There are many factors affecting the onset and progress of osteoporosis and nutrition is one of them. Excretion of calcium in urine has been found to increase with greater salt (NaCl) consumption (11). In a few studies, consuming salt has also been linked to bone loss (resorption).
Cross-sectional studies have generally not found any link between salt consumption and bone mineral density (BMD) (12).
An increase in sodium excretion in urine (indicating increased sodium consumption) was linked to diminished BMD in the hips, however, in a study of postmenopausal women over two years (13). Following a low-sodium diet (2 g/day) over a period of six months could produce considerable reductions in sodium and calcium excretion, as well as a reduction in a marker of bone resorption, as shown by a longitudinal study of 40 postmenopausal women(14).
To ascertain whether reducing salt intake has an effect on BMD and the risk of fractures in osteoporosis sufferers, more long-term prospective studies are required.
Kidney stones are largely made up of calcium. An increase in salt (NaCl) intake increases calcium excretion in urine, which in turn increases the risk of kidney stones (15, 16).
Women who consume an average of 4.9 g of sodium per day (12.6 g/day salt) have a 30% higher risk of developing kidney stones than women who consume an average of 1.5 grams of sodium per day (4.0 g/day salt), according to a large prospective study that tracked over 90,000 women for 12 years (17).
A comparable study in men did not however find any link between salt consumption and kidney stones (18).
A reduction in salt intake (NaCl) in turn reduces urinary calcium in people prone to calcium stones, studies reveal (19). A randomized controlled trial that spanned five years and focused on two different diets in men with recurrent calcium stones demonstrated that a low-sodium diet and animal protein reduced stone recurrence considerably compared to a low-calcium diet (20).
Consuming large quantities of salted, smoked, and pickled foods increases the risk of stomach (gastric) cancer according to epidemiological studies carried out in Asia (6, 7). While the salt (NaCl) these foods contain is probably the biggest culprit, they may also contain cancer-causing substances (carcinogens) like nitrosamines. Fruit and vegetables are thought to reduce the risk of gastric cancer so another contributing factor might be that populations that consume large quantities of salty foods tend to consume less of these (8).
Factors such as chronic inflammation of the stomach and infection with Helicobacter pylori bacteria increase the risk of stomach cancer. High salt intake can potentially damage cells in the stomach lining, increasing the risk of H. pylori infection and cancer-causing damage to DNA.
Consuming large quantities of salted foods (like salted fish) can increase the risk of stomach cancer, even though there is not a lot of evidence that salt (NaCl) is in itself a carcinogen (7, 9, 10).
Sodium intake reduction
INTERSALT – the largest, most consistent observational study monitoring salt (NaCl) and blood pressure – encompassed over 10,000 people in 32 countries. That increased salt intake is linked to higher blood pressure was the outcome of several cross-population and intra-population analyses (21). The relationship was shown to be even greater that had been previously anticipated through further analyses employing improved statistical techniques (22).
The effect of reducing sodium intake from food on blood pressure in people with unusually high blood pressure (hypertension) and non-hypertensive individuals who are most likely pre-hypertensive has been investigated in many randomized controlled trials.
Speculation as to the effect of reducing dietary salt on blood pressure did not differ greatly in meta-analyses (23,24, 25, 26, 27, 28). However, the number and varieties of trials included were exceptionally heterogeneous. The results of appropriate salt reduction from 20 trials studying participants with hypertension and 11 trials studying participants without hypertension (28) were assessed in one particular meta-analysis. Appropriate salt reduction (by 1.7 to 1.8 g/day sodium) decreased systolic and diastolic blood pressure by about 5.1/2.7 mm Hg in participants with high blood pressure and 2.0/1.0 mm Hg in participants without high blood pressure.
TONE (29) and TOHP-Phase II were two large-scale, high blood pressure prevention trials with significant results(30). For older participants who were already on hypertension medication, a simple reduction in sodium consumption by approximately 1.0 g/day resulted in better control of high blood pressure, according to TONE. TOHP-Phase II demonstrated that a comparable reduction of salt intake reduced systolic and diastolic blood pressure by 1.2/1.6 mm Hg in overweight, non-hypertensive participants, and also, after four years, reduced the development of high blood pressure by 14%.
