolality by freezing point depression osmometry (Uosm; Advanced Instruments Inc., Model 3320, Norwood MA); urine color, using a previously published color chart [22]; and urine specific gravity with a hand-held refractometer (Atago, A300 CL, Tokyo, Japan). Because indisputable thresholds for biological variables rarely exist, the U800 was determined in the present investigation, via data spreadsheet or graphs, as the minimum 24-h TFI or minimum 24-h urine volume that maintained Uosm below 800 mOsm/kg for all individuals in the test group (i.e., avoiding high urine concentration, reducing the risk of urolithiasis, and delaying the decline of renal function with age) [10–15,18,23,24].
Methods
Results
The data provided in this manuscript were collected according to the guidelines described in the Declaration of Helsinki, and all procedures involving human subjects were approved by the University of Connecticut Institutional Review Board for human studies. Written informed consent was obtained from all subjects. This retrospective analysis stems from a larger research study, organized in our laboratory [21], which involved measurements and analyses other than TFI and urinalysis. Figures 1–3 have not appeared in any previous publication. All subjects were college-aged females who had used oral contraceptives for at least 2 mo before enrolling. Eighty subjects qualified to participate in this investigation; of these, 28 volunteered to continue testing and were selected because they were either high volume drinkers (HIGH, n ¼ 14; top tertile; TFI, 2.0-4.0 L d1) or low volume drinkers (LOW, n ¼ 14; bottom tertile; 1.2 L/d). Exclusionary criteria included use of tobacco or nicotine; history of a disease or illness that alters normal fluid-electrolyte balance; fainting or becoming nauseous upon viewing needles or blood; being outside the age range of 19 to 34 y; not taking oral contraceptives for at least 90 d before enrollment; medication (other than oral contraceptives) that alter normal fluid–electrolyte balance, plasma osmolality, or urinary osmolality; diagnosis of type 1 or type 2 diabetes; reported caffeine intake >500 mg per day; engaging in >7 h moderate aerobic endurance training per week; or unwilling to abstain from alcohol during and the 2 d preceding this study. Fluid and food consumption were recorded across 6 consecutive days. On the morning of day 1, participants were instructed how to properly keep a 1-d diet record and to maintain their regular diet over the next 5 d (days 1–5). Participants returned to the laboratory each morning (days 2–6) between 0530 and 0800 h to review the previous day’s diet record with a counselor. Daily TFI was calculated by adding beverage volume, water volume, plus moisture content analysis of solid foods (Nutritionist Pro, Axxya Systems, Stafford TX). The data of HIGH and LOW were combined, to demonstrate the U800 method presented in this manuscript. These women reported to the laboratory between 05.30 and 08.00 h each day. Within each subject, arrival time did not differ more than 1 h throughout the course of observations. Day 1 was standardized for each
Average personal characteristics for the 28 female participants were as follows: age, 20 2 y; mass, 60.8 11.7 kg; and height 164 9 cm. Figures 1–3 depict relationships between
Fig. 1. Determination of U800 for 24-h total fluid intake (L/d; n ¼ 195).
Fig. 2. Determination of U800 for 24-h total fluid intake, expressed relative to body weight (mL/kg/d; n ¼ 195).
Please cite this article in press as: Armstrong LE, et al., An empirical method to determine inadequacy of dietary water, Nutrition (2015), http://dx.doi.org/10.1016/j.nut.2015.07.013
L. E. Armstrong et al. / Nutrition xxx (2015) 1–4
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External support for the U800 method
Fig. 3. Determination of U800 for 24-h urine volume (L/d; n ¼ 195).
Uosm, TFI, and urine volume. These figures illustrate the TFI and urine volume that correspond to the U800 threshold. For example, Figure 1 illustrates that a 24-h TFI 2.4 L/d (see vertical line) is required by healthy young women to maintain urine osmolality below U800. Figure 2 expresses the volume of TFI relative to body weight and shows that the TFI is 39 mL/kg/d for daily water intake to be adequate (i.e., below U800). Figure 3 illustrates that a daily urine volume of 1.3 L/d (vertical line) maintains Uosm below U800. All variables in Figures 1–3 represent 24-h measurement periods. Uosm was significantly correlated (P ¼ 0.00001) with urine specific gravity (R2 ¼ 0.94), urine color (R2 ¼ 0.67), TFI (R2 ¼ 0.50), and 24-h beverage intake (R2 ¼ 0.52).
