Post by Cliff Harvey ND
We've all been told time and time again that sodium (salt) is bad for us; that it's a cause of heart disease and stroke and that we should reduce our intake. The Dietitians NZ sodium fact sheet states: "Too much salt, and therefore sodium, can lead to more fluid being retained in the body. This means the heart is working harder, pumping more blood around the body, increasing pressure. High blood pressure increases the chance of having a heart attack or stroke, still two of the biggest killers in the Western world." and go on to say that "Reducing our salt intake by just a third, from around 9g (3460mg sodium) to the recommended maximum of 6g (2300mg sodium), could, in time, save over 900 Kiwi lives a year."
While this goal is admirable, the evidence seems to suggest that this isn't the case, and in fact not only is our current sodium intake safe and healthy, but reducing sodium too drastically could be related to several negative health outcomes.
The New Zealand Ministry of Health dietary recommendation to ‘choose and prepare foods that are low in salt (sodium)’ is based on the use of blood pressure as a surrogate for cardiovascular health. Indeed adequate intakes (AI) and the tolerable Upper Limit (UL) for sodium as set by the Institutes of Medicine of the United States National Academies (1) and endorsed by the Ministry of Health (2) recommendations are based upon this correlation. Reducing salt intake reduces blood pressure by only between 1 and 3.5% (3), and the evidence linking salt (sodium) reduction with improved mortality and morbidity is lacking and is insufficient to translate to public health recommendations.
The evidence instead shows that:
1) Reducing salt intake has no effect on population morbidity or mortality prevalence;
2) Low salt intakes are negatively associated with health outcomes in some population groups; and
3) Population health guidelines that are not underpinned by evidence may serve to confuse end users further, thus reducing compliance with (legitimate and scientifically robust) guidelines;
4) Reducing salt intakes further may negatively affect iodine status
Reducing sodium intake has no effect on mortality nor morbidity
A 2011 meta-analysis of RCTs of at least 6 months did not find evidence for reduced mortality or CVD mortality, and concluded that there was no evidence available to support dietary advice to reduce salt intake. In addition, they noted an increase in all-cause mortality in those with heart failure who were advised to reduce their intake (4).
Although not supporting that low sodium intakes were positively correlated with morbidity or mortality in general, the Institute of Medicine of the National Academies Sodium Intake in Populations: Assessment of Evidence (5) noted that the evidence suggests that outcomes for those with congestive heart failure are worsened by reductions in sodium and suggested a risk of adverse health outcomes associated with sodium intake levels in ranges approximating 1,500 to 2,300 mg per day in other disease-specific population subgroups, specifically those with diabetes, chronic kidney disease (CKD), or pre-existing CVD, and noted no significant correlation between improved health outcomes and reductions in dietary sodium.
Current sodium intakes are safe
Mortality and morbidity are increased at both high and low levels of sodium intake .
The average intake of sodium in New Zealand has been estimated at 3900mg per day (6), a level well within the range indicated as having no effect (positive or negative) on mortality and morbidity and so the recommendation to reduce sodium intake is confusing and unnecessary.
Further reductions in salt intake may increase iodine deficiency
Iodised salt has played a major role in reducing iodine deficiency and goitre in New Zealand. Dietary exposure to iodine has steadily decreased since 1982 (7). Thomson (8) in a review of selenium and iodine status in New Zealand, found that iodine levels have been falling since the 1980’s, and this is correlated with clinical measures of thyroid status, and that public health interventions to reduce salt intake may further reduce iodine status.
So government and dietetic guidelines to reduce sodium intake are unnecessary and confusing to the public, and they may encourage drastic sodium reductions that actually worsen outcomes for those in society that are most at risk.
1. Institute of Medicine of the National Academies. (2005). Dietary reference intakes for water, potassium, sodium, chloride and sulphate. Washington, D.C.
2. Medicine, I. o. (2013). Sodium Intake in Populations: Assessment of Evidence. Washington DC. USA: National Academies Press.
3. Graudal, N., Hubeck-Graudal, T., & Jurgens, G. (2011). Effects of low sodium diet versus high sodium diet on blood pressure, renin, aldosterone, catecholamines, cholesterol, and triglyceride. Cochrane Database of Systematic Reviews, 11.
4. Taylor, R. S., Ashton, K. E., Moxham, T., Hooper, L., & Ebrahim, S. (2011). Reduced Dietary Salt for the Prevention of Cardiovascular Disease: A Meta-Analysis of Randomized Controlled Trials (Cochrane Review). American Journal of Hypertension, 24(8), 843-853. doi: 10.1038/ajh.2011.115
5. Institute of Medicine of the National Academies. (2013). Sodium intake in populations: Assessment of evidence. Washington, D.C.
6. McLean, R., Williams, S., Mann, J., & Parnell, W. (2012). 1051 Estimates of New Zealand Population Sodium Intake: Use of Spot Urine in the 2008/09 Adult Nutrition Survey. Journal of Hypertension, 30, e306 310.1097/1001.hjh.0000420510.0000493854.ca.
7. Thomson, B. M., Vannoort, R. W., & Haslemore, R. M. (2008). Dietary exposure and trends of exposure to nutrient elements iodine, iron, selenium and sodium from the 2003–4 New Zealand Total Diet Survey. British Journal of Nutrition, 99(03), 614-625.
8. Thomson, C. D. (2004). Selenium and iodine intakes and status in New Zealand and Australia. British Journal of Nutrition, 91(05), 661-672.