The need for a comprehensive salt reduction policy in the UK

The hidden salt content of our food is alarming ? around 80 per cent of the salt consumed in the UK comes from ready-made foods, with only around 15 per cent added in cooking or at the table (the remaining 5 per cent occurs naturally in food).

Although we might expect some foods, like crisps and salted nuts, to be relatively high in salt, it is in fact many of the “healthy” foods that contain the greatest amounts. Weight for weight, cornflakes contain more salt than crisps; some ready-made soups have the same salt concentration as seawater; and many shop-bought sandwiches contain over a half a day?s recommended salt intake for an adult in a single serving.

Blood pressure and cardiovascular disease

Cardiovascular disease is the commonest cause of death and disability in the UK. It accounts for just under half of all deaths, and the three major established risk factors are cigarette smoking, abnormal lipid profiles and blood pressure.

Increasing blood pressure throughout the range in developed countries is a major accelerating factor¹. Blood pressure also has direct effects: it is the main cause of strokes, through cerebral haemorrhage and small vessel disease. It also causes atherosclerosis, and is now the main cause of heart failure.

Some of the best evidence for primary prevention of disease comes from the many trials that have been carried out over the last 30 years, which demonstrate that reducing blood pressure is of great benefit in cutting strokes, heart failure and coronary heart disease². Recent meta-analyses of these studies have shown that there is equal benefit whatever means are used to reduce blood pressure, and that the benefit is proportional to the degree of reduction³.

High blood pressure is extremely common in western countries. The UK is no exception and just under 40 per cent of the adult population has it (systolic >=140 mmHg and/or diastolic >=90 mmHg). About half of those aged 50-59 have high blood pressure, and this rises to 70 per cent at the age of 60-79⁴. At the same time, studies in developed countries have shown a consistent pattern of an increase in blood pressure with increasing age.

Evidence on salt and blood pressure

There are several types of evidence that relate salt to the development of high blood pressure:

Epidemiological studies

Reports and studies in undeveloped societies that do not, or did not, have access to salt show that blood pressure is lower than in developed societies and that there is no rise in blood pressure with age^5-10. While there may be other factors that account for the lower blood pressure, several studies have clearly demonstrated the profound importance of salt^9,10. A study in the Pacific Islands, where one community used salt water in their food and another did not, showed the community using salt had higher blood pressure⁹.

In study of two rural communities in Nigeria, one of which had access to salt from a salt lake and the other did not, showed differences in salt intake and differences in blood pressure, and yet in all other aspects of lifestyle and diet the two communities were similar¹¹. The Qash’qai, an undeveloped tribe living in Iran who have access to salt deposits on the ground, develop high blood pressure and a rise in blood pressure with age similar to that which occurs in western communities, but in all aspects live a lifestyle similar to undeveloped countries¹⁰.

Intervention studies

A well-controlled double-blind study in just under 500 new born babies¹² showed that when salt intake was reduced by about 30 per cent in the reduced salt group, as judged by spot urinary sodium concentrations, there was a progressive difference in systolic blood pressure. At the end of six months, when the study was discontinued, the babies on the lower salt intake had a 2.1 mmHg lower systolic blood pressure (P<0.01).

Many of these babies were examined 15 years later¹³, and there remained a significant difference in blood pressure, when adjusted, between those babies who in the first six months of life had had a reduced salt intake and those that had not, suggesting that there was a programming effect to salt intake in early life. This conclusion fits with several studies in animals.

In a study in Portugal¹⁴, the population of a village was given information on how to reduce salt intake, and given processed foods with less salt. The population?s intake was reduced by just under half, as judged by 24-hour urinary sodium excretion, and after a year, there was a difference in blood pressure. At two years both systolic and diastolic pressure showed very significant falls compared to those in the population of the control village.

Population studies

In the late 1950s the Japanese became aware that certain parts of the country, particularly the north, had a high salt consumption and that deaths from stroke due to brain haemorrhage were amongst the highest in the world. They found that the rate of stroke across the country was directly related to the salt intake. The government campaigned to reduce salt intake, and succeeded over the following decade in cutting it from an average of 13.5 to 12.1 g per day. At the same time, there was a fall in blood pressure both in adults and children, and an 80 per cent reduction in stroke mortality¹⁵. At the time of this fall and the reduction in stroke mortality, there were large population increases in fat intake, cigarette smoking, alcohol consumption and an increase in weight, along with a reduction in fruit and vegetable consumption and less exercise being taken. It would appear that the western influence which was rapidly being felt Japan seemed to have little effect on blood pressure, provided salt intake was reduced.

In Finland 14,000 men aged 35-64 were given advice about reducing fat and salt intake, increasing their fruit and vegetable consumption and reducing their smoking, while the Finnish food industry contributed processed foods with less salt. Over the 20-year period of the study, which started in 1972, cholesterol was reduced by 13 per cent, diastolic pressure by 9 per cent and smoking by 15 per cent. At the same time, there was a reduction in stoke mortality of 66 per cent and coronary heart disease mortality of 55 per cent. The reduction in the three major risk factors explained four-fifths of the fall in coronary heart disease mortality and two-thirds of the fall in stroke mortality^{16, 17}. It has been estimated that about half of these changes were due to the change in diet and that the reduction in salt intake played an important role in the fall in blood pressure and the reduction in strokes that occurred.

The Japanese and the Finnish experience suggest that where the right advice is given to the public and where processed foods are provided with less salt, it is relatively easy for the whole population to reduce their salt intake.

Treatment trials

Studies carried out since the early 1900s have shown that in those subjects with high blood pressure, restricting salt intake causes large falls in blood pressure. The classic studies of Kempner clearly showed major effects¹⁸, even in accelerated hypertension, which was subsequently shown by carefully controlled studies to be due to the reduction in salt intake¹⁹.