Discussion Physiologists and dietitians have proposed that Uosm distinguishes dehydration from euhydration in healthy individuals when it exceeds 800 mOsm/kg [10–12,14,15]. However, approximately 40% of Uosm is due to urea, depending on dietary macronutrients. Because diet was not controlled in the present study, the variance of urine osmolality in Figures 1–3 (see all y-axes) results from interindividual differences of both TFI and dietary osmolar content. Thus, the U800 level may be achieved with markedly different 24-h urine volumes (range, 0.3–1.3 L/d) and 24-h TFI (0.6–2.3 L/d), and U800 may or may not represent impending dehydration. However, U800 is meaningful in terms of kidney health, in that nephrologists 1) recognize that a low daily water intake increases the risk of urolithiasis and chronic kidney disease [25,26], and 2) promote increased 24-h TFI as the simplest and most effective means of reducing the risk of renal disease [23,27–29]. Their rationale recognizes that underconsumption of water results in urinary supersaturation, the driving force for crystallization and kidney stone formation [30]. Employing the vertical lines in Figures 1–3, the U800 describes the TFI or urine volumes (i.e., when exceeded) at which no person will experience chronically concentrated urine.
The U800 method is supported by international organizations and TFI data from other laboratories. Figure 1, for example, illustrates that a TFI 2.4 L/d corresponds to the U800 threshold for young women. This value is similar to the recommended daily water intake for women of 2.0 L/d, published by EFSA [7]; 2.2 L/d recommended by the World Health Organization [31]; and 2.7 L/d published by the Institute of Medicine, National Academy of Sciences, USA [1]; as well as the minimum TFI of 2.5 L/d recommended by Siener and Hesse [32] to prevent formation of renal stones in healthy men and women. These published water intake values also agree well with the relative TFI values in Figure 2, expressed as mL/kg of body weight. Figure 3 demonstrates that a urine volume of 1.3 L/d corresponds to the U800 threshold in young women. This is similar to the 1.2 L/d value that was identified by Hess [30] as a critical urine volume, below which the incidence of renal stones increased sharply in men and women. Similarly, a randomized prospective clinical trial determined that the average baseline urine volume of 101 middle-aged female control subjects was 1.24 0.44 L/d, whereas that of 199 female urolithiasis patients was 0.99 0.23 L/d shortly after the first stone formation [33]. The urolithiasis patients were subsequently divided into two groups (group 1, no change of daily water intake or diet; group 2, increased daily intake of water but no change of diet). A 5-year follow-up determined that the incidence of stone formation was 2.2-fold greater in group 1 than in group 2, and the mean urine volumes were 1.01 0.20 and 2.62 0.44 L/d, respectively. Advantages and limitations Compared with determinations of adequate water intake [1,7] and urinary hydration biomarkers [4,6,8], this method offers four distinct advantages. First, because a 24-h urine collection represents the sum of all behavioral and neuroendocrine responses that influence renal concentration or dilution [5], the U800 method is valid regardless of dietary content (i.e., renal solute load), TFI, body size, or physical activity. Second, in contrast to AI values (which meet or exceed the needs of most healthy individuals in a specific life-stage and sex) [1,7], the U800 value determines a 24-h TFI or 24-h urine volume that will attenuate risk for urolithiasis [25,26] for all individuals in the population sample. Third, U800 is useful despite the large variance of urine osmolality (Figs. 1–3) attributable to interindividual (i.e., across days) and intraindividual differences of metabolism, solid food/water consumption, and renal function. Fourth, the U800 method does not require measurement of all avenues (i.e., stool, sweat, insensible, and respiratory) of water loss to detect hyper-concentrated urine. However, we acknowledge two limitations of the U800 method. First, external validity of the 24-h TFI or 24-h urine volume data is limited and applies only to women who are similar to those who participated in the present study; different demographic groups should be evaluated independently. Second, because precise measurements of Uosm and accurate values for 24-h TFI require expertise and laboratory instruments that the average adult cannot employ, this technique provides greatest value in research and clinical settings. Conclusions Recognizing that chronically concentrated urine imparts negative health outcomes [20,25–30], the U800 laboratory method determines the point at which 24-h water intake is
Please cite this article in press as: Armstrong LE, et al., An empirical method to determine inadequacy of dietary water, Nutrition (2015), http://dx.doi.org/10.1016/j.nut.2015.07.013
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L. E. Armstrong et al. / Nutrition xxx (2015) 1–4
inadequate, utilizing measurements of osmolality in 24-h urine samples [10–15]. This straightforward method could be applied to demographic groups with very different lifestyles including men or women, children, pregnant women, residents of a geographic region, populations who consume specialized or modified diets (e.g., low carbohydrate, high protein, low salt), and individuals who chronically lose a large volume of sweat each day (e.g., laborers, soldiers, athletes, and desert dwellers).
