Maintaining adequate hydration, in both unchallenged and dehydrated conditions, is associated with multiple health benefits. Proper hydration reduces risk for the development of chronic diseases, including cardiovascular, metabolic, and renal diseases, including the development of kidney stones [1
]. Additionally, adequate hydration is associated with reductions in cognitive [2
] and athletic performance [5
] impairments. Carbohydrate (CHO)–electrolyte solutions (CES), including sports drinks and oral rehydration solutions have traditionally been the options of choice for promoting euhydration [7
]. These solutions are designed to maintain or improve hydration status by promoting drinking, absorption of fluid from the small intestine via activation of sodium-glucose transporters [10
], and retention of fluid within the body and the vascular compartment [11
]. These pro-hydration properties are primarily a function of the carbohydrate and electrolyte composition of the beverage, as well as its total osmolality [13
Maughan et al. [14
] proposed the beverage hydration index (BHI) in 2016 as a measure of the hydrating capacity or efficacy of a given beverage relative to water. Since that time, BHI has been used to compare the hydration efficacy of a variety of beverages [14
]. This index is able to assess how a beverage impacts post-ingestion body fluid balance in individuals independent of sex or body mass [19
]. In calculating BHI, cumulative urine output at various time points following consumption of 1 L of water is set to a value of 1.0. Following beverage consumption of the same volume as water, beverages that elicit a greater urinary excretion than water over a fixed period have a BHI less than 1.0, while those that elicit greater fluid retention and attenuated urinary excretion have a BHI greater than 1.0. This measurement thereby enables comparisons of the hydration capacity of various beverages both within and across studies. Although the 2-h post-ingestion time point was proposed by Maughan as the standard for comparison [14
], additional information can be gleaned from data throughout the entire 4-h testing period, especially for older adults [18
] for whom ingested beverages are retained for a longer period of time, or after consumption of beverages that maintain positive fluid balance beyond 2 h.
Dairy-based beverages have been suggested as efficacious alternatives to traditional sports drinks [21
]. Maughan et al. determined that the BHI was higher for both whole milk and skim milk compared to water, and similar to that of an oral rehydration solution (ORS) [14
]. Those investigators opined that the high BHI of milk was likely due to its high protein (and perhaps fat) content, while the elevated BHI of the ORS was due to its carbohydrate and electrolyte content. To harness the hydrating qualities of dairy, attempts have been made to develop hydration beverages from byproducts produced during ultrafiltration of milk and cheese products [22
]. For example, large quantities of milk permeate are produced as a byproduct of the ultrafiltration of milk. Milk permeate is a protein-free, fat-free liquid that contains the approximate carbohydrate and mineral content of milk [24
]. It is also high in sodium and potassium and has a relatively high osmolality (primarily due to its total mineral content). Therefore, a solution containing milk permeate, developed primarily for use during exercise and other dehydrating conditions, may have hydration characteristics that are, at a minimum, similar to that of a traditional CES beverage. However, it remains to be determined how a milk permeate-based solution (MPS) impacts hydration status in humans and how this hydration efficacy compares to that of other commercial hydration solutions.
Determining the BHI of various beverages is an important first step in determining hydration efficacy, since the conditions under which this index is calculated are highly standardized and well-described [14
], and because it has been measured in individuals varying in size, sex, and age [14
]. Therefore, the primary purposes of the present study were: (1) to determine the hydration efficacy of a novel beverage containing milk permeate relative to water and CES, as measured by net fluid balance and BHI; and (2) to determine the extent to which fluid and electrolytes are retained in the vascular space after ingestion of each solution. We hypothesized that both CES and MPS would demonstrate a higher BHI than water over a 4-h period after standardized beverage ingestion in euhydrated subjects and that the BHI of the MPS beverage would be similar to that of the CES beverage.
The present investigation examined the hydration efficacy of a novel milk permeate-based (MPS) solution in comparison to water and a traditional carbohydrate-based electrolyte sports drink (CES). Compared to CES, MPS had a lower carbohydrate content (4% vs. 6%), a similar sodium concentration (20 vs. 21 mmol/L), a higher potassium concentration (28 vs. 3.2 mmol/L), and higher osmolality (621 vs. 326 mOsm/L). The primary finding of the study was that 1 L of the milk permeate solution, consumed in a euhydrated state, was retained in the body longer compared to water and CES. Calculated BHI was significantly higher for MPS compared to both water and CES across the 4 h post-ingestion period, accompanied by a similar vascular fluid compartment expansion. Finally, there were blunted plasma glucose concentration excursions (i.e., immediate post-drinking increase and subsequent decrease below baseline at 1 h) following MPS ingestion compared to CES.
