Evidence for salt reduction

We need less than 1g of salt per day for normal physiological function (1), however around the world we’re eating on average 10g salt per day, ranging from 4g in Kenya to 15g in Kazakhstan (2), and it’s putting our health at risk. As such, the World Health Organization recommends adults reduce their salt intake to less than 5g salt per day, and even less for children (3).

Salt reduction has been recognized as one of the most cost-effective interventions for reducing the burden of non communicable diseases (NCDs), like cardio vascular disease, due its high impact on health, high feasibility, and low implementation costs (4). It has been considered on a global scale, as a priority intervention and a best buy in NCD disease prevention (5).

Salt and blood pressure

The clearest adverse effect of salt is on blood pressure; salt raises blood pressure (6,7). The more salt eaten the higher the blood pressure and the greater the increase in blood pressure with age (8,9).

Observational studies comparing individuals and populations that eat different amounts of salt show a striking association of salt consumption with blood pressure (8,9). The more salt eaten the higher is the blood pressure and the greater is the increase in blood pressure with age (9). And this is true not just for people with hypertension but for the majority of the world’s population. Salt consumption is above the 1.4g required for good health in most of the world’s population (1,2). Salt consumption above this level will cause some rise in the blood pressure of billions of people worldwide every year increasing the risk of stroke, heart attack, heart failure and kidney disease (10).

Salt and CVD (stroke, coronary heart disease and heart failure)

High salt diets have been linked with high blood pressure which is the major risk factor for cardiovascular disease including stroke, heart disease and heart failure (10,11).

Raised blood pressure is responsible for 62% of strokes and 49% of coronary heart disease (12). According to the 2010 Global Burden of Disease report 1.65 million deaths from cardiovascular causes have been attributed to excess salt intake (13). It has been estimated that a modest and long-term reduction in population salt intake (reduction of 6g of salt per day) would reduce stroke by 14% and coronary heart disease by 9% in hypertensives, and reduce stroke and coronary deaths by 6% and 4%, respectively, in people within the normal ranges of blood pressure (14).

It is estimated that reducing salt intake from 10g to 5g per day would reduce stroke rate by 23% and overall CVD by 17%. This is equivalent to 1.25 million deaths from stroke and almost 3 million deaths from CVD averted each year (11).

Salt and Kidney Disease

A high salt diet alters the sodium potassium balance, causing the kidneys to have a reduced function, and remove less water – resulting in higher blood pressure (15,16). This can put a strain on kidneys, and increase risk of kidney disease.

High salt intake increases the amount of protein in the urine, (15, 16) which is a major risk factor for the decline of kidney function, and accelerates the rate of renal functional deterioration (16).

In patients with hypertension, an increase in salt intake often increases glomerular filtration rate, vascular resistance, calculated intraglomerular capillary pressure and protein excretion. There is also increasing evidence that high salt intake increases deterioration of kidney diseases in people already suffering from kidney problems (16). Patients with progressive renal disease on a diet containing 2300mg of sodium/day have lower protein excretion and slower fall in glomerular filtration rate than those who eat a diet containing 4600mg of sodium/day (17).

A randomized double-blind trial in 40 black hypertensive individuals showed that reducing salt intake from approximately 10 to 5g/day reduced urine protein excretion by 19.4% (18).  A longer-term modest reduction in salt intake reduces urinary albumin excretion in white, black and Asian hypertensive individuals (19).

Salt and Stomach Cancer

Salt has been found to increase the action of Helictobacter pylori, which is the major risk factor for stomach cancer as it can lead to inflammation and gastic ulcers which can progress in to stomach cancer (20,21).

A high salt diet increases Helicobacter pylori colonization (20,21). Studies have shown a positive association between H. pylori and gastric ulcer amounting to a 2-3-fold increase in risk (22). Salt may also act as an irritant/inflammatory agent of the stomach lining and is known to cause gastritis, which can expose the stomach to carcinogens. A meta-analysis of prospective studies demonstrated a relationship between increasing salt intake and risk of gastric cancer- those with high salt intakes had a 68% higher risk of developing gastric cancer compared to those with lower intakes (23).

