The Effects of Salt and Water Loading on Kidney Function in Healthy Undergraduates

Abstract

One-hundred forty-five students volunteered to test the effects of salt and water loading on kidney function. Students were investigated beginning at 8:40 a.m., 1:40 p.m., and 6:40 p.m. They were approximately evenly distributed between male and female genders and were of multiple cultural/ethnic backgrounds. Upon entering the laboratory baseline data were collected. Approximately half of the volunteers then consumed 200 mL of a 2.0% NaCl solution (n=77). The others directly consumed 1.0 L of tap water (n=68). Upon entering the laboratory baseline data were collected. The others directly consumed 1.0 L of tap water. Kidney function was monitored each 30 minutes for the next 90 minutes. Under baseline conditions, urine osmolality was elevated in all students (≥ 280 mOsm/kg) but was significantly (P<0.05) greater in the 8:40 a.m. group than in either of the p.m. groups. Urine specific gravity and sodium excretion were also elevated in the 8:40 a.m. group compared to the others. Urine flow rate was least in the 8:40 group and greatest in the 1:40 p.m. group. We conclude that in these students renal function was influenced by a diurnal pattern. We also conclude that without compelling incentives (e.g. financial, gradeinfluencing) it was virtually impossible to get students to comply with pre-experimental instructions (e.g. no salt-laden meals after 6:00 p.m. before the day of experimentation).