By Martina McGrath, MD
October 18, 2017
The typical US diet is comprised of 3.4g of sodium per day on average, whereas current recommendations suggest that sodium intake should be limited to <2g/day for the general population1 and possibly lower for those with hypertension or cardiovascular disease.2
Chronic kidney disease is associated with extracellular volume expansion and salt avidity. A large proportion of patients have salt-sensitive hypertension, where dietary sodium restriction is likely to be beneficial for BP control. While other studies of sodium restriction have provided all food to patients during the study period, to ensure compliance, there are few studies where interview support was used to encourage patients to comply with dietary restrictions.
A group of researchers at University of Michigan (UM), Ann Arbor, and University of North Carolina (UNC), Chapel Hill, carried out a randomized crossover study of salt restriction in patients with CKD Stage 3 and 4.3 Their aim was to assess changes in extracellular volume (ECV), blood pressure, and albuminuria with sodium restriction. Patients were randomized to sodium restriction, <2g/day, versus usual diet and continued for four weeks, followed by a two-week washout period, and then crossed over to the opposite arm. Patients met with a nutritionist for counseling using motivational interviewing techniques at screening, baseline, week-2 and week-4 visit of each arm. Food diaries were collected to assess compliance and guide dietary counseling. Patients also were contacted by phone to encourage compliance and improve motivation.
Fifty-eight patients with eGFR 15-60mls/min/1.73m2 were enrolled. Measurements included ECV and total body water, as assessed by bioimpedence spectroscopy. Changes in weight, BP, and albuminuria were assessed at baseline, week 2 and week 4. All patients had labs at baseline, 2 and 4 weeks. Compliance was assessed by 24-hour urine collection for sodium.
Compliance with diet was variable, but a decrease in urine sodium excretion (indicating decreased intake) was seen in 79% of participants. Sixty-five percent decreased their sodium intake by >20%. However, there was quite marked variation in compliance between the two sites, with a decrease in urine sodium exertion of 77mEq/24hrs in UM versus only 22.4mEq/24hrs in UNC.
In terms of the effect of the intervention, patients following salt restriction experienced on average a 1L decrease in extracellular fluid volume, a 2.3kg decrease in weight and a mean decrease in 24-hour systolic ambulatory BP of 10.8mmHg.3 No significant effect was observed on albuminuria during the short follow-up. Greater effect size was observed in those patients with greater compliance with salt restriction but the differences underline the difficulty in achieving these targets, even with intensive support from a dedicated nutritionist service.
Despite its small size and short duration, the findings confirm the benefit of salt restriction on volume status and blood pressure in patients with reduced GFR. The magnitude of the blood pressure benefit is similar to addition of an additional antihypertensive and is well within ranges previously proven to benefit long-term cardiovascular health.4 Prior studies have shown changes in albuminuria with salt restriction, not observed here, possibly due to relatively low levels of albuminuria, and high rates of ACE inhibition in this cohort. However, the results beg the question of how long-term sodium restriction would impact progression of renal disease and cardiovascular outcomes in this high-risk population; a larger long-term study would be required to fully address this question.
- http://www.who.int/nutrition/publications/guidelines/sodium_intake_printversion.pdf. Accessed 15th October 2017.
- Whelton P.K, Appel L et al. Sodium, Blood Pressure, and Cardiovascular Disease. Circulation, 2012;126:2880-2889.
- Saran, R. et al. A Randomized Crossover Trial of Dietary Sodium Restriction in Stage 3–4 CKD. CJASN, March 07, 2017, vol. 12, no.3, 399-407.
- Blood Pressure Lowering Treatment Trialists’ Collaboration. Blood pressure lowering and major cardiovascular events in people with and without chronic kidney disease: meta-analysis of randomised controlled trials. BMJ 2013;347:f5680
Dr. Martina McGrath is an Instructor in Medicine at Harvard Medical School, and a member of the Renal Division, Department of Medicine, at Brigham and Women’s Hospital, both in Boston. Dr. McGrath is the Medical Editor for the Trends in Medicine blog.