Maintaining proper hydration status at rest is important for its health benefits and in the prevention of developing chronic disease [1
] and deficits in cognitive function [2
] and athletic performance [5
]. Carbohydrate-based electrolyte solutions have traditionally been recommended for promoting fluid retention and restoring euhydration [7
], especially during and following physical activity that is accompanied by profuse sweating. The beverage hydration index (BHI), based on cumulative urine output and net fluid balance, is an innovative approach for assessing the hydration efficacy of different beverages [14
]. This index is not impacted by differences in sex or body mass [19
], allowing its application to the general population, including older adults [18
]. In addition, results can be compared across studies that have followed the standardized BHI protocol published by Maughan et al. [14
]. For example, following consumption of a traditional carbohydrate-based sports beverage, BHI values in our study were similar to those reported in young men and women by Clarke et al. using a similar CES beverage [18
Prior investigations have suggested that whole or skim milk is an effective hydration solution at rest and its BHI is comparable to that of an oral rehydration solution beverage [14
]. Milk contains electrolytes, proteins, minerals, and other solutes that, when absorbed in the small intestine, promote fluid retention. The milk permeate beverage contains the approximate carbohydrate and mineral content of milk [24
], but without fats or proteins. The milk permeate solution tested here comprised approximately 21 mmol/L of sodium, which is similar to that of traditional carbohydrate-based hydration solutions, including the CES beverage utilized in this study (20 mmol/L). However, the potassium concentration of the MPS beverage (28 mmol/L) was considerably higher than the CES beverage (3.2 mmol/L). The MPS beverage also had a higher osmolality (621 ± 5 mosm/kg) compared to the CES beverage (326 ± 3 mosm/kg). The additional osmolar constituents of MPS consisted of chloride, magnesium, phosphorous, and calcium. The greater osmolality of MPS compared to CES or water likely contributed to the reduced urine production and greater fluid retention.
Indeed, our findings show that the cumulative urine output over the 4 h after ingestion of MPS was significantly lower than either water or CES (Figure 2
A) and was accompanied by a longer time spent in positive fluid balance (Figure 2
B). These results are consistent with prior studies showing that beverages with higher electrolyte concentrations and osmolality promote increased fluid retention in young adults [20
]. The increased fluid retention with MPS resulted in an increased BHI across the entire 4 h time course compared to water and CES (Figure 3
). Such an increase in fluid retention could be due to slower gastric emptying following ingestion of a high-osmotic solution, and we cannot rule out that possibility based on the data collected in this study [30
However, it is unlikely that differences in gastric emptying played a major role in influencing the BHI because other investigators have reported that the primary factor affecting gastric emptying is the energy content of the beverage, even when the osmolalities of the test beverages varied widely. Consumption of 500 mL of isocaloric beverages displayed similar gastric emptying rates, despite large differences in beverage osmolalities [31
], and impairments in gastric emptying were not seen at or below beverage glucose concentrations of 6% and osmolalities of 350 mosm/kg [32
]. The energy content of the MPS tested here was 27% lower than that of the CES, suggesting that fluid from MPS did not remain in the stomach longer. Additional research is warranted to discern how these differences in BHI are influenced by differences in the rates of gastric emptying, absorption in the proximal small intestine, and renal urine production [33
Serum osmolality was significantly elevated in the MPS trial compared to both CES and water at 60 min post-ingestion. This was driven, at least in part, by an increased osmolality of the MPS beverage. Intra-individual differences in serum osmolality between trials may explain some of the within-subject variability in urine output between beverages at earlier timepoints (i.e., 120 min), thus influencing BHI, although this variability appears to decrease at later timepoints (i.e., 240 min). Serum potassium concentration in the MPS trial was likewise elevated compared to water and CES in the first hour post-consumption. On the other hand, serum sodium concentration was only significantly elevated in the MPS trial compared to water and CES at 30 min, and was actually lowered compared to the CES trial at 60 min.
One important aspect of efficacious hydration is an expansion of extracellular fluid, specifically in the vascular compartment. There was an expansion of plasma volume (∆PV) in the MPS trial beginning at 30 min and continuing for the duration of the study, though this mildly increased plasma volume was not different from that of the water trial at any time point. Sustained elevations in PV, i.e., in 120- and 240-min responses are shown in Figure 5
. The %∆PV was lower for MPS than CES over the initial 10–15 min after drinking but results were highly variable (data not shown), which may reflect a slower initial rate of gastric emptying in response to the higher osmolality of the MPS beverage [32
]. Although subjects were in negative net fluid balance by the end of the study, indicating a greater excretion of fluid than what was consumed, there was a sustained plasma volume expansion of approximately 3–5% in the CES and MPS beverages at 240 min. The sustained mild PV expansion over at least 4 h after drinking MPS and CES likely indicates that the increased concentration of serum electrolytes helped retain some of the ingested fluid in the vascular space and possibly promoted some osmotic pull of water from the intracellular space into the vascular space [34
], allowing for hemodilution, an important component of efficacious hydration [35
The dairy-based MPS beverage in this study was approximately 4% glucose/galactose. In comparison, the carbohydrate–electrolyte solution (CES) tested in this study was 6% sucrose/glucose. It is thus important to elucidate potential differences in the glycemic load stemming from these differences in carbohydrate composition between beverages, which may be a particularly important consideration for populations who are at risk of metabolic dysfunction. The immediate rise in glucose concentration following MPS ingestion was blunted relative to the CES trial (Figure 5
). Additionally, the plasma glucose concentrations returned to baseline sooner after MPS consumption compared to CES consumption, which showed an overshoot below baseline values at 60 min. These plasma glucose profiles may be attributable to the lowered total carbohydrate load and lower glucose concentration in the MPS beverage compared to the CES beverage, supporting its efficacy as a potential lower-glycemic alternative to traditional carbohydrate-based sports beverages.
