Altered dietary salt intake for people with chronic kidney disease

People with CKD are at increased risk of heart disease and deteriorating kidney health which can lead to need for dialysis or kidney transplantation to survive. Reducing risk of heart disease and preserving kidney function are important treatment goals.

High salt intake is linked to risk factors for both heart disease and worsening kidney function, including high blood pressure, excess protein in the urine (proteinuria) and fluid overload. It is thought to be particularly important for people with CKD to have a low salt intake due to kidneys' role in salt balance. We aimed to find out if altering salt in the diet was beneficial for people with CKD.

We searched the literature for studies that looked at the effects of restricting salt in the diets of people with CKD up to January 2015. We found eight studies that involved 258 people which met our inclusion criteria. Study participants included people in the early stages of CKD (six studies), who were on peritoneal dialysis (one study), or were kidney transplant recipients (one study). The average study duration was six weeks, and ranged from one to 26 weeks. We did not find any studies that measured the effect of salt intake on the incidence of death, heart disease, or need to begin dialysis.

We found that reducing salt intake reduced 24 hour sodium excretion, blood pressure. One study reported restricting salt intake reduced the risk of oedema (swelling). Antihypertensive medication dosage was significantly reduced with a low salt diet. There was no significant difference in kidney function measures or body weight. There was no significant change in total cholesterol or hypotension.

Long-term effects of salt restriction in people with CKD is lacking that meant we were unable to determine the direct effects of sodium restriction on primary endpoints such as mortality and progression to end-stage kidney disease (ESKD). Research into the long-term effects of sodium-restricted diet for people with CKD is warranted, as is investigation into adherence to low salt diet.

Authors' conclusions: 

We found a critical evidence gap in long-term effects of salt restriction in people with CKD that meant we were unable to determine the direct effects of sodium restriction on primary endpoints such as mortality and progression to end-stage kidney disease (ESKD). We found that salt reduction in people with CKD reduced blood pressure considerably and consistently reduced proteinuria. If such reductions could be maintained long-term, this effect may translate to clinically significant reductions in ESKD incidence and cardiovascular events. Research into the long-term effects of sodium-restricted diet for people with CKD is warranted, as is investigation into adherence to a low salt diet.

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Background: 

Salt intake shows great promise as a modifiable risk factor for reducing heart disease incidence and delaying kidney function decline in people with chronic kidney disease (CKD). However, a clear consensus of the benefits of reducing salt in people with CKD is lacking.

Objectives: 

This review evaluated the benefits and harms of altering dietary salt intake in people with CKD.

Search strategy: 

We searched the Cochrane Renal Group's Specialised Register to 13 January 2015 through contact with the Trials' Search Co-ordinator using search terms relevant to this review.

Selection criteria: 

We included randomised controlled trials (RCTs) that compared two or more levels of salt intake in people with any stage of CKD.

Data collection and analysis: 

Two authors independently assessed studies for eligibility and conducted risk of bias evaluation. Results were expressed as risk ratios (RR) and their 95% confidence intervals (CI) for dichotomous outcomes, and mean difference (MD) and 95% CI for continuous outcomes. Mean effect sizes were calculated using the random-effects models.

Main results: 

We included eight studies (24 reports, 258 participants). Because duration of the included studies was too short (1 to 26 weeks) to test the effect of salt restriction on endpoints such as mortality, cardiovascular events or CKD progression, changes in salt intake on blood pressure and other secondary risk factors were applied. Three studies were parallel RCTs and five were cross-over studies. Selection bias was low in five studies and unclear in three. Performance and detection biases were low in two studies and unclear in six. Attrition and reporting biases were low in four studies and unclear in four. One study had the potential for high carryover effect; three had high risk of bias from baseline characteristics (change of medication or diet) and two studies were industry funded.

There was a significant reduction in 24 hour sodium excretion associated with low salt interventions (range 52 to 141 mmol) (8 studies, 258 participants: MD -105.86 mmol/d, 95% CI -119.20 to -92.51; I2 = 51%). Reducing salt intake significantly reduced systolic blood pressure (8 studies, 258 participants: MD -8.75 mm Hg, 95% CI -11.33 to -6.16; I2 = 0%) and diastolic blood pressure (8 studies, 258 participants: MD -3.70 mm Hg, 95% CI -5.09 to -2.30; I2 = 0%). One study reported restricting salt intake reduced the risk of oedema by 56%. Salt restriction significantly increased plasma renin activity (2 studies, 71 participants: MD 1.08 ng/mL/h, 95% CI 0.51 to 1.65; I2 = 0%) and serum aldosterone (2 studies, 71 participants: 6.20 ng/dL (95% CI 3.82 to 8.58; I2 = 0%). Antihypertensive medication dosage was significantly reduced with a low salt diet (2 studies, 52 participants): RR 5.48, 95% CI 1.27 to 23.66; I2 = 0%). There was no significant difference in eGFR (2 studies, 68 participants: MD -1.14 mL/min/1.73 m2, 95% CI -4.38 to 2.11; I2 = 0%), creatinine clearance (3 studies, 85 participants): MD -4.60 mL/min, 95% CI -11.78 to 2.57; I2 = 0%), serum creatinine (5 studies, 151 participants: MD 5.14 µmol/L, 95% CI -8.98 to 19.26; I2 = 59%) or body weight (5 studies, 139 participants: MD -1.46 kg; 95% CI -4.55 to 1.64; I2 = 0%). There was no significant change in total cholesterol in relation to salt restriction (3 studies, 105 participants: MD -0.23 mmol/L, 95% CI -0.57 to 0.10; I2 = 0%) or symptomatic hypotension (2 studies, 72 participants: RR 6.60, 95% CI 0.77 to 56.55; I2 = 0%). Salt restriction significantly reduced urinary protein excretion in all studies that reported proteinuria as an outcome, however data could not be meta-analysed.

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