Salt and blood pressure

Introduction

While some evidence suggests that obesity coupled with a lack of exercise is an important factor involved in the development of high blood pressure, stronger evidence indicates that salt intake is more strongly related to the development of hypertension, particularly the rise in blood pressure with age (1). Hypertension can be defined as a blood pressure of 140/90 mmHg or higher. However, the risk of cardiovascular disease increases throughout the range of blood pressure starting from a systolic blood pressure of 115mmHg (2,3) which 80% of the adult population exceeds. Raised blood pressure is the major cause of the development of cardiovascular disease, and accounts for 60% of all strokes and 50% of all heart disease.(4) Blood pressure is the biggest cause of death in the world through the strokes and heart attacks it causes.  Approximately one in three adults in the UK, 16 million adults, have high blood pressure. Five million people don’t know they have the condition.(5)

Who is most at risk of high blood pressure?

Anyone is at risk of a high blood pressure, as blood pressure starts to increase from childhood. People with a high salt diet, pregnant women and people of black African descent are particularly susceptible to high blood pressure.(6)

High blood pressure is an important risk factor for a range of conditions. These include strokes, vascular dementia, diabetes, kidney disease, heart disease and mobility problems.(6) People who already have these conditions may find a reduced salt diet beneficial in the long run.

Figure 1 - Prevalence of hypertension for men and women
Source: Health Survey for England, 2006.20

How does salt contribute?

A high salt diet disrupts the natural sodium balance in the body. This causes fluid retention which increases the pressure exerted by the blood against blood vessel walls (high blood pressure). For every one gram of salt we cut from our average daily intake, there would be approximately 6,000 fewer deaths from strokes and heart attacks each year in the UK.  It has been estimated that a reduction in salt intake from 10g a day to 6g will reduce blood pressure and could lead to a 16% reduction in deaths from strokes and a 12% reduction in deaths from coronary heart disease. This would prevent approximately 19,000 stroke and heart attack deaths in the UK each year and 2.6 million each year worldwide.  Reducing salt is one of the quickest ways to reduce your blood pressure, particularly if you already have high blood pressure.(7,8)

Figure 2. - The relation of salt excretion to the slope of the rise in systolic blood pressure with age in 52 centres in the INTERSALT study. (Adapted from INTERSALT Study, 1988).

Evidence

A large number of studies have been conducted which have consistently shown that salt intake is the major factor increasing population blood pressure. The evidence is greater for the effect of salt on blood pressure than any other dietary and lifestyle factors including a low consumption of fruit and vegetables (i.e. low potassium intake), obesity, excess alcohol intake and lack of physical exercise.

A large study of 52 communities throughout the world - the InterSalt Study – has also been conducted (Fig. 2) by taking standardised measurements of blood pressure and 24 hour urinary sodium and potassium. The study showed a positive relationship between salt intake (using 24 hour urinary sodium excretion) and blood pressure. There was also a positive and highly significant relationship between salt intake and the increase in blood pressure with age (Fig 1). This relationship showed that an increase of 6 grams of salt per day over 30 years would lead to an increase in systolic pressure by 9 mmHg. (9,11) This represents a large increase in population blood pressure.


 

Two studies (10,11), both 4 weeks in duration, have compared the effect of different salt intakes on blood pressure (3, 6, and 12 g/day in one, 4, 6 and 8 g/day in the other). Both showed a clear dose-response relationship, i.e. the lower the salt intake achieved, the lower the blood pressure. 

The most recent meta-analysis focusing on salt and blood pressure looked at 28 randomised trials (17 in hypertensives, 11 in normotensives (normal blood pressure)) with a duration of one month or longer. It showed that a modest reduction in salt intake led to a significant reduction in blood pressure, both in people with and without high blood pressure.  A dose-response relationship between the reduction in salt intake and the decrease in blood pressure was also seen. It is estimated that a reduction of salt by 6 g/d would lower blood pressure by 7/4 mmHg in individuals with high blood pressure and 4/2 mmHg in those with normal blood pressure (8).

The relationships seen in these studies provide evidence that the recommendation to reduce salt intake to 5-6g/day will have a major impact on blood pressure and a further reduction to 3-4 d/day will have a much greater effect.

Current salt intake & dietary advice

Almost everyone in the UK (and the rest of the Western world) eats too much salt. The daily recommended amount in the UK is no more than 6 grams a day; the current average salt intake is 8.1g salt a day although many people are eating more than this.

People with or considered at risk of high blood pressure should take extra care to ensure that they keep their salt intake below the recommended maximum of 6g. This can be achieved by simple changes, such as consuming less processed foods and checking product labels before purchase.

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References

1. MacGregor GA. Nutrition and blood pressure. Nutr Metab Cardiovasc Dis. 1999; 9, 6-151.
2. He FJ, MacGregor GA. Fortnightly review: beneficial effects of potassium. British Medical Journal. 2001; 323, 497-501
3. Singh RB et al. Hypertension and stroked in Asia: prevalence, control and strategies in developing countries for prevention. Journal of Human Hypertension. 2000; 14, 749-763
4. World Health Report 2002. Reducing risks, promoting healthy life. World Health Organisation, 2002.
5. BPA News. 07/09/2009. Could you be the one in three at risk of the silent killer? http://www.bpassoc.org.uk/mediacentre/Bloodpressurenews/CouldYOUbetheoneinthreeatriskofthesilentkiller [accessed 07/08/09]
6. Blood Pressure Association. 2008. What Blood Pressure can do to you. http://www.bpassoc.org.uk/BloodPressureandyou/Yourbody [accessed 07/08/09]
7. He FJ  & MacGregor GA. Importance of determining blood pressure in children: Meta-analysis of controlled trials. Hypertension. 2006; 48, 861-869
8. He FJ & MacGregor GA. Effect of modest salt reduction on blood pressure: a meta-analysis of randomised trials. Implications for public health. Journal of Human Hypertension. 2002; 16, 761-770
9. INTERSALT. Intersalt: an international study of electrolyte excretion and blood pressure. Results for 24 hour urinary sodium and potassium excretion. Intersalt Cooperative Research Group. British Medical Journal. 1988; 297,  319-28.
10. MacGregor GA et al. Double-blind study of three sodium intakes and long-term effects of sodium restriction in essential hypertension. Lancet. 1989; 334,1244-1247, 1989
11. Sacks FM et al. Effects on blood pressure of reduced dietary sodium and the Dietary Approaches to Stop Hypertension (DASH) diet. DASH-Sodium Collaborative Research Group. New England Journal of Medicine. 2001; 344, 3-10
12. Cutler JA et al. Randomised trials of sodium reduction: an overview. American Journal of Clinical Nutrition. 1997; 65, 643S-651S
13. Elliott P et al. Intersalt revisited: further analyses of 24 hour sodium excretion and blood pressure within and across populations. Intersalt Cooperative Research Group. British Medical Journal. 1996; 312,1249-53 20.
14. Craig R , Mindell J . Health Survey for England,2006. Volume 1, Cardiovascular disease and risk factors in adults. Available at http://www.ic.nhs.uk/pubs/hse06cvdandriskfactors [accessed 14/07/2010]