- Dietary Sodium Intake: Scientific Basis for Public Policy
- Health Risks
- cardiovascular diseases
- Scientific Basis for Guideline Recommendations
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Dietary Sodium Intake: Scientific Basis for Public Policy
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Obesity Silver Spring. Relevant to risk of cardiovascular disease, salt is highly involved with the maintenance of body fluid volume, including osmotic balance in the blood, extracellular and intracellular fluids, and resting membrane potential. The well known effect of sodium on blood pressure can be explained by comparing blood to a solution with its salinity changed by ingested salt.
Artery walls are analogous to a selectively permeable membrane , and they allow solutes, including sodium and chloride, to pass through or not , depending on osmosis. Circulating water and solutes in the body maintain blood pressure in the blood, as well as other functions such as regulation of body temperature. The water potential in blood will decrease due to the increase solutes, and blood osmotic pressure will increase. While the kidney reacts to excrete excess sodium and chloride in the body, water retention causes blood pressure to increase. Both studies were designed and conducted by the National Heart, Lung, and Blood Institute in the United States, each involving a large, randomized sample.
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 is 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 eating plan at the lowest sodium level 1, milligrams per day.
In agreement with studies regarding salt sensitivity, participants of African descent showed high reductions in blood pressure. See sodium sensitivity below. There has been strong evidence from epidemiological studies , human and animal intervention experiments supporting the links between high rate of salt intake and hypertension. There is scientific controversy over whether reducing sodium intake lowers cardiovascular disease incidence and mortality.
Scientific Basis for Guideline Recommendations
Of the 10 systematic reviews surveyed by Trinquart and others in , five were supportive of the hypothesis that reduced sodium intake lowers cardiovascular disease incidence and mortality, three contradicted this hypothesis, and two found insufficient evidence to reach a conclusion. The survey found 27 primary studies and letters in academic journals in support of the salt hypothesis, 34 primary studies and 51 letters contradicting the hypothesis, and 7 primary studies and 19 letters that were inconclusive.
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The survey found that the proponents and opponents of the salt hypothesis tended to cite a few key studies and researchers in their own respective camps and avoided citing conflicting studies, and that systematic reviews selected primary studies in an inconsistent fashion. On one hand, there are several long-term studies which found that groups with sodium-reduced diets have lower incidences of cardiovascular disease in all demographics. Cook and others suggest that some other studies may have contradicted or were inconclusive of the link between sodium intake and cardiovascular disease because they inaccurately measured sodium intake by relying on one measurement or subjects' recall of what they ate.
Cook and others also point out that many observational studies are not large enough, nor last long enough to provide conclusions on clinical outcomes for the effect of dietary sodium intake on morbidity and mortality. Some studies have also shown that salt consumption increases the risk of myocardial infarction , stroke , arterial stiffness and heart failure , independent of the effects of sodium on blood pressure. On the other hand, some researchers cast doubts on the link between lowering sodium intake and the health of a given population:.
Rather than create drastic salt policies based on conflicting data, Alderman and his colleague Hillel Cohen propose that the government sponsor a large, controlled clinical trial to see what happens to people who follow low-salt diets over time. Appel responds that such a trial "cannot and will not be done," in part because it would be so expensive. But unless we have clear data, evangelical antisalt campaigns are not just based on shaky science; they are ultimately unfair. But it is "based on wild extrapolations.
They do also not support the current recommendations of a generalized and indiscriminate reduction of salt intake at the population level. However, they do not negate the blood pressure-lowering effects of a dietary salt reduction in hypertensive patients. Moreover, the traditional Japanese diet is very high in salt intake, and yet the Japanese have the highest life expectancy in the world, and low rates of cardiovascular disease. Shimazu and others note, "we have found that the Japanese dietary pattern is associated with lower CVD mortality, despite the fact that the Japanese dietary pattern appeared to be related to higher sodium intake and high prevalence of hypertension.
Despite the scientific uncertainty, most physicians and clinical scientists, the European Food Safety Authority and the US Centers for Disease Control recommend that consumers use less salt in their diets, mainly to reduce the risk of high blood pressure and associated cardiovascular diseases in adults and children. None of the surveyed guidelines were deemed to rule against the link between sodium intake and cardiovascular disease.
In , the United States Centers for Disease Control and Prevention began an initiative encouraging Americans to reduce their consumption of salty foods.
In the European Union , half of the member states legislated change in the form of taxation, mandatory nutrition labeling, and regulated nutrition and health claims to address overconsumption of sodium  in response to a EU Salt Reduction Framework. A diet high in sodium increases the risk of hypertension in people with sodium sensitivity, corresponding to an increase in health risks associated with hypertensions including cardiovascular disease.
Unfortunately, there is no universal definition of sodium sensitivity; the method to assess sodium sensitivity varies from one study to another. In most studies, sodium sensitivity is defined as the change in mean blood pressure corresponding to a decrease or increase of sodium intake.
The method to assess sodium sensitivity includes the measurement of circulating fluid volume and peripheral vascular resistance. Several studies have shown a relationship between sodium sensitivity and the increase of circulating fluid volume or peripheral vascular resistance. A number of factors have been found to be associated with sodium sensitivity.
Demographic factors which affect sodium sensitivity include race, gender, and age. The difference in genetic makeup and family history has a significant impact on salt sensitivity, and is being studied more with improvement on the efficiencies and techniques of genetic testing. More specifically, haptoglobin phenotypes contribute to the characteristic of sodium-resistance in humans.
The influence of physiological factors including renal function and insulin levels on sodium sensitivity are shown in various studies. Possible mechanisms by which high intakes of dietary potassium can decrease risk of hypertension and instances of cardiovascular disease have been proposed but not extensively studied. The recommended dietary intake of potassium is higher than that of sodium.
It has been hypothesized that the ratio of potassium to sodium intake accounts for the large difference in the occurrence of hypertension between primitive cultures eating diets made up of mostly unprocessed foods and Western diets which tend to include highly processed foods. The growing awareness of excessive sodium consumption in connection with hypertension and cardiovascular disease has increased the usage of salt substitutes at both a consumer and industrial level. On a consumer level, salt substitutes, which usually substitute a portion of sodium chloride content with potassium chloride , can be used to increase the potassium to sodium consumption ratio.
In the food industry, processes have been developed to create low-sodium versions of existing products. There have been concerns with certain populations' use of potassium chloride as a substitute for salt as high potassium loads are dangerous for groups with diabetes , renal diseases , or heart failure. From Wikipedia, the free encyclopedia. Main article: DASH diet. Main article: Salt substitute. Adv Nutr. J Public Health Manag Pract.
US Centers for Disease Control.