Acknowledgment Calculations of U800 were determined via retrospective analysis of data, from an unpublished investigation funded by Danone Research, Palaiseau, France. Author contributions were as follows: All authors contributed to the research design; LA, EJ and CM participated in data collection; LA and EJ performed the data analysis; LA wrote the manuscript; and EJ, AM, and CM helped in editorial review of the manuscript. LA is a Danone Research Scientific Advisory Board member, paid consultant. EJ, AM, and CM declare no conflict of interest. The authors gratefully acknowledge the insightful review comments of Erica Perrier, Ph.D., Danone Research, France. Evan Johnson presently is a faculty member at the University of Wyoming, Division of Kinesiology & Health, Laramie, Wyoming, USA. Colleen Munoz presently is a faculty member at the University of Hartford, Department of Health Sciences & Nursing, Hartford, Connecticut, USA.
References [1] Institute of Medicine (US). Panel on Dietary Reference Intakes for Electrolytes, & Water. DRI, dietary reference intakes for water, potassium, sodium, chloride, and sulfate. Washington, D.C.: National Academy Press; 2005:1–617. [2] Sawka MN, Cheuvront SN, Carter R. Human water needs. Nutr Rev 2005;63:S30–9. [3] Greenleaf JE. Problem: Thirst, drinking behavior, and involuntary dehydration. Med Sci Sports Exerc 1992;24:645–56. [4] Oppliger R, Bartok C. Hydration testing of athletes. Sports Med 2002;32:959–71. [5] Manz F, Wentz A. 24-h hydration status: parameters, epidemiology and recommendations. Eur J Clin Nutr 2003;57:S10–8. [6] Armstrong LE. Assessing hydration status: the elusive gold standard. J Am Coll Nutr 2007;26:S575–84. [7] European Food Safety Authority. Scientific opinion on dietary reference values for water. EFSA J 2010;8:1459–506. [8] Kavouras SA. Assessing hydration status. Curr Opin Clin Nutr Metab Care 2002;5:519–24. [9] Cheuvront SN, Ely BR, Kenefick RW, Sawka MN. Biological variation and diagnostic accuracy of dehydration assessment markers. Am J Clin Nutr 2010;92:565–73. [10] Katz AI, Massry S, Agmon J, Toor M. Concentration and dilution of urine in permanent inhabitants of hot regions. Isr J Med Sci 1965;1:968–78.
