Confusion over salt research
“Salt is good for us after all,” according to the Daily Express. The newspaper said that a “controversial new study suggests that salt in the diet can lessen our chances of suffering heart disease and strokes”.
The Express story itself should be taken with a large pinch of salt, as the study doesn't suggest that eating high levels of salt is good for us. Instead, the research found that in white people with normal blood pressure, a low-salt diet only slightly reduced blood pressure but led to small increases in substances such as cholesterol.
The study itself was designed to pool and analyse the results of previous studies to determine how low-salt and high-salt diets affected blood pressure and a range of substances in the blood. Crucially though, many of the studies included in the review lasted only a few weeks and none were designed to look at the effect of salt reduction on longer-term health outcomes such as heart disease and stroke. Salt reduction is often intended to be a long-term measure, and therefore results looking at the practice over longer periods would be preferable.
This research does not change the current NHS advice, which is that all adults should eat no more than 6g of salt a day. The National Institute for Health and Clinical Excellence says that reducing the maximum average intake of salt per adult to 6g a day, and then to 3g by 2025, would result in 15,000-20,000 fewer deaths from heart disease and stroke annually in the UK.
Where did the story come from?
The study was carried out by researchers from the Cochrane Collaboration, an international organisation that undertakes systematic reviews of evidence. There was no external funding. The study was published in the peer-reviewed American Journal of Hypertension.
The Daily Express headline, “Salt is good for us after all” is misleading. The study suggests that cutting back on salt may, in the short-term, increase the presence of potentially harmful chemicals in the blood and questions whether doing so has any net benefit for white people with normal blood pressure. However, the study does not suggest that eating lots of salt is good for our health. The study also has some limitations, such as a lack of long-term results, that mean that its findings are arguably of limited use.
Both the Express and The Daily Telegraph included comments from external experts in their reports, some of which were critical of the study.
What kind of research was this?
This was a systematic review and meta-analysis combining and analysing 167 studies examining the effect of sodium intake on blood pressure, lipids and certain chemicals. It drew comparisons between people with a low salt intake and people eating a high amount of salt.
The authors point out that there is not yet a consensus over whether reducing sodium intake can reduce health problems in people with normal blood pressure. They say that although reducing salt intake has been widely promoted for good health, in population studies the effect of reduced sodium intake on illness and death have been conflicting. While randomised trials have shown that reducing sodium lowers blood pressure, they have also shown that it can raise other potentially harmful chemicals in the blood, including lipids such as cholesterol.
What did the research involve?
In their review the authors only included randomised controlled trials allocating patients to either a high- or low-sodium diet, in which sodium intake was estimated by measuring 24-hour urinary sodium excretion. People with normal or high blood pressure and of any race or age were included. Studies looking at patients with other diseases were excluded.
To be included, the studies had to have evaluated the effects of low-sodium and high-sodium diets on one of the following outcomes:
- blood pressure (both systolic and diastolic)
- LDL and HDL cholesterol
To gather studies they searched various established databases to identify eligible studies and performed a literature search for all relevant studies published from 1950 through to 2011. The search was not limited to English-language studies.
They extracted all relevant data, assessed the risk of bias using an established method and analysed the data using established statistical methods.
What were the basic results?
The researchers found 167 studies which met their criteria. Overall, their meta-analysis compared the effect of a dietary sodium intake, which they defined as either lower than normal (120mmol, or about 2.7 g of sodium) or as normal or above normal (150mmol or 3.4 g of sodium).
The researchers found that:
- In white people with normal blood pressure, sodium reduction resulted in a less than 1% reduction in blood pressure.
- In white people with high blood pressure, short-term sodium reduction reduced blood pressure by about 2%-2.5%. When all studies of all populations were included, the reduction in blood pressure was 3.5%.
- In Asian and black populations the effect of a low salt intake on reducing blood pressure was generally larger.
- Low-sodium diets resulted in a significant increase in blood levels of renin, aldosterone, adrenaline and noradrenaline, a 2.5% in cholesterol and a 7% increase in triglycerides. Higher levels of many of these substances are associated with health problems.
- The effects were similar in trials of four weeks or longer.
How did the researchers interpret the results?
The researchers said their meta-analysis shows that in people with normal blood pressure the benefit of a small reduction in blood pressure may be cancelled out by the increase in lipids and other hormones associated with a low-sodium diet.
The results, they said, do not support the idea that sodium reduction has a net benefit in white people with normal blood pressure. For people with high blood pressure salt reduction may be used alongside other treatments for high blood pressure.
While the data suggests that Asian and black people are more sensitive to sodium reduction, further studies will be needed to confirm this.
They said that it is not known if reducing our salt intake improves or worsens health outcomes in people with normal blood pressure in the long-term. Further research on reduced salt intake is required, particularly in non-white populations.
This review raises questions about the potential health effects of low-sodium diets, and therefore it will be studied with interest by those involved with dietary strategies for preventing ill health.
However, many of the studies included in the review lasted only a few weeks and none were designed to look at effect of salt reduction on longer-term health outcomes such as heart disease and stroke. This makes it harder to assess the impact of salt reduction in the long-term for someone with chronic high blood pressure. Also, assessing levels of various chemical markers rather than events such as strokes or heart attacks means we cannot directly tell whether salt reduction affects the risk of debilitating or potentially fatal outcomes; a very important factor in whether salt reduction is appropriate.
Most of the people who took part in the studies were white, so the researchers suggest that there is a need for more research in non-white populations. This is of particular importance given the higher rate of conditions such as diabetes and heart disease among black and Asian populations.
As none of the important health outcomes were measured in this study it is crucial that any further research in this area determines how far any reductions in salt intake lead to the long-term lowering of stroke and heart disease incidents.
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