J.A. Yep, C.L. Chapman, S.M. Holt, S.C. Brazelton, C.T. O’Connell, H.N. Medved, W.A.B. Howells, M.R. Gradow, A.D. Wayne, E.L. Reed, K. Wiedenfeld Needham, J.R. Halliwill, FACSM, C.T. Minson, FACSM

University of Oregon, Eugene, OR

Oral protein consumption in a hydrated state (i.e., euhydration) stimulates urine concentrating mechanisms, reducing free water clearance (CH2O) and conserving water. This response may be abated during a hyperhydrated state as evidenced by blunted post-prandial changes in renal hemodynamics with oral protein consumption. On the other end of the hydration spectrum, it remains unknown whether urine concentrating ability is altered when oral protein is consumed during mild hypohydration, a physiological state associated with negative CH2O. PURPOSE: To test the hypothesis that oral protein loading during mild hypohydration attenuates reductions in CH2O compared to during euhydration. METHODS: In a block-randomized crossover design, twenty healthy adults [9 females, 11 males; age: 21 (3) years] completed 24 hours fluid deprivation (HYPO) and 24 hours normal fluid consumption (EUHY). Protocols were separated by ³72 hours. Participants ingested a whey protein shake (1.0 g protein and 10 ml water per kg body mass) within 10 minutes. Body fluid loss was estimated via the percent change in body mass (∆BM) over 24 hours. Blood and urine samples collected at pre- (baseline) and 150-min post-protein consumption (POST) were analyzed osmolality for the calculation of CH2O. Data are presented as mean with 95% confidence intervals. RESULTS: ∆BM was reduced in HYPO vs. EUHY [-2.6% (-3.0, -2.2) vs. 0.1% (-0.3, 0.4), P<0.0001]. Baseline CH2O was lower in HYPO vs. EUHY [-1.6 ml/min (-1.8, -1.4) vs. 4.8 ml/min (3.5, 6.1), P<0.0001]. There were no differences in CH2O between conditions at POST [HYPO: -2.4 ml/min (-2.7, -2.1); EUHY: -2.1 ml/min (-2.4, -2.1), P=0.0758]. Compared to baseline, CH2O at POST was reduced in EUHY (P<0.0001) but was not different in HYPO (P=0.1464). CONCLUSION: These findings indicate oral protein loading does not enhance urine concentrating ability during prolonged mild hypohydration. It is unclear whether the lack of further reductions in CH2O during HYPO, as opposed to EUHY, reflect a “ceiling effect” of having reached the physiological maximal ability to concentrate urine whereby the already negative CH2O is not able to be further reduced with oral protein loading.

Supported by NIH R01HL144128 and F32HL164021.

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