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Changing diet substantially lowers blood pressure

 

The current average intake of sodium in the UK is 3.2 g, equivalent to about 8 g of salt, which can lead to raised blood pressure. The recommended daily amount of sodium (in the UK) is 1.6 g/ 70 mmol (equivalent to about 4 g of salt).

In the US, the DASH (Dietary Approaches to Stop Hypertension) Study found that a diet that emphasises fruit, vegetables, low-fat dairy foods and wholegrain products and reduces the amount of red meat, fats and sweets lowers blood pressure, compared with a typical US diet.

This trial studies the effect of reducing sodium intake in combination with the DASH diet on blood pressure in adults with and without hypertension.

Bottom line

The combination of lowering sodium intake with the DASH diet produces greater reductions in blood pressure than either the diet or reducing sodium alone. This combination is worth recommending for both the prevention and treatment of high blood pressure.

Reference

FM Sacks et al. Effects on blood pressure of reduced dietary sodium and the dietary approaches to stop hypertension (DASH) diet. The New England Journal of Medicine 2001 344: 3-10.

Study

Participants were 412 adults who were at least 22 years of age and whose blood pressure exceeded 120/80 mm Hg. Exclusion criteria were heart disease, renal insufficiency, poorly controlled hyperlipidemia or diabetes mellitus, diabetes requiring insulin, special dietary requirements, intake of more than 14 alcoholic drinks per week or the use of antihypertensive drugs or other medications that would affect blood pressure or nutrient metabolism.

This was a randomised trial comparing three levels of sodium intake in two diets. The three sodium levels were defined as high (a target of 150 mmol per day), intermediate (a target of 100 mmol per day) and low ( a target of 50 mmol per day). The two diets were a control diet (typical of that consumed in the US) and the DASH diet. Specific diets were designed to achieve the high, intermediate and low levels of sodium in both the control and the DASH diets and participants were provided with all of their food, including snacks.

During a two-week run-in period, participants ate the high-sodium control diet. They were then randomly assigned to follow either the control diet or the DASH diet and ate their assigned diet at each of the three sodium levels for 30 consecutive days in random order. The outcome was blood pressure at the end of each 30-day period. Blood pressure was measured five times during the screening and run-in periods (the average of these measurements was baseline blood pressure) and eight times during each intervention period (the average of the last five measurements was used for the outcome blood pressure).

Participants and dietary staff were unaware of the outcome measurements and the personnel involved in the collection of the measurements were unaware of participants’ diet assignment. Participants’ adherence to the diets was assessed by reviewing their daily food diaries, having them eat their weekday lunches or dinners on site and measuring 24-hour urinary excretion of sodium, potassium, phosphorus and urea nitrogen.

Results

Ninety-five percent of participants assigned to the DASH diet (198 of 208) and ninety-four percent of those assigned to the control diet (192 of 204) completed the study. The average age of participants was 48 years with a baseline blood pressure of 135/86 mm Hg. Approximately 50% were black, 40% were white and about 50% were women. Average urinary sodium levels were 142 mmol per day during the high-sodium period, 107 mmol per day during the intermediate-sodium period and 65 mmol per day during the low-sodium period. Levels of urinary potassium, phosphorus, and urea nitrogen (reflective of intake of fruit and vegetables, dairy products and protein respectively) were higher in the DASH diet group than in the control group and were similar for all three sodium levels.

The reduction of sodium intake significantly lowered systolic and diastolic blood pressure with both the control diet and the DASH diet (shown in Figure 1). Compared with the control diet, the DASH diet resulted in a significantly lower systolic blood pressure at every sodium level and in a significantly lower diastolic blood pressure at the high and intermediate sodium levels. Compared with the high-sodium control diet, the low-sodium DASH diet produced greater reductions in systolic and diastolic blood pressure than either the DASH diet alone or a reduction in sodium alone.

Figure 1. The effect of reduced sodium intake and the DASH diet on systolic blood pressure.


Reducing sodium intake from the high to the low level (with either the control or the DASH diet) reduced systolic blood pressure in participants with and without hypertension, among blacks and those of other races, and in men and women. Compared with the control diet with a high sodium level, the DASH diet with a low sodium level lowered systolic blood pressure by 11.5 mm Hg in participants with hypertension (12.6 mm Hg for blacks; 9.5 mm Hg for others), by 7.1 mm Hg in participants without hypertension (7.2 mm Hg for blacks; 6.9 mm Hg for others) and by 6.8 mm Hg in men and 10.5 mm Hg in women.

Comment

This study was meticulously designed and carried out. Its results have important implications for both the prevention and treatment of hypertension.

  1. The combination of a low sodium intake and the DASH diet achieved the greatest effect on blood pressure. The DASH diet and the low sodium level were acceptable among participants with no increase in symptoms (which were monitored by questionnaires). However, the long-term benefits remain to be demonstrated and will largely depend on the ability of people to make long-lasting dietary changes.
  2. The DASH diet lowered blood pressure at high, intermediate and low sodium levels, demonstrating that the DASH diet is beneficial at all levels of sodium intake.
  3. A reduction in sodium lowered blood pressure in those with and without hypertension so it is also worthwhile for people who do not have high blood pressure to reduce their salt intake.

The DASH diet emphasises fruit, vegetables and low-fat dairy foods; includes whole grains, poultry, fish and nuts; contains smaller amounts of red meat, total and saturated fat, cholesterol and sugar; and larger amounts of potassium, calcium, magnesium, dietary fibre and protein, compared with a typical diet. As such it is not just a diet for people trying to prevent or treat high blood pressure. It is a healthy diet which everyone should be recommended to follow.

For more information about the DASH diet visit the DASH diet home page at http://dash.bwh.harvard.edu .