Q: My friend brags about her low blood pressure, and yet I see her put salt on almost everything she eats. My brother claims one salty meal makes his blood pressure rise. Why the difference?
A: Your friend is probably salt-resistant, which means her blood pressure doesn’t rise very much in response to a diet high in salt (sodium chloride). In contrast, other people are salt-sensitive like your brother, which means their blood pressure rises by 5 points or more if they switch from a low-sodium to a high-sodium diet.
About 60% of people with high blood pressure are thought to be salt-sensitive. So are about a quarter of people with normal blood pressure, although they may develop high blood pressure later, since salt sensitivity increases with age and weight gain.
About a third of people with high blood pressure appear to be salt-resistant. And about 4% to 5% of people have what’s known as reverse salt sensitivity, which means their blood pressure actually gets lower when they eat salt.
The genetic variants associated with salt-sensitive high blood pressure involve a number of different mechanisms. For example, some affect an enzyme called renin, which is secreted and stored in the kidneys. Others influence the production of aldosterone (a hormone that increases blood volume) or affect the transport of sodium and other minerals within the body. Blood pressure may also reflect genetic differences in how blood vessels constrict or relax, which is unrelated to sodium.
The medications doctors choose to treat people with high blood pressure do consider these above issues. But the process is still somewhat haphazard. Most often doctors try one drug and see if it works. If it doesn’t, then the patient will try a second, and maybe a third, and more often than not a mixture of two or more.
One day, medication decisions for treating high blood pressure will be fine-tuned to reflect a person’s unique genetic variants. If future clinical trials confirm the most important variants, a simple blood test could reveal which drugs would work best for you — as well as how much salt is safe for you to consume.
Right now, there isn’t an easy test to determine who is salt-sensitive. But we know that Americans eat far more salt than they actually need, so it makes sense to advise everyone cut back on salt.
Howard LeWine, M.D., is an internist at Brigham and Women’s Hospital in Boston and assistant professor at Harvard Medical School. For additional consumer health information, please visit www.health.harvard.edu.
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