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Saudi Journal of Kidney Diseases and Transplantation
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Year : 1998  |  Volume : 9  |  Issue : 1  |  Page : 1-3
Salt and Hypertension in Man


Consultant Nephrologist, Bangalore Hospital, Bangalore, India

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How to cite this article:
Ramprasad K S. Salt and Hypertension in Man. Saudi J Kidney Dis Transpl 1998;9:1-3

How to cite this URL:
Ramprasad K S. Salt and Hypertension in Man. Saudi J Kidney Dis Transpl [serial online] 1998 [cited 2019 Dec 13];9:1-3. Available from: http://www.sjkdt.org/text.asp?1998/9/1/1/39292

   History and Introduction Top


Salt has fascinated man from time immemorial. It has been referred to in ancient Arabic literature in the proverb "Salt in not worm-eaten" signifying its imperishable nature. Its importance in day to day life could be judged when a sixth century King said "one can do without gold but not without salt". It was believed that salt was essential to promote good health and the Latin works salus and salubris, which mean health and healthy are actually driven from sal (salt). Also, in ancient Rome, soldiers were paid in the form of salarium (salt money) from which today's "salary" is derived. Despite all these, the ancient Chinese in their wisdom noted in 3000 B.C., "if too much salt is used in food, the pulse hardens; tears make their appearance and the complexion changes". Today after nearly three millennia, we are still arguing over this statement.


   Relation of Salt and Hypertension Top


Numerous reports, based on epidemiologic and demographic data, have pointed to the relationship between sodium intake and hypertension [1] . It seems likely that increased salt intake may be contributory but not sufficient cause for hypertension. Also, there is considerable heterogeneity of blood pressure response to salt intake [1] . Certain features such as African-American race and older age have been shown to be associated with increased salt sensitivity [1] . It has been shown that the hypertensive response to salt restriction is most prominent in patients with advanced disease [2] . Thus, the decline in blood pressure with salt restriction is small in patients with mild hypertension [3],[4],[5] and the full hypertensive response requires five weeks or more, and the factors responsible for the delay are not fully understood [2] .

The Intersalt Study [6] was conducted to investigate the correlation of blood pressure to sodium excretion in 52-countries world­wide. Based on the results of this study it was estimated that the systolic blood pressure should decline by 3.1 mm Hg in the general population for a decrease of 100 mmol of dietary salt. However, other studies such as the Scottish Health Study [7] failed to confirm this relationship. Many previous studies have also reported that dietary salt loading elevates blood pressure in some "salt sensitive" but all subjects [8],[9] . However, De la Sieraa et al have demonstrated with 24 hours monitoring of blood pressure that the "salt resistance" individuals had higher sleeping blood pressures [10] .


   Mechanism of Salt Sensitivity Top


The exact mechanism of salt sensitive hypertension is incompletely understood. There is evidence that hypertension results from a defect in the salt excretory capacity of the kidney [11],[12] . Genetic factors such as alterations in structure or regulation of sodium transport in the nephron have been proposed [13] . Recently, it has been proposed that hypertension consists of two phases [14] . The first phase, which is mediated by a hyperactive sympathetic and rennin-angiotensin systems, results in elevation in blood pressure which is episodic. Elevation of blood pressure in such individuals may be viewed as physiologically appropriate, since the increase in renal perfusion pressure enhances sodium excretion and restores normovolemia. However, these repeated increases in blood pressure causes structural and functional adaptive responses in the kidney blunting its ability to excrete salt. This acquired defect in excreting salt results in salt-sensitive, persistent hypertension [14] .

Other factors that may contribute to salt sensitivity in hypertension include:

a) Selective activation of intra-renal angiotensin system leading to renal vasoconstriction and increased sodium reabsorption [15] .

b) Increases activity of the sodium-hydrogen exchanger which mediates a large portion of sodium reabsoption in the proximal tubule [16] .

c) Lack of occurrence of reduced sympathetic activity after salt intake [17] . This may partly be due to an outbain-like hormone released in the brain following high salt intake which may raise the blood pressure directly by increasing sympathetic outflow [18] .

d) Salt retention leads to release of a natriuretic hormone which causes inhibition of the Na-K-ATPase pump resulting in increases cell sodium concentration which in turn, diminishes passive sodium entry into the cells. This reduces the activity of sodium-calcium exchanger in the cell membrane resulting in an increase in intracellular calcium concentration. These changes in the vascular smooth muscle result in vasoconstriction and hypertension. Also, a high salt intake increases the excretion of calcium and the negative calcium balance thus created, may cause increased intra cellular calcium and vasoconstriction. This response is probably mediated by enhanced releases of parathyroid hormone and calcitriol [19] . This hypothesis is supported by the observation that a high calcium intake can minimize the salt­induced raise in blood pressure [20] .

e) Chloride also appears to be important since sodium when given alone or with another anion such as citrate does not result in value expansion and hypertension [21],[22] .

The importance of salt restriction has come into focus in recent years due to its potential adverse effects including the demonstration that it can cause an elevation of LDL and total cholesterol levels [23] . Also, an association between low salt intake and myocardial infarction has been reported [24] . This may be because of the compensatory systems set into play by reducing salt intake, including increased rennin-angiotensin and sympathetic nervous system activity.

In conclusion, the relationship between salt intake and blood pressure is complex and susceptibility to salt induced hypertension in humans is variable. Although current studies indicate the need for modest restriction more objective studies on larger number of patients are needed to clarify the issue.