[11] Bar-David Y, Urkin J, Kozminsky E. The effect of voluntary dehydration on cognitive functions of elementary school children. Acta Pediatrica 2005;94:1667–73. [12] Bar-David Y, Urkin J, Landau D, Bar-David Z, Pulpal D. Voluntary dehydration among elementary school children residing in a hot arid environment. J Hum Nutr Diet 2009;22:455–60. CM, et al. Effects [13] Fadda R, Rapinett G, Grathwoh D, Parisi M, Fanari R, Calo of drinking supplementary water at school on cognitive performance in children. Appetite 2012;59:730–7. [14] Bonnet F, Picard EM, Catherin L, Likelier C, Constant F, Haile N, et al. French children start their school day with a hydration deficit. Ann Nutr Metab 2012;60:257–63. [15] Stookey JD, Brass B, Holliday A, Arieff A. What is the cell hydration status of healthy children in the USA? Preliminary data on urine osmolality and water intake. Public Health Nutr 2012;15:2148–56. [16] Manz F, Wentz A, Sichert-Hellert W. The most essential nutrient: defining the adequate intake of water. J Pediatr 2002;14:587–92. [17] Armstrong LE, Pumerantz AC, Fiala KA, Roti MW, Kavouras SA, Casa DJ, et al. Human hydration indices: acute and longitudinal reference values. Int J Sport Nutr Exerc Metab 2010;20:145–53. [18] Perrier E, Vergne S, Klein A, Poupin M, Rondeau P, Le Bellego L, et al. Hydration biomarkers in free-living adults with different levels of habitual fluid consumption. Br J Nutr 2012;109:1678–87. [19] Vokes T. Water homeostasis. Annu Rev Nutr 1987;7:383–406. [20] Armstrong LE, Johnson EC, McKenzie AL, Munoz CX. Interpreting common hydration biomarkers on the basis of solute and water excretion. Eur J Clin Nutr 2013;67:249–53. ~ oz CX, Le Bellego L, Klein A, Casa DJ, Maresh CM, et al. [21] Johnson EC, Mun Markers of the hydration process during fluid volume modification in women with habitual high or low daily fluid intakes. Eur J Appl Physiol 2015;115:1067–74. [22] Armstrong LE, Maresh CM, Castellani J, Bergeron M, Kenefick RW, LaGasse KE, et al. Urinary indices of hydration status. Int J Sport Nutr 1994;4:265–79. [23] Armstrong LE. The challenges of linking chronic dehydration and fluid consumption to health outcomes. Nutr Rev 2012;70:121–7. [24] Perrier ET, Buendia-Jimenez I, Vecchio M, Armstrong LE, Tack I, Klein A. Twenty-four-hour urine osmolality as a physiological index of adequate water intake. Dis Markers 2015;2015:231063. [25] Strippoli GF, Craig JC, Rochtchina E, Flood VM, Wang JJ, Mitchell P. Fluid and nutrient intake and risk of chronic kidney disease. Nephrology 2011;16:326–34. [26] Clark WF, Sontrop JM, Macnab JJ, Suri RS, Moist L, Salvadori M, et al. Urine volume and change in estimated GFR in a community-based cohort study. Clin J Am Soc Nephrol 2011;6:2634–41. [27] Tiselius HG. Aspects on estimation of the risk of calcium oxalate crystallization in urine. Urol Int 1991;47:255–9. [28] Borghi L, Meschi T, Schianchi T, Allegri F, Guerra A, Maggiore U, et al. Medical treatment of nephrolithiasis. Endocrinol Metab Clin North Am 2002;31:1051–64. [29] Tack I. Effects of water consumption on kidney function and excretion. Nutr Today 2010;45:S37–40. [30] Hess B. Nutritional aspects of stone disease. Endocrinol Metab Clin North Am 2002;31:1017–30. [31] Howard G, Bartram J. Domestic water quantity, service level and health. Geneva, Switzerland: World Health Organization; 2003:1–33. [32] Siener R, Hesse A. The effect of different diets on urine composition and the risk of calcium oxalate crystallization in healthy subjects. Eur Urol 2002;42:289–96. [33] Borghi L, Meschi T, Amato F, Briganti A, Novarini A, Giannini A. Urinary volume, water and recurrences in idiopathic calcium nephrolithiasis: a 5-year randomized prospective study. J Urol 1996;155: 839–43.
