SALT (SODIUM CHLORIDE)
The relationship between dietary salt consumption and elevated blood pressure is well
own. The prevalence of hypertension and its consequences is linearly related to dietary 3alt intake in societies throughout the world. Hypertension and its sequelae are virtually absent ~ societies in which habitual salt intake is <50 - 100 mmol/d. However, there are other differences between these groups and those that habitually consume larger amounts of salt. The "low-salt" societ-ies tend to be isolated, genetically homogeneous, physically fit, and consume increased amounts of potassium and calcium in the form of fresh fruits and vegetables. Increased salt intake is associated with societal "acculturation." This implies a crowded and sedentary lifestyle as well as many other behavioral factors that may affect blood pressure and cardiovascular risk.
In addition, the age-related increase in blood pressure is observed only in societies in which salt intake is high. Many of the elderly individuals in low-salt cultures have blood pressure levels that are no higher than those of young adults. Despite this convincing evidence, controversy still exists concerning the importance of salt in human blood pressure in general, in hypertension, and as a treatment modality. Without considering the various reasons for this controversy, suffice it to say that the magnitude of the effect of salt intake on blood pressure is diluted by the fact that there is substantial heterogeneity in the blood pressure responses of humans to alterations in salt intake. Numerous studies have demonstrated that salt-sensitive and salt-resistant individuals can be identified within both the hypertensive and normotensive populations.
Salt-sensitive subjects will demonstrate a decrease in blood pressure with dietary sodium reduction, usually to the level of 100 mmol/d (2.4 g/d). The human need for sodium is about 10 mmol/d (230 mg/d). Thus, the threshold for blood pressure responsiveness to a reduction in salt intake is many times higher than the physiological requirements. It is often difficult to differentiate between salt-sensitive and Ba1t-re~i~tant subjects without sophisticated research techniques. However, a trial of modest dietary salt restriction or diuretic administraticn should' identify those most likely to benefit from this dietary intervention. Moreover, there have been no adverse reports when a modest reduction in salt intake such as 80 - 100 mmol/d have been followed. Certain population groups have been reported to be more likely to be salt sensitive than others. Hypertensive individuals are more salt sensitive than those with normal blood pressure. Among hypertensive subjects, salt sensitivity of blood pressure is more frequent among African-Americans (75%) than Caucasians (50%) and increases with increasing age. The latter finding is also observed in the normotensive population, with the finding that significant salt sensitivity of blood pressure is not seen until the age decade of 60 yr or more.
Individuals with reduced renin responses to sodium and volume depletion, the so-called low-renin subjects, are more likely to be salt sensitive than those with brisk renin responses. In addition to a possible permissive effect of sluggish renin responses to salt sensitivity, a variety of substances have been reported to be involved in the pathophysiology of salt sensitivity of blood pressure.
An extensive scientific critique of the many studies that have been conducted in this area is beyond the scope of this chapter; however, note that the sympathetic nervous system, endothelin, insulin sensitivity, atrial natriuretic factor, alterations in renal hemodynamics, and leptin have all been implicated in the pathophysiology of salt, sensitivity. It remains to be determined which of these many factors are primary events and which are simply compensatory responses or epiphenomena. It has been shown that salt sensitivity requires the administration of sodium as the chloride salt and that other forms of sodium do not have the same pressor effect. However, this is a moot point because over 95% of the sodium found in foods is in the chloride form.
Moreover, most of the salt found in food is added in the preparation, processing, and preservation of food and only 15% is added as the discretionary form (as table salt). Thus, it is important for the food preparer as well as the patient to become familiar with identifying the salt content of foods at the grocery store and restaurant as well as in cooking. Another important recent finding related to salt and blood pressure is the observation that long-term follow-up of salt-sensitive normotensive subjects over a period of 10 yr or more demonstrated an eightfold greater rate of blood pressure increase compared with those who were initially salt-resistant. This finding supports the epidemiological observations relating the age-associated rise in blood pressure to increased salt intake.
