Salt and the kidneys
Proteinuria/renal disease
In animal studies, increasing salt intake increases proteinuria in the urine and markedly increases the rate of deterioration of renal function in experimental forms of renal disease. Studies where salt intake has been reduced in animals with experimental renal disease show a slowing of the rate of progression of the renal disease. Studies in humans have now shown that salt intake relates on a population basis to the amount of protein or albuminuria (1,2) and this is a major risk factor for developing kidney disease and cardiovascular disease. Reducing salt intake in a double blind study has now been shown to reduce proteinuria (Fig 1) (3) . This is even more marked when combined with an ACE inhibitor (4) . Therefore, individuals who develop kidney disease should restrict their salt intake because in nearly all forms of kidney disease the kidney retains sodium and water in the body. The increase in sodium retention causes an increase in blood pressure and makes the proteinuria worse and causes a further deterioration in renal function. Reducing salt intake will lower blood pressure and almost certainly reduce the rate of deterioration in renal function. Patients who are on dialysis need to restrict their salt intake as this reduces the amount of fluid that they drink between dialysis. This particularly applies to haemodialysis patients, where blood pressure is a major problem and studies have clearly shown that if they restrict salt intake there is less gain in weight between dialysis, less fluctuation in blood pressure and blood pressure is easier to control.
Figure 1. Change in urinary sodium and protein excretion with a modest reduction in salt intake in 40 hypertensive blacks. (Adapted from Ref 3) .
References
(1) du Cailar G, Ribstein J, Mimran A. Dietary sodium and target organ damage in essential hypertension. Am J Hypertens. 2002;15:222-9.
(2) Verhave J C, Hillege H L, Burgerhof J G, Janssen W M, Gansevoort R T, Navis G J, de Zeeuw D, de Jong P E. Sodium intake affects urinary albumin excretion especially in overweight subjects. J Intern Med. 2004;256:324-30.
(3) Swift P A, Markandu N D, Sagnella G A, He F J, Macgregor G A. Modest Salt Reduction Reduces Blood Pressure and Urine Protein Excretion in Black Hypertensives. A Randomized Control Trial. Hypertension. 2005; 46:308-12.
(4) Heeg J E, de Jong P E, van der Hem G K, de Zeeuw D. Efficacy and variability of the antiproteinuric effect of ACE inhibition by lisinopril. Kidney Int. 1989;36:272-9.