Hacker News Reader

Comment by ecopoesis | Hacker News Reader

The link between sodium and high blood pressure is not conclusive: http://www.todayifoundout.com/index.php/2013/02/why-does-sal...

View on HN

In short: your blood is mostly water. Your blood is surrounded by a selectively permeable membrane (your vascular tissue). Solutes increase osmotic pressure (see: biology 101). Sodium chloride (table salt) is highly soluble in water (see: chemistry). Therefore, sodium and chloride increase the osmolarity of your blood. Water passes through the semi-permeable membrane back into your blood to decrease osmotic pressure caused by the excess of sodium/chloride in your blood versus in the surrounding cells. Your blood pressure increases because the water increases the volume of your blood while the vascular tissue tries to contain and mediate this reaction (see: physics).

Blog posts are not a substitute for an education in biology, medicine, and science.

If anyone is going to nitpick about things in this post, please understand that I am striving to be understood. While I could put an asterisk after every sentence ("*except for...,") it would not be productive to do so. If anyone wishes for clarification, I would be glad to provide.

I agree with your underlying sentiment, but this blood volume hypothesis is not supported by evidence.

Instead, current research indicates that high blood pressure after excess sodium intake may be caused by activation of the sympathetic nervous system, secondary to increased sodium loading on ion channels.

I remember it was in a relatively recent paper. This mechanism might help explain why sodium hasn't consistently been shown to have an adverse effect in humans while at the same time still being strongly associated with an increased risk of cardiovascular disease. Come to think of it, it's also a good explanation for why cultures in temperate climates tend to start their day with a hearty (salty) meal ;)

I would also like to point out, since above a comment was made about glutamate containing "a shitload" of sodium, that the actual sodium content of MSG is only around 13% by weight, compared to table salt where it's nearly 40%.

This explanation, while straightfoward and common-sensical, ignores the possibility that the body has some ability to eliminate excess solutes and/or excess fluids. Certainly the case is quite good that "more fluid in circulatory system yields higher blood pressure."

Indeed, and this possibility is called the kidney. Young people with 'healthy' kidneys are able to eat and drink almost anything they want and are much more likely to have well-regulated blood pressure. However, as we age, our kidneys perform more poorly (a simple explanation as to why older people are more likely to take blood pressure medications than younger people). As our physiology weakens, our diet (and exercise, among other things) become more important to our physiological health. This, in conjunction with the hardening of our arteries due to age and atherosclerosis, means that our diet (particularly our sodium intake) becomes exponentially more important in our older age than our younger age.

If this sounds unlikely, understand that the kidney is the direct or indirect target of many medications for blood pressure. Diuretics such as hydrochlorothiazide and loop diuretics such as furosemide act very directly on the kidney. Drugs such as ACE inhibitors (e.g. lisinopril), ARBs (e.g. losaratan), and renin inhibitors (aliskiren) act on the renin-angiotensin-aldosterone system, which directly involves the kidneys in a different manner. (I am a pharmacist).

>Blog posts are not a substitute for an education in biology, medicine, and science.

Turns out that an education in biology, medicine and science in no substitute for actually doing research. Your explanation for why salt raises blood pressure and increases mortality was considered so obvious that it wasn't worth studying. When it was studied, it turned out not to be in any way true. (see: reality).

As a person with HBP who actively monitors it, I can tell you this much. Sodium intake definitely affects my blood pressure. It goes like this, the higher my intake the shittier I feel. I'm not saying sodium is the main or only cause of HBP, it seems to affect some more then others, and there may be other underlining causes. But to call it a myth is downright ridiculous.

The link between sodium and edema, water retention, and kidney stress is not inconclusive. Large doses of sodium clearly have some impact. The question is, is that impact sufficient to explain the symptoms of Chinese Restaurant Syndrome, not whether the average person needs to moderate sodium consumption to avoid hypertension.

Reply on news.ycombinator.com