Please cite this article in press as: Armstrong LE, et al., An empirical method to determine inadequacy of dietary water, Nutrition (2015), http://dx.doi.org/10.1016/j.nut.2015.07.013
Contents lists available at ScienceDirect
Nutrition journal homepage: www.nutritionjrnl.com
Applied nutritional investigation
An empirical method to determine inadequacy of dietary water Lawrence E. Armstrong Ph.D. a, b, *, Evan C. Johnson Ph.D. a, Amy L. McKenzie Ph.D. a, ~ oz Ph.D. a Colleen X. Mun a b
University of Connecticut, Department of Kinesiology, Human Performance Laboratory, Storrs, Connecticut, USA University of Connecticut, Department of Nutritional Sciences, Storrs, Connecticut, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2015 Accepted 28 July 2015
Objectives: The physiological regulation of total body water and fluid concentrations is complex and dynamic. The human daily water requirement varies because of differences in body size, dietary solute load, exercise, and activities. Although chronically concentrated urine increases the risk of renal diseases, an empirical method to determine inadequate daily water consumption has not been described for any demographic group; instead, statistical analyses are applied to estimate nutritional guidelines (i.e., adequate intake). This investigation describes a novel empirical method to determine the 24-h total fluid intake (TFI; TFI ¼ water þ beverages þ moisture in food) and 24-h urine volume, which correspond to inadequate 24-h water intake (defined as urine osmolality of 800 mOsm/kg; U800). Methods: Healthy young women (mean standard deviation; age, 20 2 y, mass, 60.8 11.7 kg; n ¼ 28) were observed for 7 consecutive days. A 24-h urine sample was analyzed for volume and osmolality. Diet records were analyzed to determine 24-h TFI. Results: For these 28 healthy young women, the U800 corresponded to a TFI 2.4 L/d (39 mL/kg/ d) and a urine volume 1.3 L/d. Conclusions: The U800 method could be employed to empirically determine 24-h TFI and 24-h urine volumes that correspond to inadequate water intake in diverse demographic groups, residents of specific geographic regions, and individuals who consume specialized diets or experience large daily water turnover. Because laboratory expertise and instrumentation are required, this technique provides greatest value in research and clinical settings. Ó 2015 Elsevier Inc. All rights reserved.
Keywords: Fluid intake Urine volume Urine osmolality Hydration assessment Renal
Introduction The human body cannot produce adequate metabolic water (250–350 mL/d) [1] to offset fluid losses from the kidneys, lungs, and skin (adult range, 1.1–3.1 L/d) [2]. The thirst drive does not stimulate drinking until water loss reaches 1% to 2% of body weight [3], and solid food provides only approximately 20% of daily total fluid intake [1]. Because of these diverse factors, as well as differences in body size and dietary intake, daily water requirement varies from person to person. Further complicating the matter, the 24-h fluid intake of adult women [1] ranges from 1.3 (lowest decile; n ¼ 3091) to 6.1 (highest decile) L/d, suggesting that drinking behavior involves a sizeable volitional component. Despite the advanced technological capabilities that science employs today, a universal daily adequate water intake has not been defined precisely [3] for any demographic group (i.e., adults, * Corresponding author. Tel.: þ011-860-486-2647; fax: þ011-860-486-1123. E-mail address: [email protected] (L. E. Armstrong).
children, pregnant women) because body water turnover is complex and dynamic [4–6]. The European Food Safety Authority (EFSA) [7] approached this problem by considering fundamental principles of urinary water and solute excretion. Specifically, EFSA determined adequate intakes (AI) for water (i.e., the AI meets or exceeds the needs of most healthy individuals in a specific life-stage and sex) by combining median water consumption data with a theoretical desirable urine osmolality [7]. This approach is justified, in part, by the fact that urine is the only avenue of fluid loss which is homeostatically regulated by the brain. Although numerous indices of hydration status are utilized today (e.g., body weight change; plasma osmolality; and urine volume, color, specific gravity, and osmolality) [8], no single biomarker unequivocally represents human hydration status in all settings and in all persons [6,9]. However, a number of physiologists and dietitians have proposed that urine osmolality (Uosm) distinguishes mild dehydration from euhydration when it exceeds 800 mOsm/kg [10–15]. For example, in a population study of German children that analyzed urine collections (n ¼ 718) excreted
http://dx.doi.org/10.1016/j.nut.2015.07.013 0899-9007/Ó 2015 Elsevier Inc. All rights reserved.