Salt and Meniere’s Disease

A high salt diet increases fluid retention in the body, which may increase fluid retained in the inner ear. This retained fluid exerts pressure, which can cause Meniere’s disease or worsen it’s symptoms (24).

Meniere’s disease is a progressive disease which damages the ear. Meniere’s disease can be caused by metabolic imbalances of sodium in the inner ear fluid, which can be contributed by a high salt diet. A lower salt diet, reducing salt intake to less than 3g of salt per day, can be highly effective at reducing the symptoms of Meniere’s disease (24).

Salt and Bone Density/Osteoporosis

High salt diets increase calcium losses in the urine, some of which will be directly lost from the bone (25).

Urinary sodium excretion controls the urinary output of calcium. A high dietary salt intake may lead to increased calcium losses in the urine, which in long term may lead to calcium mobilization from the bones. 90% of calcium is stored in the bones. Calcium lost from the bones results in weakening of the bones, and ultimately osteoporosis.

Studies in healthy men and women aged  20-79 years have shown the higher the dietary salt intake, the higher the loss of urinary calcium. An increase in dietary sodium of 2300mg/day was associated with an increase in calcium excretion of 0.6-1.0 mmol (25).

In post-menopausal women, it was calculated that an increased sodium intake of 1150mg sodium/day for 10 years would deplete calcium stores by about 7.5% (26).

Further studies suggest that if the daily excretion of sodium were halved, it would have an equivalent effect on reducing bone loss to that of increasing calcium intake by 891mg/day (27).

Another study has suggested that reduction in salt intake among post-menopausal women with a sodium intake ≥3.4g/day would benefit their bone health (28). In a study among white, black and Asian mild hypertensives, it was found that a modest reduction in salt intake reduces 24-hour urinary calcium and calcium/creatinine ratio. Thus, a lower salt intake may be beneficial to people in terms of preventing osteoporosis (19).

Salt and Obesity

Salt is not a direct cause of obesity but a major influencing factor through it’s effect on sugar sweetened beverage consumption (29).

Thirst is an unavoidable consequence of eating foods with a high salt content; where thirst is relieved with high-sugar beverages, it may contribute to weight gain (29).

A study found that there was a significant association between salt intake and total fluid, as well as sugar-sweetened soft drink consumption. A difference of 1g/day in salt intake was associated with a difference of 100 and 27g/day in total fluid and sugar-sweetened soft drink consumption, respectively (29). In the US, a study found that the per capita use of salt increased by approximately 55% from the mid-1980s to the late 1990 paralleled by an increase in the per capita use of sweetened, carbonated soft drinks during the same period increased by 45%. These led to an increased intake of calories during the same period (30).

Salt and Asthma

A high salt diet can aggravate the symptoms of asthma (31-34).

  • A high salt diet can aggravate symptoms of asthma.
  • An observational study on 138 men with initial to moderate asthma showed that bronchial reactivity was strongly related to 24-hour urinary sodium excretion (31).
  • Two trials conducted in men (27 patients in the first trial and 36 in the second) showed that an increase in dietary sodium, 80mmol/day in one and 204mmol/day in the other, increased both the severity of asthma and bronchial reactivity (32, 33).
  • Among children aged 6-7 years, adding salt to foods was strongly associated with an increased risk of respiratory symptoms such as wheezing and asthma (34).
  • There is no evidence that salt intake affects airway responsiveness in normal subjects (16).

Salt and Cognitive Decline

Vascular dementia (VD) is caused by a blockage in one of the brains blood vessels and can occur either suddenly, following a stroke, or over time, through a series of small strokes. Because of the link to stroke, hypertension and a high salt diet are important risk factors for VD (35).

In people with low physical activity levels, a low sodium intake may improve cognitive performance over time than those with higher sodium intake.

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