What the trials actually show about the DASH diet and blood pressure

The DASH diet — a pattern rich in fruit, vegetables and low-fat dairy products, with reduced saturated and total fat — lowers blood pressure, and the evidence for that comes from controlled-feeding trials rather than survey data [s1]. The effect is real but graded: it is largest in people who already have high blood pressure, smaller in those who do not, and larger again when sodium is cut at the same time [s1][s2].

The founding trial

The Dietary Approaches to Stop Hypertension trial, published in 1997, enrolled 459 adults with systolic pressures below 160 mm Hg and diastolic pressures of 80 to 95 mm Hg [s1]. For three weeks everyone ate a control diet low in fruit, vegetables and dairy, with a fat content typical of the average American diet [s1]. Participants were then randomly assigned for eight weeks to continue that control diet, to a diet rich in fruit and vegetables, or to a "combination" diet rich in fruit, vegetables and low-fat dairy with reduced saturated and total fat — the pattern now called DASH [s1].

The design's strength is what it held constant. Sodium intake and body weight were kept at fixed levels across all three groups [s1], so the blood-pressure changes could be attributed to the dietary pattern itself rather than to salt reduction or weight loss — the two things that usually confound diet-and-blood-pressure studies.

What it found

Against the control diet, the combination diet reduced systolic blood pressure by 5.5 mm Hg and diastolic by 3.0 mm Hg more, both highly significant [s1]. The fruit-and-vegetables diet did less: 2.8 mm Hg systolic and 1.1 mm Hg diastolic beyond control, the diastolic change not reaching significance [s1]. Adding low-fat dairy and cutting fat, in other words, accounted for much of the effect.

The averages conceal the more useful finding. Among the 133 participants who had hypertension at baseline, the combination diet lowered systolic and diastolic pressure by 11.4 and 5.5 mm Hg more than control [s1]. Among the 326 without hypertension, the corresponding reductions were 3.5 and 2.1 mm Hg [s1]. The people with the most to gain gained the most — a pattern that recurs across blood-pressure interventions.

An 11.4 mm Hg systolic reduction is in the range of what a single blood-pressure drug delivers, achieved here by diet alone in a controlled setting. That last qualifier matters: participants were fed prepared meals, not left to shop and cook, which is the standing gap between a feeding trial and everyday life.

Adding salt to the picture

The 1997 trial deliberately fixed sodium, leaving open how DASH and salt reduction interact. The DASH-Sodium trial, published in 2001, answered that. It randomly assigned 412 participants to either a control diet or the DASH diet, and within each diet had them eat high, intermediate and low levels of sodium for 30 days each, in random order [s2].

Cutting sodium helped on both diets, but more on the control diet. Reducing sodium from the high to the intermediate level lowered systolic pressure by 2.1 mm Hg on the control diet and 1.3 mm Hg on DASH; going from intermediate to low added a further 4.6 mm Hg on control and 1.7 mm Hg on DASH [s2]. The two levers overlap: a diet already high in potassium-rich produce leaves less room for salt reduction to work.

Combined, they were most powerful. Compared with the control diet at a high sodium level, the DASH diet at a low sodium level produced a systolic pressure 7.1 mm Hg lower in participants without hypertension and 11.5 mm Hg lower in those with it [s2]. The effects held across men and women and across Black participants and those of other races [s2].

The honest limits

Both trials were short — eight weeks and roughly three months of feeding — and both provided the food. That answers whether the diet works, not whether people can sustain it. The DASH-Sodium authors said as much: long-term benefit depends on people's ability to make lasting dietary changes and on the availability of lower-sodium foods [s2]. Neither trial measured hard outcomes such as heart attacks or strokes; both used blood pressure as a stand-in, on the well-supported assumption that lowering it reduces cardiovascular risk.

The pattern shares its logic with other tested diets. The Mediterranean pattern has the largest cardiovascular-outcome trial behind it, and the MIND diet, built partly on DASH, has been tested for cognition. DASH's specific claim is narrower and better nailed down than most: fed to people in a trial, it lowers blood pressure, most in those who need it most.

This is not medical advice. Blood pressure and its management are matters for a clinician who knows the individual's history.

Sources

• A Clinical Trial of the Effects of Dietary Patterns on Blood Pressure — New England Journal of Medicine, 1997-04-17
• Effects on Blood Pressure of Reduced Dietary Sodium and the Dietary Approaches to Stop Hypertension (DASH) Diet — New England Journal of Medicine, 2001-01-04