The present study compares different hydration effects from consumption of water, sports drinks, and ORS. We examined water absorption and plasma volume change in athletes based on the measurements of body weight change, cumulative urine output, BHI, and PVC.
Body weight change was significantly smaller for ORS than for water, SpD1, and SpD2, indicating that bodily water loss is smaller following the consumption of ORS than SpD1, and SpD, and water. Cumulative urine output differed only slightly in testing time, although cumulative urine output in 4 h was lower for ORS, SpD1, and SpD2 than for water, and it was lower for ORS than for SpD2. Thus, we believe that bodily water loss is smaller for ORS than for water and SpD2. According to a study by Millard-Stafford et al. [14], the cumulative urine output from 1 to 4 h after water intake was greater than that of drinks containing carbohydrates and electrolytes, and at 3 and 4 h, there is more water than a drink containing dipeptide and electrolyte, which supports the fact that the sports drink show in this study produced less cumulative urine than water.
The BHI is the urine output after consumption of water divided by the urine output after beverage consumption, and it is an index commonly used to verify the hydration effect of a beverage [13, 18]. In this study, although BHI was slightly different for each time period, ORS, SpD1, and SpD2 were higher than water from 1 to 4 h after water intake.According to Bechke et al. [2], BHI was higher in sports drinks than water at 3 and 4 h after water intake, and in a study by Millard-Stafford et al. [14], BHI was high in drinks containing carbohydrate and electrolyte and drinks containing dipeptide and electrolyte after 4 h after water intake, which supports the result of this study. On the other hand, Sollanek et al. [18] reported that ORS showed higher BHI than water, but sports drinks did not have a statistically significant difference. This means that when comparing the hydration effects of water, sports drinks, and ORS, ORS can have the greatest effect. Also, in this study, ORS showed higher BHI than SpD2 at 3 and 4 h after water intake, which is consistent with previous studies showing that sports drinks with low osmolality had a greater hydration effect [16]. Clarke et al. [3] found that ORS with a high Na+ content showed high BHI values, and this study also confirmed that ORS with a relatively high Na+ content produced high BHI values and BHI value was high in drinks with less cumulative urine output. In other words, it was confirmed that the cumulative urine output and the BHI result were inversely proportional, which could re-prove the study [13, 18] that verified that BHI is a useful index for identifying the hydration effect.
Our study compared PVC to verify the hydration effects of different beverages, but we found no significant difference. According to Clarke et al. [3], PVC showed a rapid increase immediately after beverage consumption (0–15 min) and then plateaued after 1 h. We measured PVC at ≥ 1 h after beverage consumption, which may have been responsible for the insignificant difference. Furthermore, the coefficient of variation (CV) of PVC was 2.35, which is relatively high, possibly preventing the detection of smaller differences in PVC. For participants, indicating that PVC should be an index with higher sensitivity to an individual’s physical characteristics.
Studies that have verified the hydration effects of beverages on athletes have been steadily conducted. As a result of conducting Systematic Meta-Analysis of 28 studies on the hydration effects of sports drinks and water which Rowlands et al. [16] reported, when comparing ingested hypertonic (> 300 mOsmol kg−1), isotonic (275–300 mOsmol kg−1) and hypotonic (< 275 mOsmol kg−1) drinks, it is reported that hypotonic drinks have the greatest effect. With the classification criteria for beverages in this study, SpD1 corresponds to isotonic drinks, SpD2 corresponds to ingested hypertonic drinks, and ORS corresponds to hypotonic drinks. Therefore, it supports the result that ORS, the main result of this study, is more effective for hydration than SpD2.
The results of this study are significant in the point that it tried to verify the hydration effect of ORS in athletes. Previous studies that tried to confirm the hydration effect of athletes have been primarily studies using sports drinks. Therefore, the results of this study may suggest ORS as a drink that can help improve athletes’ performance along with sports drink. Nevertheless, this study has the following limitations. First, participants in this study were limited to athletes in throwing, bowling, and gymnastics events, so it is not easy to broadly interpret the effects of ORS for athletes in all sports. It is expected that a follow-up study will verify the effect of ORS in athletes in various sports. Second, in the case of female athletes, it was judged that external factors could be included in the study results due to their physical characteristics (menstruation), so this study selected male athletes as the study subjects. Thus, there is a need to verify the effectiveness of ORS in female athletes in follow-up studies. Third, the study subjects who participated in this study were university students, so ORS effect may be different for younger athletes. Therefore, it is expected that a follow-up study will verify the effectiveness of ORS in various age groups.