Kimmel PL, Lew SQ, Bosch JP. Department of Medicine, George Washington University Medical Center, Washington, DC 20037, USA.
Nephrol Dial Transplant 1996;11 Suppl 9:85-88

Due to the paucity of long-term observational data, it is still unclear whether the decreased glomerular filtration rate (GFR) noted in older humans is a results of true changes in renal function over time. The fact that a carefully characterized subpopulation of subjects showed no decrease in GFR over time suggests that the 'physiological decrease' in GFR noted in the elderly is not inevitable. In studies in our patient population, there was a significant correlation between creatinine clearance and nutritional protein intake in elderly patients without renal disease. In our studies, elderly subjects without renal disease who ingested > 1 g/kg day of protein had creatinine clearances in the range of 90-100 ml/min/1.73 m2, while those with lower protein intakes had lower creatinine clearances. Our studies suggest that GFR is not a fixed function, and that its value may change both over short periods and over decades in humans, with these changes being associated with changes in nutritional protein intake. Low levels of GFR are not necessarily equivalent to a diagnosis of renal disease. Diet must be considered in the assessment of renal function in elderly patients before a diagnosis of renal insufficiency can be made. Decreased GFR is not an inevitable consequence of ageing in humans.