“The government's maximum daily salt intake target has been set too high for people to avoid unnecessary stroke and heart deaths,” the BBC reported. Newspapers also said that cutting the amount of salt in your diet by a teaspoon (5g) a day can reduce your chances of having a stroke.
This news is based on a large, high-quality review of salt intake and cardiovascular disease, which found that higher intake of salt was associated with a 23% increase in the risk of stroke. The advice to cut intake by 5g a day is based on an estimate that people in the West eat 10g a day. This cut would bring people into line with the World Health Organization’s recommended 5g a day.
The important point for individuals is that too much salt is bad for you and increases the risk of health problems. Although the UK has a recommended salt intake of 6g a day, this is the maximum recommended amount and eating less would do no harm. In fact, the Food Standards Agency (FSA) states, “there is little or no evidence that having a low intake of salt has a negative effect on health.”
Where did the story come from?
This research was carried out by Pasquale Stazzullo and colleagues from the University of Naples and University of Warwick. The study was supported by an EC grant, and the publication is reported as not necessarily reflecting the position of WHO. The study was published in the peer-reviewed British Medical Journal .
An editorial states that the evidence that salt raises blood pressure is now indisputable and calls for a reduction in salt intake.
Generally, the study has been accurately reported. It states that average daily salt consumption in Western countries is about 10g, and that reducing this by about 5g (one teaspoon) to roughly the WHO target of 5g a day could reduce the risk of stroke. The study does not criticise the FSA’s recommended level for salt of 6g a day, nor does it compare the FSA and WHO targets.
What kind of research was this?
A systematic review aims to combine the findings of all available previous studies on a subject, and is the most effective and respected type of study for examining the current evidence on a particular issue. By necessity, the studies included in the review were observational studies, as due to the health implications it would be unfeasible and unethical to experiment with people’s salt intake.
There are inherent drawbacks to pooling observational studies. Even a well-designed systematic review has limitations as the included studies are likely to have slightly different methods, follow-up and measurement of outcome and exposure, and not all of them may have accounted for all possible confounders.
What did the research involve?
To find appropriate studies for the review, the researchers conducted a search of several medical databases for cohort (group) studies published between 1966 and 2008. The studies all assessed salt intake at their beginning and recorded incidences of either stroke or total cardiovascular disease (the outcome) at least three years later.
A total of 13 studies (out of a possible 3,246 publications) met the inclusion criteria and were suitable for meta-analysis. Details were collected on the studies’ populations, their methods of assessing and categorising salt intake, follow-up, and outcomes assessed (stroke and cardiovascular disease). Some studies reported only stroke outcomes while others looked only at total cardiovascular events or deaths. A combined risk ratio was calculated, using statistical methods to take into account differences between the studies.
The review was particularly thorough and well executed, and the researchers had specific inclusion criteria to ensure that studies had a minimum follow-up, specified a cardiovascular outcome and had categorised salt intake. All studies were also assessed for quality. These steps limited the chances of introducing errors because of differences between the studies, and allowed the researchers to look at the effects of the different methods that were used.
For example, how salt intake was measured varied considerably and included 24-hour dietary recall, food frequency questionnaire, 24-hour urine excretion and questionnaire. Risk was also reported differently. For example, some studies gave the number of events for each salt exposure category, while others specifically reported differences in event rate per 100mmol/day difference in salt intake. Attempts to account for these differences were made in the combined analysis.
What were the basic results?
The 13 studies involved a total of 177,025 participants, and follow-up varied between 3.5 and 19 years. During this time, there were over 11,000 vascular events (such as stroke or heart attack).
Higher salt intake was associated with a 23% increased risk of stroke (relative risk [RR] 1.23, 95% confidence interval [CI] 1.06 to 1.43). There was no increased risk of cardiovascular disease as a whole with higher salt intake, although when one study with outlying results (highly different findings from all the other studies) was excluded, there was a borderline significant increased risk (RR 1.17, 95% CI 1.02 to 1.34). The relationships between stroke and cardiovascular outcome were reported to be stronger with a greater difference in salt intake and with longer follow-up.
How did the researchers interpret the results?
The researchers conclude that high salt intake is associated with significantly increased risk of stroke and total cardiovascular disease.
In addition, they argue that, as salt intake was imprecisely measured in most studies, the effects “are likely to be underestimated”. They say these findings support the requirement for a “substantial population reduction in salt intake for the prevention of cardiovascular disease”.
This well-conducted review examined a collectively large population for the relationship between salt intake and stroke and cardiovascular disease. In addition, it collected detailed information from each individual study on its methods, findings and quality, and attempted to take into account the differences between these in its analysis.
However, the review does have some inherent limitations:
- Although the researchers attempted to include only studies where participants had a definite prior salt exposure and a later cardiovascular outcome, there were some differences between the studies, particularly in their length of follow-up and how they assessed salt exposure and risk.
- In the meta-analysis, there was calculated to be significant heterogeneity between studies indicating differences between them in their results, which could be due to the different methods used. Although all studies showed a trend towards increased risk of stroke with higher salt intake, the relationship was only significant in four individual studies. Only three out of ten individual studies found a significant increased risk of cardiovascular disease. However, when the results are combined in meta-analysis, the trend across studies leads to a significant increased risk.
- It is also probable that a person’s day-to-day salt intake can vary considerably. As only a single measurement of salt levels was taken, either through 24-hour urine excretion or a food frequency questionnaire, this cannot be reliably accounted for.
- There were a number of possible confounders that were not taken into account in the different studies but which may have affected the relationships. For example, few studies analysed men and women separately. The confounders that the studies took into account also varied and included blood pressure, BMI, age, diabetes, smoking, cholesterol and alcohol.
- It is not clear how or if the participants were tested for cardiovascular disease at the beginning of all the studies. This would be necessary to establish whether the outcome (CVD) definitely followed the exposure (salt).
It should be noted that 5g of salt a day is the WHO recommendation, while the UK recommendation is 6g a day. Although this research supports a move to reduce current UK salt intake, it does not criticise the UK salt limit, nor does it compare the FSA and WHO recommendations directly or suggest what the daily intake should be.
The important finding of this study for individuals is that too much salt is bad for you and increases your risk of stroke and cardiovascular problems. The UK’s 6g daily salt allowance is the maximum recommended level, and eating less than this would do no harm. In fact, the FSA says, “there is little or no evidence that having a low intake of salt has a negative effect on health.”