Salt intake: the lower the better

Clinical bottom line

Reducing salt intake by 3 g per day lowers blood pressure, but the effect is doubled with a 6 g per day reduction and tripled with a 9 g per day reduction.

Background

Too much salt can cause high blood pressure, which in turn is one of the most important risk factors for cardiovascular disease. The average salt intake in the UK is between 9 and 12 g per day. Current recommendations are to reduce salt intake to 6 g per day or less. This paper asks whether these recommendations are low enough and whether further reductions in salt intake would have greater effects on blood pressure (i.e. whether there is a dose-response to salt reduction).

Reference

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

Systematic review

This paper re-analysed a meta-analysis. To be included, studies had to meet the following criteria:

Random allocation to either reduced salt intake or usual salt intake;
No simultaneous interventions in either group;
Net reduction in 24-hour urinary sodium less than or equal to 40 mmol (2.4 g of salt per day);
Salt reduction for at least four weeks;
Study participants not children or pregnant.

Results

Hypertensive trials

734 hypertensives were studied in 17 trials. Median age was 50 (ranging from 24 to 73 years). Study duration varied from four weeks to one year (median six weeks).
Median 24-hour urinary sodium on:
- usual salt intake was 161 mmol or 9.5 g per day (range 7.4 to 11.2 g);
- reduced salt intake: 87 mmol or 5.1 g per day (range 3.4 to 6.9 g);
- net change: reduction of 78 mmol or 4.6 g per day (range 3.1 to 6.9 g).
Pooled estimates of changes in blood pressure were -5.0 mmHg for systolic (95% CI -5.8 to -4.2) and -2.7 mmHg for diastolic pressure (95% CI -3.2 to -2.3).

Hypertensive trials

2,220 normotensives were studied in 11 trials. Median age was 47 (ranging from 22 to 67 years). Study duration varied from four weeks to three years (median four weeks).
Median 24-hour urinary sodium on:
- usual salt intake was 154 mmol or 9.1 g per day (range 7.5 to 11.8 g);
- reduced salt intake: 82 mmol or 4.8 g per day (range 3.3 to 7.9 g);
- net change : reduction of 74 mmol or 4.4 g per day (range 2.4 to 6.9 g).
Pooled estimates of changes in blood pressure were -2.0 mmHg for systolic (95% CI -2.6 to -1.5) and -1.0 mmHg for diastolic pressure (95% CI -1.4 to -0.6).

Blood pressure results

Hypertensives
A reduced salt intake of:	Predicted a fall in systolic and diastolic blood pressure of:
50 mmol or 3 g/d	3.6/1.9 mmHg
100 mmol or 6 g/d	7.1/3.9 mmHg
150 mmol or 9 g/d	10.7/5.8 mmHg

Normotensives
A reduced salt intake of:	Predicted a fall in systolic and diastolic blood pressure of:
50 mmol or 3 g/d	1.8/0.8 mmHg
100 mmol or 6 g/d	3.6/1.7 mmHg
150 mmol or 9 g/d	5.4/2.5 mmHg

The impact on cardiovascular disease

The impact of reducing salt intake on cardiovascular disease was estimated using falls in blood pressure from both hypertensives and normotensives. The results are shown in Table 1, where SBP = systolic blood pressure; DBP = diastolic blood pressure; and IHD = ischaemic heart disease.

Table 1. Predicted reductions in stroke and ischaemic heart disease deaths\

Measure	Reduction in salt intake
	3 g/d		6 g/d		9 g/d
	SBP	DBP	SBP	DBP	SBP	DBP
Fall in BP in all participants	2.5	1.4	5	2.8	7.5	4.2
Reduction in stroke death, %	12	14	23	25	32	36
Stroke deaths prevented in UK	7,300	8,300	13,700	15,500	19,300	21,600
Reduction in IHD, %	9	10	16	19	23	27
IHD deaths prevented in UK	10,600	12,400	20,300	23,600	29,100	33,700

Comment

It was estimated that a reduction of 3 g/d in salt intake would lower blood pressure by 2.5/1.4 mmHg, which would reduce strokes by 12 to 14% and ischaemic heart disease by 9% to 10%. In the UK, the total number of stroke deaths is 60,666 per year and the total number of ischaemic heart disease deaths is 124,037 per year. Therefore, a reduction of 3 g/d in salt intake would prevent approximately 7,300 to 8,300 stroke deaths and 10,600 to 12,400 ischaemic heart disease deaths in the UK per year. These effects are predicted to double with a 6 g/d reduction and triple with a 9 g/d reduction.

It was also estimated that even in normotensives, reducing salt intake by 3 g/d would lower blood pressure by 1.8/0.8 mmHg, which would reduce strokes by 9% and ischaemic heart disease by 6%. A reduction of 6 g/d would reduce strokes by 17% and ischaemic heart disease by 12% and a 9 g/d reduction would reduce strokes and ischaemic heart disease by 24% and 18% respectively.

How do we eat less salt?

Cut down on salty foods, e.g. crisps, nuts, salted meats (e.g. bacon and gammon), salty sauces (e.g. soy sauce) and stock cubes.
Use less salt in cooking and don’t add salt to food (2 teaspoonfuls equals 9 grams!). Try flavouring foods with herbs and spices.
Read food labels. Approximately 75 to 80% of our salt intake comes from salt added to processed foods. Choose foods with lower sodium levels.