 
   References Top

1.Weinberger MH. Salt sensitivity of blood pressure in humans. Hypertension 1996;27:481-90.  Back to cited text no. 1  [PUBMED]  [FULLTEXT]
2.Law Mr, Frost CD, Wald NJ. By how much does dietary salt reduction lower blood pressure? I-analysis of observational data among populations; III-Analysis of data from trials of salt reduction. BMJ 1991;302:811-24.  Back to cited text no. 2    
3.Australian National Health and Mediacla Research council Dietary Salt Management committee. Fall in blood pressure with modest reduction in dietary salt intake in mild hypertension. Lancet 1989;1:399-402.  Back to cited text no. 3    
4.Cutler JA, follmann D, Elliott P, Such I. An overview of randomized trials of sodium reduction and blood pressure. Hypertension 1991;17(suppl):127-33.  Back to cited text no. 4    
5.Macgregor GA, Markandu ND, Sangnella GA, singer DR, Cappuccio FP, double-blind study of three socium intakes and long-term effects of sodium restriction in essential hypertension. Lancet 1989;2:1244-7.  Back to cited text no. 5    
6.Intersalt cooperative Research Group. An international study of electrolyte excretion and blood pressure. Results for 24 hours urinary sodium and potassium excretion. BMJ 1988;297:319-28.  Back to cited text no. 6    
7.Smith WC, Cromgbie IK, tavendale RT, Gulland SK, Tunstall-Pedoe HD. Urinary electrolyte excretion, alcohol eonsumption, and blood pressure in the Scottish heart health study. BMJ 1988;297-30.  Back to cited text no. 7    
8.Egan BM, Stepnialkowski K, Nazzaro P. Insulin levels are similar in obese salt-sensitive and salt-resistant hypertensive subjects. Hypertension 1994;23(Suppl):11-17.  Back to cited text no. 8    
9.Zoccali C, Mallamaci F, Parlongo S. The influence of salt intake on plasma calcitonin gene related peptide in subjects with mild essential hypertension. J Hypertens 1994;12:1249-53.  Back to cited text no. 9  [PUBMED]  
10.De la Sierra A, Lluch MM, coca A, et al. Assessment of salt sensitivity in essential hypertension by 24 h ambulatory blood pressure monitoring. Am J Hypertens 1995;8:970-7.  Back to cited text no. 10  [PUBMED]  
11.Cowley AW Jr, Roman RJ. The role of the kidney in hypertension. JAMA 1996;275:1581-9.  Back to cited text no. 11  [PUBMED]  
12.De Wardener HE. The primary role of th ekdiney and salt intake in the etiology of essential hypetension: Part 1. clin Sci Colch 1990;79:193-200.  Back to cited text no. 12  [PUBMED]  
13.Lifton RP. Molecular genetics of human blood pressure variation. Science 1996;272:676-80.  Back to cited text no. 13  [PUBMED]  [FULLTEXT]
14.Johnson RJ, Schreiner GF. Hypothesis: The role of acquired tubulointerstitial disease in the pathogenesis of salt­dependent hypertension. Kidney Int 1997;52:1169-79.  Back to cited text no. 14  [PUBMED]  
15.Redgrave J, Rabinowe S, Hollenberg NK, Williams GH, correction of abnormal renal blood flow response to angiotensin II by converting enzyme inhibition in essential hypertension. J Clin Invest 1985;75:1285-90.  Back to cited text no. 15    
16.Siffert W, Dusing R. Sodium-proton exchange and primary hypertension. An update. Hypertension 1995;26:649-55.  Back to cited text no. 16    
17.Campese VM, Romoff MS, Levitan D, et al. Abnormal relationship between sodium intake and sympathetic nervous system activity in salt-sensitive patients with essential hypertension. Kidney Int 1982;21:371.  Back to cited text no. 17  [PUBMED]  
18.Huang Bs, Leenen FH. Brain "oubain" mediates the sympathoexcitatory and hypertensive effects of high sodium intake in Dahl salt-sensitive rats. Circ Res 1994;74:586-95.  Back to cited text no. 18  [PUBMED]  
19.Sowers JR, Zemel MB, Zemel PC, Standley PR. Calcium metabolism and dietary calcium in salt sensitive hypertension. Am J Hypertens 1991;4:557-63.  Back to cited text no. 19  [PUBMED]  
20.Rich GM, McCullough M, Olmedo A, Malarick C, More TJ. Blood pressure and renal blood flow responses to dietary calcium and sodium intake in human. Am J Hypertens 1991;4(11):642S-5S.  Back to cited text no. 20    
21.Kurtz TW, Al-Bandar HA, Morris RC Jr. "Salt sensitive" essential hypertension in men. Is the sodium ion alone important? N Engl J Med 1987;317:1043-8.  Back to cited text no. 21    
22.Boegehold MA, Kotchen TA, Relative contributions of Na+ and Cl- to salt­sensitive hypertension. Hypertension 1989;14:579-83.  Back to cited text no. 22    
23.Sharma AM, Arntz HR, Kribben A, Schatenfroh S, Distler A. dietary sodium restriction: adverse effect on plasma lipids. Klin Wochenschr 1990;68:664-8.  Back to cited text no. 23    
24.Alderman MH, Madhavan S, Cohen H, Sealey JE, Laragh JH. Low urinary sodium is associated with greater risk of myocardial infarction among treated hypertensive men. Hypertension 1995;25:1144-52.  Back to cited text no. 24  [PUBMED]  [FULLTEXT]

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Correspondence Address:
K S Ramprasad
Consultant Nephrologist, Bangalore Hospital, R.V. Road, Bangalore
India
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PMID: 18408273

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