Q:There still seems to be debate about whether or not we need salt and if so what kind, how much and how often. Is there any definitive answer in sight? HK, Cardiff..........
A:Earlier in the year the medical press went salt crazy with the publication of additional data from the Intersalt study a multicentre project covering 52 populations in 32 countries (BMJ, 1996; 312:1249-53). The group's findings were that high sodium intake is related to significantly higher blood pressure, and that the effect was most pronounced later in life. This confirms the findings of most other studies (Hypertension, 1991; 17(sup 1):3-8).
At the same time, a study in Barcelona showed that people who switch from a low salt diet (20 mmol/day) to a high one (260 mmol/day) showed a decrease in fasting glucose, overall cholesterol levels, triglycerides and uric acid. The trial was carried out on 50 hypertension patients to assess fluid and hormonal fluctuations on a high salt diet (Clin Sci 1996; 91: 155-61).
Of course, all human beings need salt. Salt is necessary for the proper functioning of nerves and muscles and helps to regulate the body's fluid balance.
That said, the modern diet is much higher in salt than in the past. Today, salt is used to add flavour to otherwise nutritionally poor and tasteless pre packaged convenience foods. It is estimated that 75 per cent of our salt intake comes from processed foods (15 per cent comes from discretionary use at the table or in cooking and 10 per cent from the natural content of food) (BMJ, 1988, 297:319-28).
It is now accepted that our average salt intake of around 10g per person per day should be cut in half. This action would, it is speculated, reduce the incidence of strokes by 22 per cent and heart attacks by 16 per cent in Britain alone conferring greater benefit than all the current conventional treatment for high blood pressure (BMJ, 1991; 302:811-24).
Other research shows that high salt intake is also an important risk factor for osteoporosis. In a recent study of post menopausal women, sodium retention was linked to greater calcium excretion and a reduction in the rate of hip bone density (Lancet, 1996; 348:250-1). The authors concluded that a modest reduction in salt intake was as effective as an increase in calcium intake of up to 900mg. A high salt intake in young girls may also reduce calcium intake in the skeleton leading to bone problems later in life (Am J Clin Nutr, 1995; 62:417-25).
That said, there also appear to be problems with the low salt regimes currently in vogue. The symptoms of sodium deficiency include nausea, vomiting, dizziness, cramps, exhaustion, apathy and, if extreme, circulatory failure.
Pregnant women are often advised to restrict their salt intake in order to prevent pre eclampsia though no evidence exists to show that this is effective. Quite the contrary, since a shortage of sodium can produce low blood pressure, compromising the fetal blood supply. According to Dr Tom Brewer in America, who developed the Brewer Diet to prevent pre eclampsia, it's possible that pregnant women need more salt because their greater blood volume and tendency to sweat more means they are excreting more salt through their skin.
Recently the BMJ (1995; 311:1486-87) reported a case of a woman who developed kidney failure and hyperkalaemia (too much potassium in the blood) while taking a non steroidal anti inflammatory drug (NSAID). Low salt preparations which contain as much as 60 per cent potassium, are often recommended for those with edema and hypertension. Hyper kalaemia has few warning symptoms and potassium levels in the blood can rise to dangerous, even fatal levels, without the individual or his or her doctor being aware of it. While rare, reports such as this highlight the potential dangers of using one extreme dietary measure to compensate for the damage cause by another.
Perhaps your best health insurance is to avoid convenience foods and to lightly use non iodized salt (see p1).