Answer: When we look at our modern way of life, we must admit we have come a long way. Yet, while we have developed a culture based on fast foods and supermarkets, our kidneys have not really changed. They were originally designed to enable ancient ancestors to concentrate urine in order to become more mobile on land, i.e. to wander away from the lake or pond to forage for food. The kidneys also adapted to climate - becoming more efficient at retaining sodium (a component of salt) in hot, dry climates. (1) In summary, the kidney was designed to balance the salt intake naturally occurring in foods with body needs. Travel throughout Africa and visit the archeological sites where Robert Leaky did his excavations. (2) You will never find a salt shaker there!

 Why salt?

Refrigeration is a novel convenience. Historically, food was salted to preserve it. Fish not salted would spoil before it was brought to market. Milk was always a vital nutritive staple, and the making of cheese, the process to preserve it, requires salt. Hence, prior to refrigeration, salt was a necessary commodity enabling agriculture to develop. (3)

 The physiology of salt:

Eating as we do, many people accumulate more salt and water than their kidneys can handle. Some people have genes that control cellular channels, enzymes and hormones at various sites in the kidney, conserving salt to enable adaptation to the hot, dry savannah. (4,5) In order to remain active, one had to control body temperature. If water and salt were scarce, the kidney would conserve salt to retain fluid used to coat the body with sweat during activity. As sweat evaporated from the skin, it would cool the skin and keep body temperature normal. Without sweating, the body would quickly overheat during activity. However, those genes necessary in our early development mistakenly conserve salt regardless of the environment. As long as excessive salt is ingested, it will be disproportionately reabsorbed in about 20 percent of the population. (6) Through a process known as osmosis, salt retains water. It also promotes thirst, as every bartender and movie theater proprietor knows. Excessive salt keeps the circulatory volume higher than it should be, exerting excess fluid pressure on blood vessel walls. These walls react to this stress by thickening and narrowing, leaving less space for the fluid already cramped in the blood compartment, raising “resistance” and requiring higher pressure to move blood to the organs. The heart has to pump against this high pressure system. Lifting free weights in the athletic center causes muscles to become harder and larger. This same phenomenon happens to the heart with one notable exception, there is no break. This 24/7 activity can cause the heart to enlarge dramatically, and dangerously. The kidney contains around one million tiny, delicate filters comprised of blood vessels. The increase in pressure transmitted to the kidneys damages its vascular system (7) leading to a disorder known as “hypertensive nephrosclerosis,” a major cause of kidney disease.

 Measuring Blood Pressure

Blood pressure measurements consist of two numbers, the top or systolic reflects pressure developed by the heart when it pumps. In diastole, the large chamber of the heart expands, resting a fraction of a second to fill with blood. The “diastolic” pressure at rest is a function of resistance, analogous to the pressure that still remains in a garden hose when shutting off the water valve.

Essential Hypertension

This genetic disorder, essential hypertension, is present in approximately 65 million Americans, and while characterized by measurements over 140/90, cardiovascular risks are already associated with blood pressures greater than 115/75. (8) It is controlled by restricting salt or using a diuretic, and more common in those whose ancestral origins are equatorial. Clinical trials reveal that diuretics (9) are the most successful medications controlling hypertension. Frequently, additional medications are necessary to achieve control. (10,11) We consider a normal blood pressure to be 130/80 for patients with CKD. (12) But, studies such as the large MDRD trial, suggest lowering the pressure to 125/75 when proteinuria is present. (13) A healthy diet with salt reduction to roughly less than five grams (100 mmols sodium) substantially lowers blood pressure. (14) Tribes in remote areas that ingest little salt in their diet have lower blood pressures. Around the world, lower salt usage is associated with lower pressure. (15,16) Restricting salt is difficult because it is as much a preservative as a flavoring condiment.

Adding salt preserves shelf life, making economic sense for the food industry, even if less healthy. Longer shelf life increases the likelihood it will be sold before it spoils. Discarded, spoiled food is a liability for the grocer, and to compete for the grocer’s business, food packagers extend the shelf life of their products with salt. The alternative, refrigeration, is a more costly but healthier choice. We hardly live in a salt-free culture, and must therefore read labels, and cut our salt intake to less than five grams per day. In addition, measuring blood pressure regularly and using medications when necessary are vital to reducing the incidence of CKD. (17)

References:

1. Young JH, Chang Y-PC, Kim JD-O, et al: Differential susceptibility to hypertension is due to selection during the out-of-Africa expansion. PLoS Genet 1:2005

2. Leakey LS, Tobias PV and Napier JR: A NEW SPECIES OF THE GENUS HOMO FROM OLDUVAI GORGE. Nature 202:7-9, 1964

3. Kurlansky M: Salt. A World History Penguin Group: New York, 2002

4. Rosskopf D, Schurks M, Rimmbach C, et al: Genetics of arterial hypertension and hypotension. Naunyn Schmiedebergs Arch Pharmacol 2007

5. Blaustein MP, Zhang J, Chen L, et al: How does salt retention raise blood pressure? Am J Physiol Regul Integr Comp Physiol 290:514-523, 2006

6. Haddy FJ: Role of dietary salt in hypertension. Life Sci 79:1585-1592, 2006

7. Cushman WC: The burden of uncontrolled hypertension: morbidity and mortality associated with disease progression. J Clin Hypertens (Greenwich) 5:14- 22, 2003

8. Chobanian AV, Bakris GL, Black HR, et al: The Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure: the JNC 7 report. JAMA 289:2560-2572, 2003

9. Major outcomes in high-risk hypertensive patients randomized to angiotensin-converting enzyme inhibitor or calcium channel blocker vs diuretic: The Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT). JAMA 288:2981- 2997, 2002

10. Wright JT, Bakris G, Greene T, et al: Effect of blood pressure lowering and antihypertensive drug class on progression of hypertensive kidney disease: results from the AASK trial. JAMA 288:2421-2431, 2002

11. Calhoun DA: Resistant or difficult-to-treat hypertension. J Clin Hypertens (Greenwich) 8:181-186, 2006

12. K/DOQI clinical practice guidelines on hypertension and antihypertensive agents in chronic kidney disease. Am J Kidney Dis 43:1-290, 2004

13. Hebert LA: Target blood pressure for antihypertensive therapy in patients with proteinuric renal disease. Curr Hypertens Rep 1:454-460, 1999

14. Sacks FM, Svetkey LP, Vollmer WM, et al: Effects on blood pressure of reduced dietary sodium and the Dietary Approaches to Stop Hypertension (DASH) diet. DASH-Sodium Collaborative Research Group. N Engl J Med 344:3-10, 2001

15. Mancilha-Carvalho JdJ and Souza e Silva NA: The Yanomami Indians in the INTERSALT Study. Arq Bras Cardiol 80:289-300, 2003

16. Elliott P, Stamler J, Nichols R, et al: Intersalt revisited: further analyses of 24 hour sodium excretion and blood pressure within and across populations. Intersalt Cooperative Research Group. BMJ 312:1249-1253, 1996

17. Flack JM, Peters R, Shafi T, et al: Prevention of hypertension and its complications: theoretical basis and guidelines for treatment. J Am Soc Nephrol 14:S92-98, 2003

 Stephen Z. Fadem, MD, FASN, FACP, serves as a member of the AAKP Medical Advisory Board and a Vice President of the AAKP Board of Directors. Dr. Fadem is a practicing nephrologist in Houston.

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