Salt and cardiovascular disease

Salt consumption has been intensely studied for its role in human physiology and impact on human health. In particular, excessive dietary salt consumption over an extended period of time has been associated with hypertension and cardiovascular disease, in addition to other adverse health effects.

Most forms of edible salt are composed primarily of sodium and chloride, as other minerals such as magnesium and calcium, make unrefined salts bitter and therefore rarely eaten. An excessive intake of the ionic compound sodium chloride has long been suspected to increase blood pressure. Sodium and chloride serum levels are both carefully controlled by the kidneys, and acute and chronic excessive intake of both ions can cause adverse health effect. However, only serum sodium is thought to be strongly correlated to blood pressure levels and cardiovascular disease.

Effect of salt on blood pressure


The human body has evolved to balance salt intake with need through means such as the renin-angiotensin system. In humans, salt has important biological functions. 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.

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 which allows sodium chloride to enter the blood stream.

Circulating water and solutes in the body maintain blood pressure in the blood, as well as other functions such as regulation of body temperature. When too much salt is ingested, it is dissolved in the blood as two separate ions - Na+ and Cl-. 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 inside blood vessel walls.

Blood pressure may continue to build as water is consumed hours after salt is ingested. As excess sodium is excreted by the kidneys, blood pressure drops accordingly. Diets that consistently contain high salt content will increase blood pressure over time. Fortunately, as many studies have shown, limiting salt intake in the diet can reverse these effects.

Since most cases of hypertension are essential hypertension, it is unlikely that a single factor can be attributed to the cause of hypertension in most hypertensive patients.

DASH-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 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 (1,500 milligrams per day). However, this study is especially significant because participants in both the control and DASH diet group 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. See sodium sensitivity below.

Hypertension and cardiovascular disease
There has been strong evidence from epidemiological studies, human and animal intervention experiments, supporting the links between high rate of salt intake, hypertension and cardiovascular diseases. Short term studies have proven that groups with sodium reduced diets show lower incidences of hypertension, irrespective of sex, race, age, physical activity level, and body mass. However, to properly study the effects of sodium intake levels on risk of development of cardiovascular disease, long term studies of large groups using both dietary and biochemical measures are necessary. Most of these studies, with a few exceptions, show statistical significance that groups with sodium reduced diets show lower incidences of cardiovascular disease in all demographics. A study by Cook and colleagues were the first to show reduction of cardiovascular disease after 15 years of sodium reduction in a randomised trial.

More data is needed to support the conclusions of observational studies which suffer from design flaws. Many of these 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. Previous mixed results and inconclusive interpretation of non-experimental studies may also root from the way sodium is measured in the study.

Current trends and campaigns
According to the 2004 Canadian Community Health Survey (CCHS) Canadians in all age groups are consuming sodium between 45.1 - 98.8% above the upper limit set for their gender and age group. The US Department of Agriculture claims that the average daily sodium intake for Americans over 2 years of age is 3436 mg. The majority of sodium consumed by North Americans is from processed and restaurant foods, while only a small portion is added during cooking or at the table.

Sodium sensitivity
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. One study shows that the American population of African descent are significantly more salt sensitive than those Caucasians. Women are found to be more sodium sensitive than men; one possible explanation is based on the fact that women have a tendency to consume more salt per unit weight, as women weigh less than men on average. Several studies have shown that the increase in age is also associated with the occurrence of sodium sensitivity.

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. In both hypertensive and non-hypertensive individuals, those with haptoglobin 1-1 phenotype are more likely to have sodium sensitivity than people with haptoglobin 2-1 or 2-2 phenotypes. More specifically, haptoglobin 2-2 phenotypes contribute to the characteristic of sodium-resistance in humans. Moreover, prevalence of a family history of hypertension is strongly linked with the occurrence of sodium sensitivity.

The influence of physiological factors including renal function and insulin levels on sodium sensitivity are shown in various studies. One study concludes that the effect of renal failure on sodium sensitivity is substantial due to the contribution of decreasing the Glomerular filtration rate (GFR) in the kidney. Moreover, insulin resistance is found to be related to sodium sensitivity; however, the actual mechanism is not still unknown.

Potassium and hypertension


Possible mechanisms by which high intakes of dietary potassium can decreases risk of hypertension and instances of cardiovascular disease have been proposed but not extensively studied. However studies have found a strong inverse association between long-term adequate to high rates of potassium intake and the development of cardiovascular diseases.

The recommended dietary intake of potassium is higher than that of sodium. Unfortunately, the average absolute intake of potassium of studied populations is lower than that of sodium intake. According to Statistics Canada, Canadian's potassium intake in all age groups are lower than recommended, while sodium intake greatly exceed recommended intake in every age group.

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.

Salt substitutes
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. This change has been shown to blunt the effects of excess salt intake on hypertension and cardiovascular disease. It has also been suggested that salt substitutes can be used to provide an essential portion of daily potassium intake, and may even be more economical than prescription potassium supplements.

In the food industry, processes have been developed to create low-sodium versions of existing products. The meat industry especially have developed and fine-tuned methods to decrease salt contents in processed meats without sacrificing consumer acceptance. Research demonstrates that salt substitutes such as potassium chloride, and synergistic compounds such as phosphates, can be used to decrease salt content in meat 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. The use of salts with minerals such as natural salts have also been tested, but like salt substitutes partially containing potassium, mineral salts produce a bitter taste above certain levels.