6 - Renal Nutrient Excretion with Emphasis on Glucose

Abstract

Summary 'Glycosuria’, by which is meant the excretion of more than usual amounts of glucose in the urine, is commonplace in healthy pregnant women. The losses can often be ten or more times the typical losses of non-pregnant women. Its major feature is a conspicuous variability both from day to day and during the course of a day, and normal non-pregnant patterns of excretion are reestablished within a week of delivery. All this strongly suggests that there can be no more than a tenuous relation to blood sugar levels and points to an intermittent renal tubular failure to reabsorb glucose. The kidney’s capacity to reabsorb glucose has been exaggerated. The classical ideas are misleading and the concept of a fixed maximal tubular reabsorptive capacity for glucose, the TmG, is not acceptable; instead reabsorption continues to rise with increasing filtered load, but at a diminishing rate. In pregnancy, under the conditions of a glucose infusion, renal reabsorption of glucose is less effective than in the non-pregnant state, and is still less effective in women who have a marked glycosuria in pregnancy. Moreover, women who develop glycosuria when pregnant have a reduced capacity for reabsorption even when they are not pregnant. It may be that women with more than usual degrees of glycosuria in pregnancy have an element of tubular damage. That concept together with that of varying nephron perfusion and function are perhaps relevant in explaining the characteristic intermittency of clinical glycosuria. Glycosuria as a screen for abnormal carbohydrate tolerance in pregnancy is without physiological foundation.