Renal functional reserve in subjects with diabetes mellitus

Diabetic nephropathy develops in approximately 35% of patients with insulin-dependent diabetes mellitus (IDDM) and in a similar proportion of patients with non-insulin-dependent diabetes mellitus (NIDDM). However, we remain at present unable to identify the susceptible subset prior to the development of microalbuminuria. Up to 25% of IDDM patients and a variable proportion of NIDDM patients manifest glomerular hyperfiltration in the first few years of diabetes. It has been debated whether this basal hyperfiltration is predictive of future renal disease and whether better prediction can be achieved by the use of the renal haemodynamic response to a protein meal, defined by some authors as renal reserve. The concept of renal functional reserve in patients with diabetes mellitus is complicated by the dependence of the GFR response on basal GFR, the influence of the prevailing metabolic conditions, and because the response differs to different stimuli. We review the factors affecting renal hemodynamics and renal hemodynamic responses in the context of supranormal, normal, and impaired renal function in diabetes. We conclude that although the measurement of renal functional reserve may help clarify important pathophysiological mechanisms, the assessment of basal GFR in clinical practice is all that is required for predictive and monitoring purposes.