Excess dietary salt (sodium chloride) consumption has been extensively studied for its potential impact on human health. Chronic, high intake of dietary salt is associated with hypertension and cardiovascular disease, among other adverse health outcomes. Major health and scientific organizations, such as the World Health Organization, the US Centers for Disease Control and Prevention, and the American Heart Association, have established high salt consumption as a major risk factor for cardiovascular diseases and stroke.
Effect of salt on blood pressure Salt fulfills several important biological functions in humans. The human body has evolved to compensate for high salt intake through regulatory systems such as the renin–angiotensin system. Salt is particularly involved with maintaining body fluid volume, including the regulation of osmotic balance in the blood, extracellular and intracellular fluids, and resting membrane potential. When salt is ingested, it is dissolved in the blood as two separate ions – Na+ and Cl−. While the kidney reacts to excrete excess sodium and chloride in the body, water retention causes blood pressure to increase.
Reducing salt intake in chronic kidney disease A 2021 Cochrane review of people with chronic kidney disease, including those on dialysis, demonstrated robust evidence that salt reduction decreases systolic and diastolic blood pressure and albuminuria. However, there was moderate certainty evidence that some people may experience hypotensive symptoms, such as dizziness, following sudden reduction of salt intake. The effect of salt restriction on extracellular fluid, edema, and total body weight was uncertain.
Dietary Approaches to Stop Hypertension-Sodium study
The DASH-Sodium study was a sequel to the original DASH (Dietary Approaches to Stop Hypertension) study. Both studies were designed and conducted by the National Heart, Lung, and Blood Institute in the United States, each involving a large, randomized sample. While the original study was designed to test the effects of several varying nutrients on blood pressure, DASH-Sodium varies only in salt content in the diet. Participants were pre-hypertensive or at stage 1 hypertension and either ate a DASH Diet or a diet reflecting an "average American Diet". During the intervention phase, participants ate their assigned diets containing three distinct levels of sodium in random order. Their blood pressure was monitored during the control period and at all three intervention phases. The study concluded that the effect of a reduced dietary sodium intake alone on blood pressure is substantial and that the largest decrease in blood pressure occurred in those eating the DASH Diet at the lowest sodium level (1,500 milligrams per day). However, this study is especially significant because participants in both the control and DASH diet groups showed lowered blood pressure with decreased sodium alone. In agreement with studies regarding salt sensitivity, participants of African descent showed high reductions in blood pressure.
Hypertension and cardiovascular disease In 2018, the American Heart Association published an advisory stating that "if people in the U.S. consume an average 1,500 mg/day sodium, it could result in a 25.6% decrease in high blood pressure and an estimated $26.2 billion in health care savings. Another estimate projects that achieving this goal would reduce cardiovascular disease deaths from 500,000 to nearly 1.2 million over the next 10 years." There has been evidence from epidemiological studies, human and animal intervention experiments, supporting the links between high rate of salt intake and hypertension. A Cochrane review and meta-analysis of clinical trials showed that reduced sodium intake reduces blood pressure in hypertensive and normotensive subjects. Since controlling hypertension is related to a reduced risk of cardiovascular disease, it is plausible that salt consumption is a risk factor for cardiovascular health. However, to properly study the effects of sodium intake levels on the risk of development of cardiovascular disease, long-term studies of large groups using both dietary and biochemical measures are necessary. As of 2019, major government research organizations, such as the US Centers for Disease Control and Prevention and the European Food Safety Authority, advise consumers to reduce their salt consumption to lower the risk of cardiovascular diseases. One 2016 review found that five studies supported the evidence that reduced sodium intake lowers cardiovascular disease incidence and mortality, three contradicted this evidence, and two found insufficient evidence to conclude. The survey found 27 primary studies and 106 letters in academic journals in support of the salt evidence, 34 primary studies and 51 letters contradicting the evidence, and 7 primary studies and 19 letters that were inconclusive. There are several long-term studies which found that groups with sodium-reduced diets have lower incidence of cardiovascular disease in all demographics. Some researchers cast doubts on the link between lowering sodium intake and the health of a given population.
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