Ever since our ancestors crawled out of the ocean we have had to maintain the salt water constancy of our milieu intérieur, our inside juices. This principal was first elucidated by Claude Bernard in the late 1800s. We do this by the mechanisms that keep the salt and water in our bodies the same as the salt and water of the ocean back then. We have written about salt and water many times in this column but a review article in this week’s New England Journal of Medicine sparks more about this ongoing saga and a new maybe breakthrough in the treatment of high blood pressure. (1)
Lately salt and water are part of the generalized anxiety. That is because we want to eat too much salt and it seems to be related to hypertension, the silent killer of way too many of us, and because of silliness about how we don’t drink enough water. It is also interesting to me because the parallels between hunger for salt and thirst for water may teach us some things about hunger for foods, a way more fraught kind of hunger.
OK so it’s complicated. We have to explain the following straightforward observations:
-Salt restriction compared to high salt intake raises blood pressure by 0.4 (mm Hg) in people without high blood pressure and 4 (ten times more but still not much) in people with high blood pressure.
-salt sensitivity, defined as a change in blood pressure of 10 or greater when salt balance is altered by a combination of diet and loop (strong) diuretics (“water pills”), only happens in people who get high blood pressure. Though that’s kind of a tricky definition to be able to figure out in the day to day practice of medicine.
-People who take a salt substitute that’s 25% potassium chloride rather than regular salt 100% sodium chloride have lower blood pressure and lower rates of stroke and major heart attacks and death from all causes.
-This benefit of the salt substitute is even more powerful in people who have congestive heart failure for whom salt is a much bigger problem.
-There is a frequently observed association between a very LOW salt intake and excess mortality. You have got to eat some salt.
-The sodium in the blood and the interstitial fluids (the fluid outside of the cells) is high and very low inside of the cells. This is a mirror image of the potassium in your body – high inside cells but low outside and in the blood.
-water in your body comes from drinking it and also about 12 ounces comes as a product of burning calories. Exercise increases substantially the amount of water you make burning calories. In hibernating bears who don’t drink anything, energy metabolism is the only way they can get any water.
-There is no evidence that drinking more water than your thirst compels is good for anything.
-It’s hard to sort out the a relationship between salt intake and water intake in people but that is probably because we always drink more than we need to.
How do these baseline facts about salt and water control in the body happen? They are mediated by powerful systems that involve the brain, the kidneys, the heart and blood vessels, the systems of hormones, the systems of immunity and inflammation and even the bacterial populations of your intestines.
Ok so way more than a normal lay person like myself can understand reading this 13 dense page review article but one major revelation to me was the part about a convoluted mechanism in the kidney that acts like a magic switch in response to your potassium intake. Eating more potassium may be all you need to do fix salt sensitivity and blood pressure. But not too much. Potassium is a notorious poison.
1. Ellison DH, Welling P. Insights into salt handling and blood pressure. N Engl J Med 2021;385:1981-93. DOI:10.1056/NEJMra2030212.
John DiTraglia M.D. is a Pediatrician in Portsmouth. He can be reached by e-mail- [email protected] or phone-354-6605.