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Stroke and heart disease

Introduction

Cardiovascular disease (CVD) is an umbrella term referring to strokes, coronary heart disease (CHD) and heart failure.

Stroke

A stroke occurs when part of the blood flow to the brain is cut off. This causes a break in the oxygen supply, causing cells to die. Globally, stroke is the second leading cause of death for those aged 60 years and over. Every year 15 million people will suffer a stroke; 6 million of these cases will lead to death and 5 million cases will lead to permanent disability (1).

Stroke has a greater disability impact compared to any other chronic disease. The outcomes of strokes are wide ranging, but sufferers can experience paralysis, speech impediment and memory problems which can be highly frustrating and difficult for both the individual and the family.  Although stroke incidence is declining in many developed countries, it is increasing in the developing world. Over the next 20 years, deaths caused by stroke could triple in Latin America, the Middle East and sub-Saharan Africa. (1)

Coronary heart disease (CHD)

CHD is the term used to describe what happens when the heart’s blood supply is reduced or blocked. Raised blood pressure is a major risk factor for heart and circulatory diseases including heart attacks and heart failure. Over time, untreated high blood pressure can lead to a thickening of the heart muscle which can reduce the effectiveness of the heart pumping action.    

CHD is the UK's biggest killer and the number one cause of mortality globally. In 2012, cardiovascular diseases killed 17.5 million people, which is equal to 31% of all global deaths (6).

Who is most at risk of cardiovascular disease?

Older people, people with high blood pressure, diabetics, people of black and South Asian descent and smokers are all at increased risk of having a stroke or heart attack.

How does salt contribute?

Raised blood pressure is a major cause of cardiovascular disease, responsible for 62% of stroke and 49% of coronary heart disease. Importantly, the risk of CVD increases throughout the range of blood pressure, starting at 115/75 mmHg.(7) Salt is the major factor that increases blood pressure and is therefore responsible for many strokes and heart attacks every year.(8) From the blood pressure reduction seen in a meta-analysis (9), it was estimated that a reduction to 5 g of salt per day would reduce stroke by 24% and coronary heart disease by 18%. This would prevent approximately 35,000 stroke and CHD deaths a year in the UK and approximately 2.5 million deaths worldwide.(10)

Two large randomised trials (Trials of Hypertension Prevention (TOHP) I and II) looked at the long-term effects of salt reduction on CVD in more than 3000 participants over an 18month (TOHP I) or 36-48 month (TOHP II) period. Compared with the control group, individuals in the intervention groups reduced their salt intake by 25% to 30% from an average of approximately 10 g/d, resulting in a fall in BP of 1.7/0.9 mm Hg in TOHP I and 1.2/0.7 mm Hg in TOHP II. A follow-up study 10 – 15 years later showed that individuals who were originally allocated to the reduced salt group had a 25% lower incidence of cardiovascular events.

A recent meta-analysis11 of 19 independent cohort samples from 13 studies, with 177,025 participants showed that a high salt intake is associated with a significant increase in risk of CVD. A reduction in salt intake from 10g to 5g per day, would reduce stroke rate by 23% and overall CVD by 17%. This would save 0.25million deaths from strokes and almost 3 million deaths from cardiovascular disease each year. These results support the role of a substantial reduction in population salt intake for the prevention of cardiovascular disease.

 

 

In June 2010, the National Institute for and Health and Clinical Excellence (NICE) published a report on the prevention of cardiovascular disease which highlighted salt reduction as the number one priority as a cardiovascular preventative measure. It also highlighted that we should be aiming for a salt intake of 6g by 2015 and 3g by 2025.(12)

Direct effect on stroke

Evidence suggests that a high salt intake also has a direct effect on strokes, independent of the effect of salt on blood pressure.(8,13-14) When different populations are compared, there is a very close correlation between salt intake and stroke mortality independent of blood pressure (Fig 2).(13) Another study has confirmed this within a single country.(8)

Direct effect on left ventricular mass

Salt intake can directly and independently lead to enlargement of the heart (Fig 2). Reducing salt intake has been shown to reduce left ventricular hypertrophy (15), which is a major risk factor for cardiovascular disease.

