Studies over the years have suggested high intakes of salt play a role in the development of high blood pressure so dietary advice for the prevention and lowering of blood pressure has focused primarily on reducing sodium or salt intake. A 1989 study looked at the response an intake of 3-12 g of salt per day had on blood pressure. The study found that modest reductions in salt, 5-6 g salt per day caused blood pressures to fall in hypertensives. The best effect was seen with only 3 g of salt per day with blood pressure falls of 11 mmHg systolic and 6 mmHg diastolic. More recently, the use of low salt diets for the prevention or treatment of high blood pressure has come into question. The Trials of Hypertension Prevention Phase II in 1997 indicated that energy intake and weight loss were more important than the restriction of dietary salt in the prevention of hypertension. A 2006 Cochrane review, which looked at the effect of longer-term modest salt reduction on blood pressure, found that modest reductions in salt intake could have a significant effect on blood pressure in those with high blood pressure, but a lesser effect on those without. It agreed that the 2007 public health recommendations of reducing salt intake from levels of 9-12 g/day to a moderate 5-6 g/day would have a beneficial effect on blood pressure and cardiovascular disease.
The effectiveness of the DASH diet for lowering blood pressure is well recognized. The 2005 Dietary Guidelines for Americans recommends the DASH Eating Plan as an example of a balanced eating plan consistent with the existing guidelines and it forms the basis for the USDA MyPyramid. DASH is also recommended in other guidelines such as those advocated by the British Nutrition Foundation, American Heart Association, and American Society for Hypertension.
Although results of the study indicated that reducing sodium and increasing potassium, calcium, and magnesium intakes play a key role on lowering blood pressure, the reasons why the DASH eating plan or the DASH-Sodium had a beneficial affect remains uncertain. The researchers suggest it may be because whole foods improve the absorption of the potassium, calcium and magnesium or it may be related to the cumulative effect of eating these nutrients together than the individual nutrients themselves. It is also speculated that it may be something else in the fruit, vegetables, and low-fat dairy products that accounts for the association between the diet and blood pressure.
The Salt Institute supports the DASH diet, but without the salt restriction. They claim that the DASH diet alone, without reduced sodium intake from manufactured foods, would achieve the desired blood pressure reduction. Their recommendation is based on the fact that there are no evidence-based studies supporting the need for dietary salt restriction for the entire population. The Cochrane review in 2006 showed that modest reductions in salt intake lowers blood pressure significantly in hypertensives, but a lesser effect on individuals with normal blood pressure. Restriction of salt for those with out hypertension is not recommended.
There is continued call for the food industry to lower their use of salt in processed foods from governments and health associations. These groups claim if the reduction of intake to 6 g salt/day is achieved by gradual reduction of salt content in manufactured foods, those with high blood pressure would gain significant health benefit, but nobody’s health would be adversely affected. In 2003, the UK Department of Health and Foods Standards Agency, several leading supermarkets and food manufacturers set a target for Page 251 an average salt reduction of 32% on 48 food categories. In June 2006, the American Medical Association (AMA) appealed for a minimum 50% reduction in the amount of sodium in processed foods, fast food products, and restaurant meals to be achieved over the next ten years.
Researchers have evaluated other dietary modifications, such as the role of potassium, magnesium, and calcium on blood pressure. Substantial evidence shows individuals with diets high in fruits and vegetables and, hence, potassium, magnesium, and calcium, such as vegetarians, tend to have lower blood pressures. However, in studies where individuals have been supplemented with these nutrients, the results on their effects on blood pressure have been inconclusive.
There is some debate on whether patients can follow the diet long-term. The 2003 premier study (a multi-center trial), which included the DASH diet when looking at the effect of diet on blood pressure, found that the DASH diet results were less than the original study. This difference is thought to be because in the DASH study participants were supplied with prepared meals, while participants on the premier study prepared their own foods. As a result, only half the fruit and vegetable intake was achieved in the premier study, which affected the overall intakes of potassium and magnesium. The researches concluded that compliance to the DASH diet in the long term is questionable, but agreed that patients should still be encouraged to adopt healthy interventions such as the DASH diet, as it does offer health benefits.
In terms of heart health, the Dash diet lowered total cholesterol and LDL cholesterol, but it was associated with a decrease in high-density lipoprotein (HDL), the “good” cholesterol. Low HDL levels are considered a risk factor for coronary heart disease (CHD) while high levels are thought to be protective of heart disease. The decrease was greatest in individuals who started with a higher level of the protective HDL. Researchers agree that the reasons for the decrease in HDL levels needs further review, but concluded that the overall effects of the DASH diet are beneficial to heart disease.
While long term health effects of the DASH diet are yet to be established, the diet closely resembles the Mediterranean diet, which has been shown to have other health benefits including a reduced risk for heart disease and cancer rates. It is thought that the DASH diet is likely to offer similar health benefits.