Please cite this article in press as: Armstrong LE, et al., An empirical method to determine inadequacy of dietary water, Nutrition (2015), http://dx.doi.org/10.1016/j.nut.2015.07.013
2
L. E. Armstrong et al. / Nutrition xxx (2015) 1–4
during a 24-h period, Manz and Wentz [16] statistically identified 830 mOsm/kg as the lower limit of dehydration. A complex statistical evaluation of hydration biomarkers [9], involving receiver operating curve analyses of sensitivity and specificity, identified 831 mOsm/kg as the criterion value for dehydration in adult men and women. Similarly, two recent publications involving healthy adults evaluated commonly used hydration biomarkers. In the first, a Uosm of 800 was categorized as mild dehydration [17]; in the second study, no man or woman (n ¼ 71) who consumed more than 2 L water/d (range, 2–4 L/d) experienced a Uosm >800 mOsm/kg during 4 d of observations [18]. These publications independently support using a 24-h urine osmolality of approximately 800 mOsm/kg as the threshold that determines dehydration. Additional support for using urine osmolality as an index of inadequate water intake arises from studies of water homeostasis, in that thirst is sensed when plasma osmolality reaches 290 to 295 mOsm/kg; this tonicity corresponds to a urine osmolality of approximately 800 mOsm/kg [12,19], suggesting that the brain initiates thirst, in part, to avoid concentrating urine excessively. Therefore, we propose that inadequate water consumption exists when urine osmolality exceeds 800 mOsm/kg in a 24-h sample (U800). Using U800 in the present investigation, we identify the 24-h total fluid intake (TFI; TFI ¼ water þ beverages þ moisture in solid foods) and 24-h urine volume that represent the threshold of inadequate daily water consumption. Although more cumbersome than a spontaneous urine sample, a 24-h urine collection is advantageous because 1) it represents the sum of all behavioral (i.e., diet and exercise) and neuroendocrine responses that influence renal concentration or dilution throughout a day [5], and 2) it represents whole-body hydration status more accurately than spontaneous measurements made at a single point in time [5,18,20]. Furthermore, the large natural variance of 24-h urine osmolality (i.e., interindividual and intraindividual) is acknowledged and incorporated into the U800 technique.
participant to occur on the first day that she consumed the first placebo pill of her contraceptive pill pack. This was done to reduce the variance of day-to-day fluctuations in total body water due to reproductive hormone changes across the menstrual cycle. On 7 consecutive days, 24-h urine output was collected and analyzed for four variables: total volume by mass (Ohaus, Ranger, Parsippany, NJ); urine osmolality by freezing point depression osmometry (Uosm; Advanced Instruments Inc., Model 3320, Norwood MA); urine color, using a previously published color chart [22]; and urine specific gravity with a hand-held refractometer (Atago, A300 CL, Tokyo, Japan). Because indisputable thresholds for biological variables rarely exist, the U800 was determined in the present investigation, via data spreadsheet or graphs, as the minimum 24-h TFI or minimum 24-h urine volume that maintained Uosm below 800 mOsm/kg for all individuals in the test group (i.e., avoiding high urine concentration, reducing the risk of urolithiasis, and delaying the decline of renal function with age) [10–15,18,23,24].
Methods
Results
The data provided in this manuscript were collected according to the guidelines described in the Declaration of Helsinki, and all procedures involving human subjects were approved by the University of Connecticut Institutional Review Board for human studies. Written informed consent was obtained from all subjects. This retrospective analysis stems from a larger research study, organized in our laboratory [21], which involved measurements and analyses other than TFI and urinalysis. Figures 1–3 have not appeared in any previous publication. All subjects were college-aged females who had used oral contraceptives for at least 2 mo before enrolling. Eighty subjects qualified to participate in this investigation; of these, 28 volunteered to continue testing and were selected because they were either high volume drinkers (HIGH, n ¼ 14; top tertile; TFI, 2.0-4.0 L d1) or low volume drinkers (LOW, n ¼ 14; bottom tertile; 1.2 L/d). Exclusionary criteria included use of tobacco or nicotine; history of a disease or illness that alters normal fluid-electrolyte balance; fainting or becoming nauseous upon viewing needles or blood; being outside the age range of 19 to 34 y; not taking oral contraceptives for at least 90 d before enrollment; medication (other than oral contraceptives) that alter normal fluid–electrolyte balance, plasma osmolality, or urinary osmolality; diagnosis of type 1 or type 2 diabetes; reported caffeine intake >500 mg per day; engaging in >7 h moderate aerobic endurance training per week; or unwilling to abstain from alcohol during and the 2 d preceding this study. Fluid and food consumption were recorded across 6 consecutive days. On the morning of day 1, participants were instructed how to properly keep a 1-d diet record and to maintain their regular diet over the next 5 d (days 1–5). Participants returned to the laboratory each morning (days 2–6) between 0530 and 0800 h to review the previous day’s diet record with a counselor. Daily TFI was calculated by adding beverage volume, water volume, plus moisture content analysis of solid foods (Nutritionist Pro, Axxya Systems, Stafford TX). The data of HIGH and LOW were combined, to demonstrate the U800 method presented in this manuscript. These women reported to the laboratory between 05.30 and 08.00 h each day. Within each subject, arrival time did not differ more than 1 h throughout the course of observations. Day 1 was standardized for each
Average personal characteristics for the 28 female participants were as follows: age, 20 2 y; mass, 60.8 11.7 kg; and height 164 9 cm. Figures 1–3 depict relationships between
Fig. 1. Determination of U800 for 24-h total fluid intake (L/d; n ¼ 195).
