Long term over consumption of salt gets a lot of attention for causing raised blood pressure, but it has also been linked with gastric cancer, osteoporosis and kidney stones. Chronic deficiency in salt has recently been linked with heart failure and increased risk of death from cardiovascular disease.
High Salt problems
Some studies have reported a reduction in blood pressure when salt levels are reduced. One of the best known was the Intersalt study(1). However this study included 4 indigenous tribes whose salt intake was so low that it skewed the results. This is admitted in the paper and when the 4 tribes were omitted from the study the association between salt intake and blood pressure was barely significant.
Most studies have shown reductions in blood pressure that are pretty small, often about 2.5mgHg. So someone with a systolic blood pressure of 140mgHg would then have 137.5mgHg. That is pretty insignificant. Indeed, many people do not show a reduction in blood pressure at all on reducing salt intake. The reduction is most pronounced for those with higher blood pressure. This could be because the groups of people with high blood contain more salt sensitive individuals.
Whether a reduction in salt intake is likely to lead to a reduced death rate from cardiovascular disease or any other disease, has been a bone of contention ever since the first studies on salt and health were produced. My view is that for most people there is not much of a link with high blood pressure. For those with established high blood pressure it may be worth reducing intake, but only if their intake is already above the average of 9g per say of salt.
Some studies have linked stomach cancer with high salt intakes. Although salt is not carcinogenic it has been suggested that it could firstly damage the stomach lining(2), and secondly encourage the growth of the bacteria, H. Pylori, a known risk factor for stomach cancer.
On balance it looks as though a very salty diet is probably likely to increase risk for stomach cancer slightly(3). Stomach cancer rates have been falling over the last 50 years in most countries, however there is still a big difference between rates in Japan and the US and UK. Japan has a stomach cancer rate 10 times greater than the US. This is associated with a much greater intake of salt as measured by urinary salt excretion over a 24 hour period.
The rates of increase in cancer in the Japanese studies suggest to me that very high salt diets (>25g/day) are a clear risk factor for stomach cancer. The level of salt (c.9g/day) in a typical UK or US diet however, is probably not going to affect your risk significantly.
Osteoporosis and kidney stones
When you consume more salt, you either excrete the amount you don't need through your kidneys, or sweat it out if you are exercising. It has been suggested that for every extra 6g of salt that is filtered through your kidneys an additional 30mg of calcium is drawn into the urine(4). It is thought that this loss of calcium can increase one's risk of both osteoporosis and kidney stones.
Considering the numbers, 30mg of extra calcium lost per day compared to a recommended daily intake of 1100mg is just 2-3%. Compared with the total amount of calcium stored in the body, 1kg, it is just 0.003%. At this rate it would take about 10 years to lose 10% of your calcium stores.
There are two factors that could cause increased loss of calcium through the kidneys. Firstly, with more salt being filtered out through the kidneys, salt is absorbed in place of calcium, and secondly an increased rate of filtration in the kidney caused by hormonal changes could lead to increased calcium losses.
However, under normal cicumstances the body adjusts for increased loss of calcium by absorbing more calcium from the gut(4). It appears that if dietary calcium is adequate and vitamin D levels are normal then for most people there is no significant danger of bone loss when consuming diets high in salt.
It's been suggested that post-menopausal women should reduce salt intake as in some studies they have shown reduced ability to normalise their calcium levels. Whether this would be the case if these women were on healthy diets rather than the standard American diet is open to question. In any case, those who want to reduce risk of osteoporosis would be best off eating more dairy (cheese and yoghurt) and unfilleted fish such as sardines. I don't generally advise taking calcium supplements as excess calcium can be dangerous if it is not absorbed alongside the full range of fat soluble vitamins A, D E and K.
The increased flux of calcium through the kidneys when a salty diet is consumed can lead to kidney stones, especially in women(5). Adequate fluid and calcium intake offset the effect. Unless you have a history of kidney stones in your family it is probably not worth reducing salt intake for this reason.
Low salt problems
A recent study showed that people with heart failure who were taking a diuretic to treat their symptoms, fare better on an average salt intake of 7.2g/day, than a lower one of 4.8g/day(6). Those with lower salt had increased levels of renin and rates of readmission to hospital.
Increased CV death rates
A number of recent studies have shown an increased risk of death from cardiovascular disease in people consuming less than the average amount of salt consumed in the UK per day, namely 9 grams. Does this mean we should all make sure we consume plenty of salt. I'm not so sure. These studies(7,8,9) included a high level proportion of the ill and the elderly. Many elderly people have trouble keeping enough salt in their blood. This is typically due to certain drugs or high blood sugar which can lead to low blood sodium, hyponatraemia. Hyponatraemia increases the risk of death from cardiovascular disease.
Is the amount of salt that the health authorities tell us to eat (6g) actually harmful for our health? I think it depends who you are. If you are old it may well be that you would do better consuming more salt. If you are an active person with salty sweat then 6g of salt is probably not good for your health also. However, for many people 6g of salt per day is perfectly reasonable.
Human beings evolved in many places with low levels of salt intake over thousands of years. That is why I believe there is not an increased risk of death from cardiovascular disease at low salt intakes unless you are prone to low levels of blood sodium.
In cases of acute sodium shortage, life threatening hyponatraemia is common. With the increase in long distance races in hot conditions, especially those involving running, hyponatraemia is becoming very common. This condition can be exacerbated by the long list of drugs, including amphetamines (ecstasy), diuretics (furosemide), NSAIDS (Ibuprofen, Naproxen), Opiates (Morphine, Codeine), SSRIs (Fluoxetine-Prozac, Paroxetine), Tricyclics (amitriptyline, Imipramine), Oxytocin and Vincristine to name a few. If you are an endurance athlete and care for your health, my advice is to avoid taking drugs to get yourself through events. Some are on the banned list anyway, but you have to ask yourself if you should be competing at all if you need medical assistance before you start the race.
Long duration intense exercise can lead to reduced sodium levels. If cramping is in more than one muscle group this can be a sign of sodium depletion. Basically there is not enough sodium in these cases to allow for proper nerve conduction(10).
If you are an endurance athlete making sure you have enough salt throughout an event lasting over 2 hours can be crucial. If you lose a lot of salt in your sweat you do need to replace it, or your performance and health will suffer.
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1) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1834069/?page=4 Intersalt study
2) http://www.ncbi.nlm.nih.gov/pubmed/3916194 animals experiments and stomach cancer
3) http://onlinelibrary.wiley.com/doi/10.1111/j.1349-7006.2005.00006.x/full review of stomach cancer evidence
4) http://www.jacn.org/content/25/suppl_3/271S.full.pdf osteoporosis
5) http://www.sciencedirect.com/science/article/pii/S0022534711060241 kidney stones
6) http://www.ncbi.nlm.nih.gov/pubmed/17688420 heart failure evidence
7) http://www.ncbi.nlm.nih.gov/pubmed/16490476 heart disease death rates in NHANES - huge population cohort
8) http://jama.jamanetwork.com/article.aspx?articleid=899663 heart disease risk in average population sample
9) http://jama.jamanetwork.com/article.aspx?articleid=1105553 sodium and potassium effects on heart disease death rates in people with CVD