Dietary Salt Reduces Blood Flow
A high salt intake is known to be the primary cause of the rise in blood pressure in age seen in nearly everyone who lives in societies where large amounts of salt are added to the food. Data from the DASH trials have led some to believe that a Mediterranean-style diet may be helpful for the treatment and prevention of hypertension even if salt intake is not limited. However, it is well known that blood pressure rises with age in all Mediterranean countries just as it does in the United States and Asian countries. Over 90% of these populations develop hypertension sooner or later. Indeed, a recent study in Spain showed that adherence to a more Mediterranean-style diet does not prevent the development of hypertension.1The lower saturated fat and higher potassium, calcium and magnesium content of the DASH diet and a more Mediterranean-style diet may mitigate some of the damage to the cardiovascular system over time and possibly slow the development of hypertension and cardiovascular disease and reduce the risk of cardiovascular events. However, as the DASH Sodium Trial showed, reducing salt intake from about 3,400 to 1,500 mg of sodium per day makes all the difference. In addition to reducing saturated fat and increasing whole grains, beans, fruits, vegetables, and low-fat dairy products, the reduction in salt lowered blood pressure more than double the reduction from diet alone in just over 8 weeks.A recent observational study looked at the risk of stroke and heart disease in over two thousand participants 30-52 years of age followed for 1.5 to 3 years. The amount of sodium and potassium in their urine was measured several times during this period and researchers found a significant increased risk of stroke and heart attacks developing in those with a higher ration of sodium to potassium in their urine.2The adverse effects of excess salt on the cardiovascular system are not limited to its effects on blood pressure. A recent study by Australian researchers examined the impact of feeding a group of 29 overweight and obese subjects a typical Western diet or a diet with less salt but the same amount of saturated fat and potassium for two weeks. Urinary sodium was measured to determine compliance with the two diets. On the normal high-salt diet, sodium excretion was about 3,600 mg per day compared to only about 1,500mg per day on the low-salt diet. The subjects had normal blood pressure and all followed both diets for two weeks in a crossover design.The researchers assessed flow-mediated dilation (FMD) of the brachial artery on the high and low salt diets. Impaired FMD is generally due to dysfunction of the endothelial cells that line the arteries and is believed to be involved in the pathology of cardiovascular disease. The results of this study showed FMD was reduced by 45% on the high-salt diet compared to the low-salt diet.3 Importantly, this study found that the impaired FMD caused by increased salt intake was independent of its effects on blood pressure.The authors conclude “These findings suggest additional cardioprotective effects of salt reduction beyond blood pressure reduction.”Bottom Line:Excessive salt intake appears to interfere with normal artery function and impair blood flow long before it leads to the development of hypertension. Clearly waiting for the development of hypertension before restricting salt intake is ill advised.By James J. Kenney, PhD, RD, FACNReferences:1. Am J Clin Epidemiol 2009;169:339-462. Arch Intern Med 2009;169:32-403.3. Am J Clin Nutr 2009;89:485-90
