Prof Risteárd Mulcahyexamines the dangers of too much salt in our diet and suggests ways in which we can cut it down
Salt (sodium chloride) is an essential part of our diet because it plays a crucial role in maintaining the normal physiological and metabolic processes of the body. Salt deprivation may lead to serious dehydration, fall in blood pressure and collapse of our metabolic processes.
It is recommended by the British government and the American Medical Association, among other authorities, that we need about 6g of salt daily, although most people eat in excess of this optimum level.
Salt is mostly excreted in the urine and the kidneys have an important and efficient mechanism to deal with salt deprivation or excess.
Smaller amounts of salt are excreted in sweat, saliva, from the bowels and during respiration. Heavy sweating during and after exercise or in very hot weather may lead to quite substantial amounts of salt loss and to serious dehydration.
Formerly in Ireland and in all western countries, salt intake was excessively high both as free salt and in foods which were preserved in salt before the era of refrigeration.
The high intake of salt in previous generations was reflected in the high incidence of salt-related diseases. There has been a substantial fall in salt consumption during the past 50 years because of the widespread adoption of refrigeration in food preservation.
The food industry, in response to pressure from health promoting organisations, has succeeded in reducing salt in canned and preserved food. There has also been a considerable decline in the use of free salt added to food at the table, again as a result of health promotion advice.
Excess salt intake is still common and is associated with high blood pressure, cancer of the stomach and osteoporosis. It may be associated with other cancers, such as colon cancer, and excess may aggravate asthma in some afflicted patients but the evidence incriminating salt in these latter conditions is far from proven.
The salt link with high blood pressure (hypertension) is no longer in doubt thanks to extensive clinical, basic and epidemiological research.
Countries with a high salt intake, such as Japan and other Asian countries, have a high incidence of hypertension as measured by its complications, including stroke caused by cerebral haemorrhage, heart failure and kidney failure.
Like other western countries, Ireland has enjoyed a substantial fall in these complications during the past 50 or more years, partly through better identification of hypertension and advances in drug treatment but mostly because of the substantial population reduction in salt intake.
Another factor which has contributed to reducing high blood pressure and its complications over the last half-century has been the elimination of nephritis or kidney inflammation with the widespread use of penicillin.
Patients have not often died from the complications of high blood pressure in latter years.
It is not so widely known that salt excess can contribute to adenocarcinoma, the commonest form of stomach cancer. There is a close correlation found between the incidence of stomach cancer and high blood pressure in the many populations which have been studied.
The importance of salt as a carcinogen is based on its interaction in the stomach with nitrates (another common food additive) to produce nitrosomes, a well-established carcinogen. Other factors may also play a part, such as the relatively newly described Helicobacter pylori infection.
There may be a complex interaction between these and other possible carcinogens which can be difficult to define. However, stomach cancer has declined during the past 50 years from being the most common form of cancer in the mid-1940s to being in eighth place, consistent with the changes in salt consumption in western countries.
There is also strong evidence that an excess of salt may contribute to osteoporosis, particularly in older and inactive women. This is a further reason why we should be moderate in our salt intake.
There are rare conditions where salt should not be restricted because of increased salt loss, such as cystic fibrosis, or occupations associated with heavy and prolonged sweating.
There is no doubt that much has been achieved in encouraging people to reduce free salt with their food just as industry has found ways and means of substituting other preservatives and flavouring agents for salt, but some health authorities still believe that more can be done to reduce salt to physiological levels in our society.
It is not difficult to eliminate free salt from our daily food. At first food will taste very bland but after a few weeks, as the salt taste threshold falls, the natural taste of food will be restored and the need for salt will diminish.
French mustard or lemon juice are some of the useful substitutes if one needs added taste. A boiled egg or raw tomato can be delicious with Dijon mustard and/or pepper.
Young children should not be allowed to develop the free salt habit as it has been shown that salt-eating children have significantly higher pressures than those who eat little or no salt.
Higher pressures in children invariably mean significant hypertension in the adult.
Food manufacturers are still under some pressure to reduce salt. The principle sources of salt are canned and preserved foods such as potato crisps, tortillas, cornflakes, ketchup, soya, pickles, processed meats, salted nuts, canned soups, Bovril and smoked fish. These foods should be eaten in moderation.
Useful information about salt content will be found on the labels while shopping.
Risteárd Mulcahy is professor of preventive cardiology (emeritus) at St Vincent's University Hospital and UCD, and is currently director of the exercise stress facility at the Charlemont Clinic, Dublin.