Important considerations for the benefits of a hydration-promoting beverage include the taste, consistency, and thirst-quenching qualities of the beverage. As such, subjects in the current study completed a sensory evaluation survey for each beverage (data not displayed) regarding qualitative aspects, such as overall likability, taste, sweetness, aroma, and thirst-quenching properties. There were no statistical differences in responses among beverages.
In prior BHI studies [14
], hydration solutions have been stored at approximately 4–6 °C. All beverages utilized in the current study were stored at 16–20 °C to prevent greater pressor responses to ingestion of cold beverages compared to room-temperature beverages [36
], which may negatively impact effective venous blood sampling within the first 30 min post-ingestion. While the present study used a prescribed drinking protocol, it has previously been reported that stimulation of cold sensitive oropharyngeal receptors results in lowered ad libitum fluid consumption in humans, and that optimal water temperature to encourage ad libitum consumption is approximately 15 °C [37
]. However, ad libitum consumption of either 4 or 20 °C water displayed no differential influence on hydration status in mildly dehydrated young adults [38
]. The current recommendation of the American College of Sports Medicine is that ingested fluids for the purpose of hydration should be at ambient temperatures between 15 and 22 °C [39
]. Therefore, we determined that storing beverages at room temperature was justified. Another limitation of this study, inherent to all BHI studies, is that the location of the fluid remaining in the body is unknown. BHI is determined by differences in the rates of gastric emptying, fluid absorption in the proximal small intestine, and renal urine production. In that regard, future studies measuring the gastric residual contents and duodenal constituent concentrations are warranted.
Although it was outside the scope of the current study to measure circulating concentrations of hormones associated with fluid balance maintenance, it is possible that circulating vasopressin influenced fluid retention in the current study. Following water consumption, there is a rapid decrease in plasma vasopressin concentrations independent of gastrointestinal absorption rate [40
]. Additionally, these changes appear to occur prior to changes in plasma osmolality [44
]. In a prior study examining the potential role of plasma vasopressin in plasma volume changes following consumption of either a glucose polymer–electrolyte solution or water, there were no differences in plasma vasopressin concentrations between the two trials despite a higher plasma volume in the glucose polymer–electrolyte trial compared to the water trial [46
]. However, there were no changes in plasma osmolalities in either trial in that study, whereas in the current study there was an increase in plasma osmolality in the MPS trial. Future investigation is warranted to examine the potential role of vasopressin in mediating changes in plasma volume following consumption of beverages with a wide range in osmolalities.
Approximately 36% of the United States population is affected by lactose malabsorption to some degree [47
], which may lead to gastrointestinal (GI) discomfort following dairy consumption. This is especially important in individuals who are exercising, as GI discomfort may impair performance. Thus, it is beneficial to explore potential dairy-based alternatives that can provide the same hydrating capacity of milk without the GI discomfort. The current MPS beverage consists of 2% glucose and 2% galactose and is protein- and fat-free. Milk typically contains 4–6% lactose. Although subjects in the current study did not report any GI discomfort at any point following consumption of the MPS beverage, no subjects in the study reported any history of intolerance to dairy-based products.
Subjects in this study started drinking in a euhydrated state and remained at rest for 4 h post-ingestion. When there is any suggestion that individuals may be even mildly dehydrated, clinicians often order prescribed drinking as a first step toward restoring or assuring euhydration. CES solutions are often the drink of choice in such conditions but cross-beverage comparisons have rarely been conducted. This adds situational validity to the BHI approach. However, it is unknown how these findings translate to maintenance of, or return to, euhydration under stressed conditions such as during and after exercise in the heat. Few investigations have examined the efficacy of either skim [48
] or low-fat [49
] milk for rehydration during or following acute bouts of exercise. Skim milk, both alone and with added sodium, resulted in a lower urine output compared to a traditional sports drink or water in the 4 h following cycling-induced dehydration [49
], though this response was attributed to the high protein concentration of milk. The solution tested in our study contained milk permeate, an ultrafiltrate of milk which is both protein- and fat-free, with the approximate mineral content of milk, and equivalent sodium concentration of common sports drinks. Future investigation is warranted to examine the efficacy of this milk permeate solution as a both a beverage for consumption during and after exercise.