a Department of Health , Human Performance, and Recreation, University of Arkansas , Fayetteville , Arkansas , USA.
b Georgia Prevention Institute, Augusta University , Augusta , Georgia , USA.
J Am Coll Nutr. 2018 Jan;37(1):17-23. doi: 10.1080/07315724.2017.1323692. Epub 2017 Oct 6.
Urine specific gravity (U) is often used to assess hydration status, particularly around athletic competition, but it is unknown whether high U is indicative of plasma volume (PV) reduction (i.e., hypohydration). We tested the hypothesis that if high U is reflective of reduced PV, subsequent fluid ingestion would increase PV.
The purpose of this study was to examine 24-hour changes in U and PV in individuals presenting with high and low spot U.
Nineteen healthy males were provided food and water over 24 hours with a total water volume of 35 ml·kg body mass. Absolute PV and blood volume (BV), measured using the CO-rebreathe technique, along with U were measured before and after a 24-hour intervention period. Based on a preintervention morning spot U, subjects were post hoc assigned to groups according to U (≤1.020 or >1.020; low and high U, respectively).
Despite presenting with an elevated spot U (1.026 ± 0.004), subsequent fluid ingestion over 24 hours did not lead to changes (∆) in PV (-75 ± 234 ml) or BV (-156 ± 370 ml) in the high U group (p > 0.05). However, a spot U after the 24-hour intervention in this group decreased (p = 0.018) to a level indicating improved hydration status (1.017 ± 0.007). In the low U group, there were no changes in PV (-39 ± 274 ml), BV (-82 ± 396 ml), or U (0.003 ± 0.007) over the 24-hour fluid intervention (all p > 0.05).
Despite a high preintervention U and subsequent decrease after 24-hour fluid intake, measures of PV and BV were not indicative of this seemingly improved hydration status. This suggests that a highly concentrated spot sample U and subsequent changes are not accurately representative of PV or BV.
尿比重（U）常被用于评估水合状态，特别是在竞技比赛前后，但目前尚不清楚高 U 是否提示血容量（PV）减少（即脱水）。我们假设如果高 U 反映了 PV 减少，那么随后的液体摄入会增加 PV。
本研究旨在检查出现高和低即时 U 的个体在 24 小时内 U 和 PV 的变化。
19 名健康男性在 24 小时内提供食物和水，总水量为 35ml·kg 体重。使用 CO 重复呼吸技术测量绝对 PV 和血容量（BV），以及干预前和干预后 24 小时的 U。根据干预前早上的即时 U，根据 U（≤1.020 或>1.020；低和高 U，分别）对受试者进行事后分组。
尽管即时 U 升高（1.026±0.004），但在高 U 组中，24 小时内的液体摄入并没有导致 PV（-75±234ml）或 BV（-156±370ml）的变化（p>0.05）。然而，该组在 24 小时干预后的即时 U 下降（p=0.018）至表明改善水合状态的水平（1.017±0.007）。在低 U 组中，24 小时液体干预期间，PV（-39±274ml）、BV（-82±396ml）或 U（0.003±0.007）均无变化（均 p>0.05）。
尽管干预前 U 高，24 小时液体摄入后 U 下降，但 PV 和 BV 的测量结果并未表明这种看似改善的水合状态。这表明高浓度即时 U 样本及其随后的变化不能准确代表 PV 或 BV。