A number of cross-sectional studies have shown a positive correlation between 24-h urinary sodium and left ventricular mass.(15-17) A reduction in salt intake has been shown to decrease left ventricular mass in hypertensive individuals.(18-21)

Direct effect on capillary density

Recent research has provided further evidence that a high salt intake has a direct damaging effect on capillary density independent of and additive to the effect it has on blood pressure. A study in individuals with mildly raised BP  found that a modest reduction in salt intake can increase skin capillary density, and the greater the reduction of salt, the greater the increase in the number of capilleries.(22)

 

 

Current salt intake & dietary advice

Many populations consume too much salt. The World Health Organisation recommends that we consume no more than 5 grams a day but many countries have intakes of 9-12 grams per day – double the recommended maximum.

It has been shown that a high salt intake, a low consumption of fruit and vegetables (which corresponds to a low potassium intake), obesity, excess alcohol intake and lack of physical exercise all contribute to the development of high blood pressure. However, the diversity and strength of the evidence is much greater for salt than for other factors. People with, or considered at risk, of stroke or heart disease should take extra care to ensure that they keep their salt intake below the recommended maximum of 5g.

References

  1. World Health Organisation (2016) Cardiovascular Diseases (CVDs) http://www.who.int/mediacentre/factsheets/fs317/en/
  2.  NHS (2009) Coronary Heart Disease. http://www.nhs.uk/Conditions/Coronary-heart-disease/Pages/Introduction.aspx
  3.  Lewington S et al. Age-specific relevance of usual blood pressure to vascular mortality: a meta-analysis of individual data for one million adults in 61 prospective studies. Lancet. 2002; 360, 1903-1913
  4. Nagata C et al. Sodium intake and risk of death from stroke in Japanese men and women. Stroke. 2004; 35,1543-7
  5.  He FJ, MacGregor GA. Effect of modest salt reduction on blood pressure: a meta-analysis of randomized trials. Implications for public health. Journal of Human Hypertension. 2002; 16, 761-770
  6. He FJ, MacGregor GA.  How far should salt intake be reduced? Hypertension.2003; 42, 1093-1099
  7. Strazullo P et al. Salt intake, Stroke and Cardiovascular Disease: meta-analysis of prospective studies. British Medical Journal. 2009; 339, b4567doi:10.1136/bmj.b4567
  8. NICE. Prevention of Cardiovascular Disease. Public health guidance 25. Published June 2010
  9.  Perry IJ, Beevers DG. Salt intake and stroke: a possible direct effect. Journal of Human Hypertension. 1992; 6, 23-25
  10. 0.    Xie JX, Sasaki S, Joossens JV, et al: The relationship between urinary cations obtained from the INTERSALT study and cerebrovascular mortality. Journal of Human Hypertension. 6:17-21, 1992
  11. 1.   Schmeider RE et al. Dietary salt intake. A determinant of cardiac involvement in essential hypertension. Circulation. 1988; 78, 951-956
  12. 2.    Kupari M et al. Correlates of left ventricular mass in a population sample aged 36 to 37 years. Focus on lifestyle and salt intake. Circulation. 1994; 89, 1041-1050
  13. 3.   du Caliar G et al. Sodium and left ventricular mass in untreated hypertensive and normotensive subjects. American Journal of Physiology. 1992; 263, H177-18
  14. 4.   Ferrara LA et al. Left ventricular mass reduction during salt depletion in arterial hypertension. Hypertension. 1984;  6, 755-759
  15. 5.   Liebson PR et al. Comparison of five antihypertensive monotherapies and placebo for change in left ventricular mass in patients receiving nutritional-hygienic therapy in the Treatment of Mild Hypertension Study (TOMHS). Circulation.1995; 91, 698-706.
  16. 6.   Jula AM et al. Effects on left ventricular hypertrophy of long-term nonpharmacological treatment with sodium restriction in mild-to-moderate essential hypertension. Circulation. 1994; 89:1023-1031
  17. 7.   Levy D et al. Prognostic implications of echocardiographically determined left ventricular mass in the Framingham Heart Study. New England Journal of Medicine. 1990; 322, 1561-1566
  18. 8.   He FJ et al. Effect of modest salt reduction on skin capillary rarefaction in White, black and South Asian individuals with mild hypertension. Hypertension. Published online June 2010. Available at; http://www.actiononsalt.org.uk/evidence/docs/na_capillary_density_hypertension_2010-1.pdf
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