Beyond blood pressure

There are two types of review articles. In one type of review article, the author(s) search for all of the research that's been done into a particular topic, then pool those results together, yielding stronger results. (Here's an example.)

Another type of review allows the authors to report on the general state of the research - identifying the most common conclusions but also noting the quality research that may contradict those conclusions. This is especially useful when the subject in question is still not well understood. The research into sodium and blood pressure is just such a subject, leading researchers at the University of Delaware to write today's review article (JACC 2015;65(10):1042-50). The research articles they reviewed focus on several aspects of sodium and the link to high blood pressure. Here are their conclusions:

Salt Sensitivity

Those patients whose blood pressure decreases when they reduce the amount of sodium in their diet (or increases with more sodium) are known as "Salt Sensitive," while those whose blood pressure does not change are known as "Salt Resistant." Those people with normal blood pressures who can be identified as Salt Sensitive appear to be more likely to develop high blood pressure later and have a higher overall risk of death from all causes, but the total number of people whose reactions are significant are very small.

Why Sodium Increases Blood Pressure

Research to understand just how sodium causes blood pressure to change is ongoing, but recent research shows that it includes kidney function; hormones that control the body's fluid balance; the heart, veins, and the heart's output; and genetic mechanisms.

Effects of Sodium on Organs

Very interesting research suggests that high levels of salt in the diet can still negatively impact various organs even when the patient does not have high blood pressure.

Arteries: Higher levels of dietary sodium actually stiffens the arteries, which impairs their ability to dilate or constrict, and damages the lining of the blood vessels (the endothelium), which among other functions helps manage the body's fluid balance.

Heart: High sodium intake can thicken the wall of the left ventricle of the heart, which is responsible for pumping blood to the rest of the body (the right ventricle pumps blood to the lungs). While this thickening is normal for those who exercise regularly or are pregnant, because they need to pump a higher volume of blood, in most people this increased mass is scar tissue, not healthy heart muscle.

Brain: Research in rats suggests that high salt in the diet makes brainstem nuclei react more strongly to various stimuli, leading to a greater variation in blood pressures even though the average blood pressure may not increase.

Salt Intake and Blood Pressure

The authors acknowledge that the ideal would be to perform a long-term, randomized controlled trial comparing different levels of salt in the diet. That's not really feasible, however - imagine trying to get thousands of people to stick to exact amounts of salt intake in their diet for multiple (dozens, perhaps) years. Meta-analyses of smaller studies as well as systematic reviews show "a strong positive association" between sodium intake and systolic blood pressure (the top number in a blood pressure reading).

Cardiovascular Disease and Illness or Death

Most of the research that analyzes the association between dietary sodium and cardiovascular disease are observational studies, which can only show that one thing is related to another - not that one thing causes another. It's also very difficult to measure dietary sodium intake very accurately over the long term, and many studies are short-lived for that reason. That said, the research looking at the opposite - that very low sodium is associated with heart disease, illness, or death - is unclear and mostly speculative.

What this means for you

The preponderance of evidence lands on the side of keeping your sodium intake within the Institute of Medicine's recommended limits, even if you do not have diagnosed high blood pressure. Here are three easy steps to reducing the sodium in your diet.

First posted: March 18, 2015