In the UK the relationship between salt and health was considered by the official Scientific Advisory Committee on Nutrition in 2003 (1). This report confirmed earlier recommendations that the average intake for the adult population should be 6 gm/day of salt (sodium chloride) which is equivalent to 2.4 gm/day of sodium. This means men should be consuming 7 gm/day and women 5 gm/day. It is generally accepted that the average intake in the UK is 9 gm/day and so to achieve the recommendation would require a reduction of about one third. However a survey of people aged over 65 found that the average intake for men was 6.8 gm/day and for women 5.3 gm/day (2).
The SACN report concluded that there is an association between the intake of salt and blood pressure and that a reduction in dietary salt intake would lower the blood pressure risk for the whole population. However it was recognised that the long-term effects on health and premature mortality were less certain.
This approach has a number of limitations which include the following:
- An association does not demonstrate cause and effect
- The focus of attention has been on blood pressure which is a risk factor for cardiovascular disease. It has been assumed that any reduction represents an improvement in health which is not necessarily true. While it is probably beneficial for those who have raised blood pressure, this is certainly no justification for recommending to the population as a whole. Furthermore there are almost certainly other parameters that are affected by altering the dietary intake of salt which may or may not have an impact on an individual’s health. As the report acknowledges it was not possible to conclude that with any degree of confidence that a reduction in salt intake will result in definite improvements in public health and/or life expectancy
- In determining what the optimum intake should be there has been no attempt to allow for the impact of other nutrients in the diet, such as calcium and potassium which undoubtedly influence the action of sodium and chloride in the body
- Salt is definitely required by the body and so there must be minimum amount which is essential. By focussing on the need to limit the intake, with respect to the whole population there is a genuine danger that some people will suffer insufficiencies as a direct result of their attempts to comply with the official advice
- This will definitely apply to those who are over 65 years of age which is when most deaths occur. As mentioned above the available evidence indicates that this group has an intake of salt which is effectively at the recommended level. As this is a population average it follows that many of them are actually consuming less and consequently may not be meeting the requirement.
In the light of these doubts it is highly significant that the Institute of Medicine (IOM) in the USA has just published a report, which has conducted a re-evaluation of the research on salt and health including much new evidence which has emerged in recent years(3). This work was initiated because of recent findings which have indicated that some sections of the population in the USA may not be consuming adequate quantities of salt, especially those over 51 years, African-Americans, those with hypertension, diabetes, chronic kidney failure or congestive heart failure. The review discovered that in a number of studies, the effects of dietary sodium on cardiovascular disease (CVD) outcomes persisted even after the elimination of blood pressure. This indicates that association between dietary sodium and the risk of CVD may be modulated through interaction with other dietary factors or through pathways in addition to blood pressure. Therefore the review body decided to focus on health outcomes instead of using blood pressure as a biomarker of health status as had been done in previous evaluations. It was noted that there was a lack of consistency in the methods used to define high and low intakes of sodium which meant that it was not possible to determine a range of intakes which could be regarded as “healthy”.
It was concluded that:
- Research on sodium intake and direct health outcomes has methodological flaws and limitations, but nevertheless indicates a possible relationship between higher levels of intake and the risk of CVD
- Evidence from studies on direct health outcomes was inconsistent regarding an association below 2.3 mg sodium/day (5.75 gm salt/day) and either benefit or risk of CVD or all-cause mortality in the general population in the USA.
These conclusions have been explained by Professor Robert Heaney of Creighton University(4). He points out that the average intake in the USA of just under 9 gm/day is probably about right. This is because at intakes below this, various mechanisms come into play which can be damaging to health. By contrast an intake of about 15 gm/day or higher is too much.
The implications for the UK are very highly significant. First of all the basis for the current recommendations is open to serious criticisms and does not stand up to rigorous examination. But much more pertinent is the real possibility that the continued promotion of these recommendations may result in increased risks of damage to health by those people who react positively to the official advice.
It is now abundantly clear that it is very difficult to determine precisely what advice on Healthy Eating should be given to the public. Previous blogs (See Blogs 8 and 14) have shown how implementation of the recommendations on fat has contributed to the “obesity epidemic” and the disastrous increase in the incidence of diabetes. It now appears that the advice on salt has also been a huge mistake. Unfortunately there is absolutely no sign that this has been recognised and appreciated by those in a position to alter the official policy. In the meantime anyone who is seriously interested in making sense of the science and the relevant implications will have to do their own research and hopefully these blogs will help!
- S Finch et al (1998) “National Diet and Nutrition Survey: People aged 65 years and over” TSO: London
- Available at http://www.nap.edu/catalog.php?record_id=18311