Decline in kidney function. Creatinine clearance, an indicator of the kidney’s efficiency in filtering toxins from the blood, can decline with age. In a classic study, Lindeman and colleagues found that in “normal” patients, the “mean decrease in [creatinine clearance] was 0.75 mL/min/year” after age 40.20 “Normal” patients were those who didn’t have renal or urinary tract disease or use diuretics or antihypertensives. (However, the authors also noted that “one-third of all subjects followed had no absolute decrease in renal function.”) This decline in function means that the kidneys are less able to concentrate
urine, so water is lost that in a younger person wouldn’t be.