Fig. 2. Determination of U800 for 24-h total fluid intake, expressed relative to body weight (mL/kg/d; n ¼ 195).
Please cite this article in press as: Armstrong LE, et al., An empirical method to determine inadequacy of dietary water, Nutrition (2015), http://dx.doi.org/10.1016/j.nut.2015.07.013
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External support for the U800 method
Fig. 3. Determination of U800 for 24-h urine volume (L/d; n ¼ 195).
Uosm, TFI, and urine volume. These figures illustrate the TFI and urine volume that correspond to the U800 threshold. For example, Figure 1 illustrates that a 24-h TFI 2.4 L/d (see vertical line) is required by healthy young women to maintain urine osmolality below U800. Figure 2 expresses the volume of TFI relative to body weight and shows that the TFI is 39 mL/kg/d for daily water intake to be adequate (i.e., below U800). Figure 3 illustrates that a daily urine volume of 1.3 L/d (vertical line) maintains Uosm below U800. All variables in Figures 1–3 represent 24-h measurement periods. Uosm was significantly correlated (P ¼ 0.00001) with urine specific gravity (R2 ¼ 0.94), urine color (R2 ¼ 0.67), TFI (R2 ¼ 0.50), and 24-h beverage intake (R2 ¼ 0.52).
Discussion Physiologists and dietitians have proposed that Uosm distinguishes dehydration from euhydration in healthy individuals when it exceeds 800 mOsm/kg [10–12,14,15]. However, approximately 40% of Uosm is due to urea, depending on dietary macronutrients. Because diet was not controlled in the present study, the variance of urine osmolality in Figures 1–3 (see all y-axes) results from interindividual differences of both TFI and dietary osmolar content. Thus, the U800 level may be achieved with markedly different 24-h urine volumes (range, 0.3–1.3 L/d) and 24-h TFI (0.6–2.3 L/d), and U800 may or may not represent impending dehydration. However, U800 is meaningful in terms of kidney health, in that nephrologists 1) recognize that a low daily water intake increases the risk of urolithiasis and chronic kidney disease [25,26], and 2) promote increased 24-h TFI as the simplest and most effective means of reducing the risk of renal disease [23,27–29]. Their rationale recognizes that underconsumption of water results in urinary supersaturation, the driving force for crystallization and kidney stone formation [30]. Employing the vertical lines in Figures 1–3, the U800 describes the TFI or urine volumes (i.e., when exceeded) at which no person will experience chronically concentrated urine.
