Allon N. Friedman, MD, Selection Committee, Nephrology and Nutrition Region
Editor's Note: The following "Scouting Report" will help you complete your bracket in the NephMadness Tournament. Read it carefully to make your selections, and discuss your thoughts in our Comments section. This article first appeared on the AJKD BLOG.
Meet the Competitors: Low Sodium Intake Is a Risk Factor for Mortality vs High Sodium Intake Is a Risk Factor for Hypertension
Sodium is the Wilt Chamberlain of nephrology boogeymen. Nothing else in nephrology will ever earn both Rookie of the Year and MVP in its debut season, and the renal world will never see another player average 50 points a game for a season like Wilt the Stilt did in 1962. But when it comes to legacy, controversy, and importance, sodium is the Wilt Chamberlain of nephrology.
Sodium has been identified as a global health burden. Restricting sodium intake in order to reduce blood pressure and cardiovascular disease is a goal of just about every professional society or government health organization that walks the earth. The World Health Organization, US Department of Agriculture, NICE public health guidelines, American Heart Association, KDIGO, the CDC, and the Institute of Medicine have all recommended lower sodium intake.
Despite all of those recommendations, US sodium intake has remained stubbornly elevated with no sign of dropping over the last 50 years. Given the ubiquity of the recommendations, one could reasonably expect the science to be settled on the ill effects of dietary sodium, but emerging data over the last few years has kept the conclusions mired in controversy.
Low Sodium Intake Is a Risk Factor for Mortality
Cross-sectional and epidemiologic data has repeatedly shown low-sodium diets to be associated with worse outcomes This was made clear when O'Donnell et al looked at sodium intake and adverse outcomes in the ONTARGET and TRANSCEND trials. Both of these trials looked at high-risk patients over the age of 55 with either established CV disease or high risk diabetes.
Average 24-hour sodium excretion was 4.8 grams (208 mmol) or roughly double the recommended sodium intake for individuals. Expectedly morbidity and mortality rose as sodium excretion went up, but surprisingly, morbidity and mortality also rose as sodium excretion went down from the average. The mortality was lowest at precisely the average sodium intake.
Data from O'Donnell et al.
The Belgians did a comprehensive evaluation of Flemish sodium habits and followed them for 8 years. Unlike just about any other study on sodium excretion, the Flemish Study on Genes, Environment, and Health Outcomes (1985-2004) and the European Project on Genes in Hypertension used honest-to-goodness 24-hour urine collections for all 3,681 participants. CV mortality was increased in the lowest tertile of sodium intake.
During the follow-up, over 500 previously normotensive people developed benign hypertension. The incidence of hypertension was not influenced by baseline sodium excretion. Though interestingly, the cross-sectional analysis showed exactly what the large epidemiologic studies have shown, that increased sodium excretion was associated with increased blood pressure.
This curious association of increased CV mortality with low sodium excretion has also been found in the analysis of the NHANES 1, 2, and 3. Low sodium diets increase renin, aldosterone, and the sympathetic nervous system activity, possibly driving the increased adverse outcomes.
High Sodium Intake Is a Risk Factor for Hypertension
He et al performed a Cochrane Systematic Review to determine the effect a reduction in dietary sodium (or more often urinary excretion of sodium) has on blood pressure and consistently found that even modest reductions of sodium for a month reduce blood pressure. In 22 trials of 1,990 people with hypertension, a reduction of salt excretion of 75 mmol (4.4 g) reduced blood pressure 5.39/2.82 mm Hg. A larger, 100 mmol (6 g) reduction in salt excretion lowered systolic blood pressure 10.8 mm Hg.
The meta-analysis examined 2,240 normotensive individuals from 12 trials. A reduction in salt excretion of 75 mmol (4.4 g) reduced blood pressure 2.4/1.0. A larger, 100 mmol (6 g) reduction in salt excretion lowered systolic blood pressure 4.4 mm Hg.
Translating these reductions in blood pressure to lives saved gives dramatic results. In the 2010 report of the Dietary Advisory Committee on the Dietary Guidelines for Americans, the authors estimated that a reduction in sodium intake of 400 mg/d would:
● Reduce heart attacks by 20,000 to 32,000 per year
● Reduce strokes by 13,000 to 20,000 per year
● Save between 17,000 and 28,000 lives every year
From a financial perspective, this represents a savings of between $12 and $20 billion dollars annually.
Various experimental studies have been done to prove the relationship of sodium intake to blood pressure, and ultimately to lives saved, but few were quite as devious as Hsing-Yi Chang's study of Taiwanese nursing homes. Chang's group secretly randomized 5 nursing home kitchens to either normal sodium chloride or a mixture of sodium and potassium chloride. Sodium intake in the control group was 5.2 g/d and 3.8 g/d in the intervention group. In total, 768 veterans were served by the kitchens with low salt and 1,213 were served by control kitchens.
After an average follow-up of 31 months there was significantly lower cardiovascular death in the intervention group (1,310 deaths vs 2,140 deaths per 100,000 person-years). This represents a reduction of CV death of about 60% compared to the control group. The authors also noted less health care expenditures in the group fed in the low-salt kitchens. Of course, the improvements in outcomes could as much be due to the increased potassium intake as to the decreased sodium intake.
The world's government, medical, and professional organizations urge low-sodium diets because despite the holes, on balance low-sodium diets deliver reduced risk of hypertension, stroke, and cardiac disease.
NKF © 2015
The National Kidney Foundation
Cite this: NephMadness 2015: Nephrology and Nutrition Region - Medscape - Mar 02, 2015.
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