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Blood Pressure, Salt, and Potassium
We know that diets low in sodium (salt) help to lower blood pressure. What you may not know is that diets that are high in potassium, such as vegetarian diets and those high in fruits in vegetables, can also help reduce blood pressure.
Reducing Sodium Reduces Blood Pressure
I know I've mentioned this idea before, but it's worth saying again: medicine is not like math. Remember back in Algebra class where if A = B and B = C, then A = C? One of the reasons that we do research in as large a group of people as we can is because not all people's bodies react the same way to certain things. More people in a study is better because it's easier to see how most people will react.
DASH diet: more than high blood pressure?
A study of 45,306 men between the ages of 45 and 79 and without a history of cancer were followed for seven years by researchers in Sweden (Am J Clin Nutr 2006;83:667-73). The study assessed their level of dairy product intake and correlated the subjects' intake to the incidence of colorectal cancers of various types: colorectum, colon, proximal colon, distal colon, and rectum.
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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:
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.
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.
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.
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).
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.
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