Can low-salt diets really cut blood pressure in one week?

Recent research published in the Journal of the American Medical Association (JAMA) found that people following a low-salt diet for just one week may experience a significant reduction in blood pressure compared to a high-salt diet.

Researchers from Vanderbilt University Medical Center (VUMC), Tennessee, studied 213 people aged 50 to 75 living in Chicago, Illinois, and Birmingham, Alabama. Most of those taking part had a history of high blood pressure (hypertension), ranging from treated and controlled to untreated and uncontrolled, and all participants were already eating a high sodium diet.

They followed one of two diets for a week, with blood pressure measured before and after:

High sodium: 2.2g of sodium per day (almost a teaspoon of salt) was added to their usual diet through two stock cubes
Low sodium: Food was provided which contained an average 0.5g of sodium per day (about a quarter of a teaspoon)

The participants then switched to the opposite diet for a further week.

After the first week, systolic blood pressure was on average 6 mmHg lower in people on the low sodium diet compared to when they followed their usual diet. This was true regardless of age, sex and ethnicity, as well as hypertension status and blood pressure medication use.

High blood pressure (hypertension) is a common and serious condition. It is linked to around half of all heart attacks and strokes, which can be life-threatening. High blood pressure is linked to a high salt intake and so UK dietary advice is not to eat more than 6 grams of salt per day to help reduce our risk. At the moment, most of us are eating more than this.

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What do the researchers say?

Norrina Allen, co-principal investigator of the study and Professor of Preventive Medicine at Northwestern University, Feinberg School of Medicine, said: “We found that 70-75% of all people, regardless of whether they are already on blood pressure medications or not, are likely to see a reduction in their blood pressure if they lower the sodium in their diet.”

Deepak Gupta, MD, MSCI, Associate Professor of Medicine at VUMC and co-principal investigator, said: “In the study, middle age to elderly participants reduced their salt intake by about one teaspoon a day compared with their usual diet. The result was a decline in systolic blood pressure by about 6 millimetres of mercury (mm Hg), which is comparable to the effect produced by a commonly used first-line medication for high blood pressure.”

“In order to live a healthy lifestyle, understanding what we eat has important health effects. Raised blood pressure contributes to one out of every eight deaths worldwide,” Gupta explained. “If people want to lower their blood pressure, attention to dietary sodium is one part of that.”

How good was the research?

The link between low salt intake and improved blood pressure is well documented. This study showed that the link remains even for those on blood pressure-lowering medications.

But the research had some limitations. It only included a small sample of 213 people from two US cities, making it difficult to extend the findings to other, larger populations. Another issue is that the researchers didn’t follow people for very long, so we can’t tell whether people would manage to follow the low-salt diet long term.

Finally, the researchers used urine samples to see if people were sticking to their assigned diet. In the low salt group, urine sodium levels were higher than expected. While this could mean participants weren’t following the low salt diet perfectly, it doesn’t undermine the findings. If anything, it could mean the true effect of a low-salt diet is even greater.

How good was the media coverage?

In general, media outlets reported on the study faithfully without making exaggerated claims. For example, the headline appearing in The Daily Mail was: “Reducing salt intake by one teaspoon a day lowers blood pressure the same amount as medication in just one week.” Meanwhile, the Yahoo News headline was: “Cutting back on salt can lower your blood pressure as much as common medications, study finds.”

Later in the Yahoo article, one sentence reads: “Cutting just a teaspoon of salt per day from your diet could lead to better heart health.” This suggests that one teaspoon of salt is a small amount, when it’s actually about 6 grams. Considering most of us in the UK exceed the daily recommended limit of 6 grams per day, cutting down by a teaspoon might be harder than this is made to sound.

The BHF verdict

Cutting down on salt is one of the best changes you can make to your diet to reduce your risk of high blood pressure. It is encouraging to see that blood pressure can improve when people start cutting back on salt.

However, reducing our salt intake can be challenging. While we can be mindful of the amount we add to our food and cooking, the truth is that most of the salt we eat has already been added to many of the foods we buy. Asides from takeaways, ready meals and processed meats, salt is also added to everyday foods like bread and breakfast cereals. Check front of pack labels and try to choose foods that have a green traffic light for salt as often as you can.