Recently more modest reductions in salt intake, ie to around 5 to 6 g per day from an intake of 10 to 12, have been shown to cause falls in blood pressure in hypertensives equivalent to single drug therapy²⁰. These falls are larger in those subjects with lower levels of plasma renin activity or angiotensin II, for example those of African origin ²¹, or older people. Furthermore, modest reductions in salt intake have been shown to be additive to drug treatment²².

A double blind study of salt reduction with three sodium intakes ? 10, 5 and 3 g per day ? in subjects with mild untreated essential hypertension showed a clear dose response to the salt reduction²³ with large falls in blood pressure on 3 g per day. Furthermore blood pressure remained controlled on the lower salt intake at the one-year follow up.

Three longer-term studies in the USA have also demonstrated that quite small reductions in salt intake not only reduce blood pressure, but may also prevent the development of hypertension, and may allow the withdrawal of drug therapy^24-26.

When all of the evidence is considered, that for the beneficial salt intake, particularly when judged against other nutrients, appears to be robust. The benefit of reducing salt intake, even modestly, from 9 to 10 to 5 to 6 g per day, could be of immense benefit in reducing and preventing cardiovascular disease. For instance, recent research in hypertension demonstrated that each 1g reduction in our average salt intake would save 6,000 lives a year and prevent a further 6,000 heart attacks and strokes that people would suffer and survive²⁷. Based on all of this evidence, most western countries have recommended a reduction in salt intake from 9 to 10 to 5 to 6 g per day.

In the UK, the COMA report of 1994, recommended a population reduction in salt intake from 9 to 6 g per day²⁸. Several other dietary recommendations intended to reduce the very high cardiovascular mortality in the UK were initially endorsed by Conservative ministers and the Department of Health, but, according to a leading article in the British Medical Journal²⁹, huge pressure was exerted by the food industry on the government to withdraw the specific recommendations on salt. In 1996 the Chief Medical Officer said there was continuing controversy concerning the relationship between salt and blood pressure, and at that time, a group of scientists, many of whom were either on the COMA committee or sub-committee on minerals, set up the Consensus Action Group on Salt and Health (CASH) with the aim of establishing a dialogue with the food industry on the reduction of salt levels in processed foods³⁰.

CASH would like to see an immediate, across the board reduction of 10-15 per in salt added to food. A reduction of this magnitude would not be detectable by taste and the food would not, therefore, be rejected by consumers. Further reductions of 10-15 per cent should follow at intervals of around 18 months, until the target of 6g per day per adult is reached.

Since the inception of CASH, the government?s Scientific Advisory Committee on Nutrition (SACN) has endorsed the 1994 COMA report and set a target salt intake of no more than 6g per day for adults, with much lower levels for children. The current government has called for the food industry to make substantial cuts in the amount of salt added to the nation?s food and has even threatened legislation if salt levels are not reduced.

At last it seems as though action will be taken to reduce our salt intake. If only the 1994 COMA guidelines had been followed, we might already be reaping the rewards of lower blood pressure in our population and several thousand of the deaths from cardiovascular disease that have occurred over the past decade might have been prevented.

References

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19 Watkin DM, Froeb HF, Hatch FT, Gutman AB. Effects of diet in essential hypertension: II. Results with unmodified Kempner rice diet in fifty hospitalised patients. Am J Med 1950;9:441-93.

20 MacGregor GA, Markandu ND, Best FE, Elder DM, Cam JM, Sagnella GA, Squires M. Double-blind randomised crossover trial of moderate sodium restriction in essential hypertension. Lancet. 1982;1:351-5.

21 Sacks R M, Svetkey LP, Vollimer VM et al. Effects on blood pressure of reduced dietary sodium and the dietary approaches to stop hypertension (DASH) diet. N Eng J Med 2001; 344: 3-10.

22 MacGregor GA, Markandu ND, Singer DRJ, Cappuccio FP, Shore AC, Sagnella GA. Moderate sodium restriction with angiotensin converting enzyme inhibitor in essential hypertension: a double blind study. BMJ 1987;294:531-4.

23 MacGregor GA, Markandu ND, Sagnella GA, Singer D, Cappuccio FP. Double-blind study of three sodium intakes and long-term effects of sodium restriction in essential hypertension. Lancet. 1989;2:1244-7.

24 The Trials of Hypertension Prevention Collaborative Research Group. The effects of nonpharmarmacologic interventions on blood pressure of persons with high normal levels: results of the Trials of Hypertension Prevention, phase I. JAMA 1992;267:1213-20.

25 The Trials of Hypertension Prevention Collaborative Research Group. Effect of weight loss and sodium reduction intervention on blood pressure and hypertension incidence in overweight people with high-normal blood pressure. The Trials of Hypertension Prevention, Phase II. Arch Intern Med 1997;157:657-67.

26 Whelton PK, Appel LJ, Eepeland MA, Applegate WB, Ettinger WH, Kostis JB, Kumanyika S, Lacy CR, Johnson KC, Folmar S, Cutler JA, for the TONE Collaborative Research Group. JAMA 1998;279:839-46.

27 He FJ, MacGregor. How far should salt intake be reduced? Hypertension. 2003; 42: 1093-1099.

28 COMA report 46: Nutritional Aspects of Cardiovascular Disease: 1994.

29 Godlee F. The food industry fights for salt. BMJ. 1996;312:1239-40.

30 MacGregor GA, Sever PS. Salt-overwhelming evidence but still no action: can a consensus be reached with the food industry? CASH (Consensus Action on Salt and Hypertension). BMJ 1996;312:1287-9.

Professor Graham MacGregor is the chair of Consensus Action on Salt and Health, and professor of cardiovascular medicine at St George?s Hospital, London.