Salt boosts blood pressure for some people. UAB study asks: Who?

UAB is participating in a multi-center clinical trial that will dig deeper into the connection between hypertension and salt intake.
Written by: Matt Windsor
Media contact: Bob Shepard

rep salt fries 550pxA new UAB study is recruiting volunteers to learn more about the effects of dietary salt on blood pressure, including who is most affected and the mechanisms behind it.High blood pressure — hypertension — is the leading underlying cause of death worldwide. More than 1.25 billion people have hypertension, including more than 100 million Americans. Most people worldwide eat more salt than recommended, too. Is there a connection?

Cora Lewis, M.D., professor and chair of the University of Alabama at Birmingham Department of Epidemiology in the School of Public Health is the UAB principal investigator for a multi-center clinical trial, now enrolling participants, that is exploring two related questions:

  • How common is salt sensitivity of blood pressure?
  • What mechanisms can explain it?

“A number of studies link high salt intake to high blood pressure and a higher risk of premature death and cardiovascular disease, including heart attacks and strokes,” Lewis said. “From randomized controlled trials, the gold-standard type of clinical study, we know that a lower-sodium diet lowers blood pressure on average. But there is wide variability in blood pressure response to salt.”

New clinical trial explores two crucial questions

Some people, for reasons not yet clear, are more salt-sensitive than others. Identifying these individuals is important, because studies show that people who are salt sensitive are at higher risk for mortality, Lewis says.

Current estimates are that around half of people with high blood pressure are sensitive to salt, while that figure is only about 25 percent for people with normal blood pressure. Lewis also adds that Black people tend to be more often salt-sensitive than white people.

But few studies, especially in recent decades, have looked at the relationship between blood pressure and salt intake in an experimental way. Instead of relying on food diaries and spot checks of blood pressure, Lewis’ study is recruiting participants to eat high-salt and low-salt diets for a week each, with blood pressure measured continuously over 24 hours using a small device that participants take home.

“There has been some controversy in certain circles over the relationship between sodium intake and cardiovascular disease, at least in part because many of the studies we have are observational and are not clinical trials,” Lewis said. “This is particularly true for the larger and longer-term studies of dietary sodium and its relationship to actual cardiovascular disease outcomes. That is where our study comes in.”

How the study works

The study, which also is taking place at Northwestern University and is coordinated by a researcher at Vanderbilt University, is recruiting volunteers aged 50-75 who have normal blood pressure or high blood pressure that is treated and controlled. Participants begin by eating their normal diet for one week to establish baseline data on salt intake, blood pressure and more. That is followed by a week each of a high-salt diet — the participant’s normal diet plus bouillon packets to make a salty soup that you could easily buy at the grocery store — and a low-salt diet, with all food for the week provided by the study researchers. Participants also get bottled distilled water to drink, since tap water can contain salt.

Participants are randomized to start with the low-salt or high-salt diet, in case the order affects results. At the end of each week, participants will wear a special blood pressure monitor for 24 hours and collect urine samples for 24 hours. Full-day urine sampling is the best way to measure how much salt a person has consumed, because it tends to be removed from the body quickly.

Gathering crucial data

The study also will dig into possible mechanisms that could explain salt’s impact on blood pressure. Blood samples, taken during four weekly visits, will allow Lewis and her co-investigators to study how the quantities of various immune cells change with diet.

“Consuming a high-salt diet might trigger inflammation,” Lewis said. “We will be studying whether the immune system responds to the high salt-containing diet compared to the low salt-containing diet.”

Inflammation, the sign of an immune response, affects the arteries and is a key factor in atherosclerosis, sometimes called ‘hardening of the arteries, which underlies heart attack and many strokes.

Reducing salt can clearly have beneficial effects. In a large study in rural China, published in the New England Journal of Medicine in 2021, researchers randomized nearly 21,000 participants to use regular salt or a salt substitute with 75 percent sodium and 25 percent potassium.

“Those assigned the substitute had lower rates of stroke, cardiovascular disease and death from any cause,” Lewis said. “The trial results stress that we need to know more about sodium and how it might affect the cardiovascular system. We’re working on that in this study of salt sensitivity of blood pressure.”