The U800 method is supported by international organizations and TFI data from other laboratories. Figure 1, for example, illustrates that a TFI 2.4 L/d corresponds to the U800 threshold for young women. This value is similar to the recommended daily water intake for women of 2.0 L/d, published by EFSA [7]; 2.2 L/d recommended by the World Health Organization [31]; and 2.7 L/d published by the Institute of Medicine, National Academy of Sciences, USA [1]; as well as the minimum TFI of 2.5 L/d recommended by Siener and Hesse [32] to prevent formation of renal stones in healthy men and women. These published water intake values also agree well with the relative TFI values in Figure 2, expressed as mL/kg of body weight. Figure 3 demonstrates that a urine volume of 1.3 L/d corresponds to the U800 threshold in young women. This is similar to the 1.2 L/d value that was identified by Hess [30] as a critical urine volume, below which the incidence of renal stones increased sharply in men and women. Similarly, a randomized prospective clinical trial determined that the average baseline urine volume of 101 middle-aged female control subjects was 1.24 0.44 L/d, whereas that of 199 female urolithiasis patients was 0.99 0.23 L/d shortly after the first stone formation [33]. The urolithiasis patients were subsequently divided into two groups (group 1, no change of daily water intake or diet; group 2, increased daily intake of water but no change of diet). A 5-year follow-up determined that the incidence of stone formation was 2.2-fold greater in group 1 than in group 2, and the mean urine volumes were 1.01 0.20 and 2.62 0.44 L/d, respectively. Advantages and limitations Compared with determinations of adequate water intake [1,7] and urinary hydration biomarkers [4,6,8], this method offers four distinct advantages. First, because a 24-h urine collection represents the sum of all behavioral and neuroendocrine responses that influence renal concentration or dilution [5], the U800 method is valid regardless of dietary content (i.e., renal solute load), TFI, body size, or physical activity. Second, in contrast to AI values (which meet or exceed the needs of most healthy individuals in a specific life-stage and sex) [1,7], the U800 value determines a 24-h TFI or 24-h urine volume that will attenuate risk for urolithiasis [25,26] for all individuals in the population sample. Third, U800 is useful despite the large variance of urine osmolality (Figs. 1–3) attributable to interindividual (i.e., across days) and intraindividual differences of metabolism, solid food/water consumption, and renal function. Fourth, the U800 method does not require measurement of all avenues (i.e., stool, sweat, insensible, and respiratory) of water loss to detect hyper-concentrated urine. However, we acknowledge two limitations of the U800 method. First, external validity of the 24-h TFI or 24-h urine volume data is limited and applies only to women who are similar to those who participated in the present study; different demographic groups should be evaluated independently. Second, because precise measurements of Uosm and accurate values for 24-h TFI require expertise and laboratory instruments that the average adult cannot employ, this technique provides greatest value in research and clinical settings. Conclusions Recognizing that chronically concentrated urine imparts negative health outcomes [20,25–30], the U800 laboratory method determines the point at which 24-h water intake is
Please cite this article in press as: Armstrong LE, et al., An empirical method to determine inadequacy of dietary water, Nutrition (2015), http://dx.doi.org/10.1016/j.nut.2015.07.013
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inadequate, utilizing measurements of osmolality in 24-h urine samples [10–15]. This straightforward method could be applied to demographic groups with very different lifestyles including men or women, children, pregnant women, residents of a geographic region, populations who consume specialized or modified diets (e.g., low carbohydrate, high protein, low salt), and individuals who chronically lose a large volume of sweat each day (e.g., laborers, soldiers, athletes, and desert dwellers).
Acknowledgment Calculations of U800 were determined via retrospective analysis of data, from an unpublished investigation funded by Danone Research, Palaiseau, France. Author contributions were as follows: All authors contributed to the research design; LA, EJ and CM participated in data collection; LA and EJ performed the data analysis; LA wrote the manuscript; and EJ, AM, and CM helped in editorial review of the manuscript. LA is a Danone Research Scientific Advisory Board member, paid consultant. EJ, AM, and CM declare no conflict of interest. The authors gratefully acknowledge the insightful review comments of Erica Perrier, Ph.D., Danone Research, France. Evan Johnson presently is a faculty member at the University of Wyoming, Division of Kinesiology & Health, Laramie, Wyoming, USA. Colleen Munoz presently is a faculty member at the University of Hartford, Department of Health Sciences & Nursing, Hartford, Connecticut, USA.
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Please cite this article in press as: Armstrong LE, et al., An empirical method to determine inadequacy of dietary water, Nutrition (2015), http://dx.doi.org/10.1016/j.nut.2015.07.013