Sunday, December 9, 2012

Salt and Blood Pressure

One of the common diet recommendations is that we should reduce our salt (sodium chloride) intake.  The rationale is that lowering salt reduces water retention, which will lower blood volume, therefore lower blood pressure. 

However, blood levels of sodium and blood pressure are some of the most tightly regulated things in the body.  There are many negative feedback mechanisms from the brain, heart, blood vessels and kidneys to maintain blood pressure and sodium levels within a normal range.  For example, on a standard blood test the recommended range for sodium is 135-145 mmol/L, which is only a deviation of 3.6% either way from 140. 

Despite salt being portrayed as the main driver of hypertension, meta-analyses of randomised controlled trials that have reduced salt have only found very mild reductions in blood pressure (see tables below), which suggests dietary sodium doesn’t cause hypertension and is consistent with the negative feedback systems.

[1]
Systolic
Diastolic
Mainly Caucasians – High Blood Pressure
-4.18
-1.98
Mainly Caucasians – Normal Blood Pressure
-1.27
-0.54
African Americans – Normal and High Blood Pressure
-6.44
-1.98

[2]
Systolic
Diastolic
Hypertension
-3.70
-0.90
Normal Blood Pressure
-1.00
-0.10

[3]
Systolic
Diastolic
High Blood Pressure
-3.90
-1.90
Normal Blood Pressure
-1.20
-0.26

[4]
Systolic
Diastolic
Normal and High Blood Pressure
-1.10
-0.60

[5]
Systolic
Diastolic
High Blood Pressure
-4.90
-2.60
Normal Blood Pressure
-1.70
-0.70

[6]
Systolic
Diastolic
Children
-1.17
-1.29
Infants
-2.47

Reducing sodium is not only ineffective for hypertension it also has some undesirable effects.  The sodium reduction in these trials roughly tripled renin and aldosterone levels, increased noradrenaline (norepinephrine) by 30% and adrenaline (epinephrine) by 12% [1] [3].  Renin and aldosterone are part of the renin-angiotensin-aldosterone system (RAAS) and noradrenaline and adrenaline are part of the sympathetic nervous system (fight or flight response).  The RAAS and noradrenaline retain sodium and water.  Reducing sodium can lead to an increase in insulin resistance through noradrenaline [7] and an increase in oxidative stress though the RAAS [8].  It seems that low sodium intakes invoke somewhat of a stress response. 

* Despite being considered ‘heart healthy’, reducing sodium increases total cholesterol by 3.0%, LDL-C by 4.6% and triglycerides by 5.9% [1] 

** A reason why sodium levels are thought to be more important than potassium in blood pressure is because there is much more sodium in the fluid outside of cells (extracellular fluid, ECF) and more potassium in the fluid inside of cells (intracellular fluid, ICF).  Blood is extracellular fluid so changes in sodium would have more effect on blood volume and blood pressure than potassium.  For example on a blood test the recommended range for potassium is 3.7-5.3 mmol/L, whereas sodium is 135-145 mmol/L.

*** Salt-resistance is the norm and means that blood pressure stays relativity constant despite variations in dietary salt, within reason.  Salt-sensitivity is the opposite and refers to blood pressure being sensitive to the amount of salt in the diet.  Salt-sensitivity can occur when the RAAS is unresponsive to changes in salt intake due to the RAAS being too high or being blocked by ACE inhibitors [9].  However, several other systems involved in sodium balance may be impaired in salt-sensitivity [10]

Further Reading
(1) Shaking Up the Salt Myth 

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