The value of reducing blood pressure slightly in people with hypertension is doubted by some doctors. There is, however, evidence to suggest that a reduction in diastolic blood pressure by about 2 mm Hg would reduce the total number of people America with high blood pressure by 17%, the risk of a heart attack by 5%, and the risk of a stroke by 15% (31). These conclusions come from reviews of observational studies and randomized controlled trials.
Significant public health benefits could therefore be brought about through slight average reductions in blood pressure.
Labeling people ‘salt-sensitive’ or ‘salt-resistant’ based on their blood pressure response to changes in sodium intake has not proven very reliable. This is because no proper population samples have been taken and the results have not been very reproducible (32). The data for salt consumption (loading and depletion) over a short period of time (a few days or up to a week) had to be heavily manipulated to achieve any intelligible results in relevant ‘salt sensitivity’ studies. No evidence has been found to suggest that such studies are at all relevant to changes in blood pressure resulting from long-term, sustained, slight changes in sodium consumption.
However, people who already have hypertension, older individuals, and African Americans are some of the population groups that tend to show greater mean blood pressure responses to changes in salt consumption (33). Genetic studies may be the best hope for finding conclusive data on salt sensitivity (34). Blood pressure can also be reduced by eating a balanced diet and losing weight (35, 36, 37). So not only genes but also lifestyle factors play an important role in determining salt sensitivity.
Eating more whole grains, vegetables, fish, poultry, nuts, and low-fat dairy products compared with a typical American diet resulted in considerably reduced blood pressure in hypertensive people (systolic blood pressure/diastolic blood pressure: 11.4 mm Hg/5.5 mm Hg) and individuals with normal (normotensive) blood pressure (3.5/2.1 mm Hg). This was demonstrated by DASH (Dietary Approaches to Stop Hypertension), arandomized controlled trial (38). In the diet used in the DASH study, sodium levels were kept the same to monitor the effects of other food components but potassium and calcium levels were significantly higher, protein levels were a little higher, and fat, saturated fat, and cholesterol levels were lower in than a typical American diet.
The DASH diet was then compared to a typical American diet as a control diet with three levels of sodium (NaCl) consumption – low (2.9 g/day), medium (5.8 g/day), and high (8.7 g/day) – in the DASH-sodium trial (39). The DASH diet was found to considerably lower systolic and diastolic blood pressure in hypertensive and normotensive participants for each of the different sodium consumption levels compared to the control diet. Reducing sodium consumption was shown to further lower blood pressure. Combining the DASH diet with lower sodium consumption was shown to be more effective than either measure on its own.
Prevention and treatment of high blood pressure are therefore most effective when combined with a healthy diet (40).
In addition, a prospective cohort study in 88,517 middle-aged women followed for 24 years found that adherence to a DASH-style diet reduced risk of coronary heart disease and stroke significantly (41).
Target organ damage
The heart, blood vessels, and kidneys are all damaged by sustained (chronic) hypertension. This significantly increases the risk of heart disease and stroke, as well as hypertensive kidney disease. Increased mortality from cardiovascular diseases has been linked to sodium (NaCl) consumption in several clinical studies, due to the abnormal thickening of the heart muscle (‘left ventricular hypertrophy’) caused by it (42).
Organ damage, e.g., changes in the structure and function of large elastic arteries, could be another result of increased sodium consumption, research shows. This effect is separate from its contribution to hypertension (43,44, 45, 46).
Though the results of the few studies that have examined how a reduction in sodium (NaCl) consumption affects cardiovascular disease and mortality have been inconclusive (47, 48, 49, 50, 51, 52), generally a direct association has been indicated (47, 48, 49).
Lower salt intake produced a tendency toward reduced incidence of heart disease in the TONE study (29).
A striking finding of one study was that 10–15 years down the line, people who did not have high blood pressure who were put on the low-sodium diet of the two previous TOHP trials were a quarter less likely to suffer a cardiovascular event (53). A further link between salt and heart disease was found during this TOHP follow-up study: the sodium-potassium ratio was linked to increased risk in a dose-response relationship (54).