Medical Matters: In the 1980s I spent three months working in the Canadian province of Newfoundland. I remember my first lunchtime in the hospital canteen in St John's, jet-lagged and befuddled, opting for one of the many fish dishes on offer, writes Dr Muiris Houston.
The cod pie was so salty it was practically inedible. I was told afterwards the saltiness was a throwback to the days when isolated coastal communities salted away their catches from the fertile fishing grounds of the Grand Banks. To the uninitiated palate, however, the taste was almost nauseating. But at the least the saltiness was overt and could be avoided, unlike the salt in processed food, which often cannot be tasted because of the sophistication of the manufacturing process.
So what is salt, and how can it affect our health? Salt is sodium, one of the minerals essential for life. Our bodies need minerals for several reasons: they are part of its structure; they help regulate bodily fluids; and, as components of enzymes, they are involved in the body's metabolic processes. Some minerals, such as zinc, copper and manganese, are antioxidants; they play a role in stopping free radicals from damaging cells. The major minerals - calcium, phosphorous, magnesium, potassium and sodium - are needed in the largest amounts. Others, such as iron, iodine and copper, are required in smaller quantities, but they are no less important.
Different foods supply different minerals, which is why eating a varied diet is essential. We all have different absorption rates, and the amount we need may change, depending on age and illness.
Sodium works in balance with potassium; a higher intake of potassium may help to counteract the effect of salt. Neither sodium nor potassium has an EU-set recommended daily amount (RDA). In the case of sodium, Britain has defined a recommended normal intake (RNI) of 1.6 grams. Medical experts agree that an intake of greater than 3.2 grams puts you at risk of high blood pressure. Because it is found in many foods and also added during processing and cooking, most of us have a higher intake than we need.
Excessive sweating can cause a temporary depletion of sodium levels. Prolonged diarrhoea and vomiting are other reasons why salt levels in the body can drop; this is the reason why in infants, whose regulatory mechanisms are less developed, dehydration should always be treated with properly formulated rehydration solutions.
Low salt levels give rise to muscle cramps in adults. A friend of mine who is on a medically prescribed low-salt diet for hypertension suffers with regular cramps in his calves. Apart from cutting back on the use of table salt, his diet involves closely watching the sodium content of processed foods. These are not always the obvious ones. Shop-bought cakes and pastries contain surprising amounts of salt. And a tablespoon of soy sauce contains over a gram of sodium.
What, then, of the medical benefits of a low-salt regime? They are largely confined to the area of kidney and heart disease. A search through the Cochrane Library, the most reliable health database, reveals that there is no evidence that dietary salt has any place in the treatment of asthma. In women undergoing IVF treatment, a trial from 1996 found no evidence that a low-salt diet improved pregnancy rates. And a reduced sodium intake did not influence whether pregnant women developed pregnancy-associated hypertension, or pre-eclampsia.
On a positive note, the Cochrane review concluded that reducing salt intake did lower blood pressure, but it was by only a small amount. The reduction was not big enough to make researchers expect "an important health benefit".
People with high blood pressure gained most from a low-salt diet, leading the reviewers to conclude that it is a useful part of a programme to reduce blood pressure. Interestingly, the accumulated research suggests that "advice to reduce salt helps to maintain lower blood pressure following the withdrawal of blood pressure medication". There is also a prolonged benefit from long-term salt reduction. A low-sodium diet can, for one, lower blood pressure for up to five years. Another study found a 28-day low-sodium diet reduced systolic blood pressure by four millimetres of mercury; the diastolic, or lowest, figure went down by an average two millimetres.
A low-salt diet has clearly measurable benefits for high blood pressure. What we now need are studies to measure the broad, long-term health benefits of such an intervention.
You can e-mail Dr Muiris Houston at mhouston@irish-times.ie. He regrets he cannot answer individual queries.