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Researchers publishing in JAMA Internal Medicine have examined the relationship between salt consumption and cardiovascular disease, heart failure, and death. The team focused on adults aged 71-80 and monitored them for ten years, finding that there wasn’t an association between the negative outcomes which they monitored and salt intake.
Prof. Anna Dominicza, Regium Professor of Medicine, University of Glasgow, said:
“This is an interesting paper, but there are several limitations that are acknowledged by the study authors. Firstly, food frequency questionnaires are not the optimal measure of sodium intake and have a poor correlation with the 24-hour urinary sodium excretion, which is a gold standard. Therefore the association between dietary sodium intake and outcomes could have been attenuated. Secondly, the Health ABC Study was not designed to answer the question regarding appropriate dietary sodium intake. This means that the analyses presented in the paper were secondary analyses not pre-specified when the study was originally designed. Such secondary analyses are prone to confounding.
“Furthermore, the study participants were volunteers with good functional capacity and thus not fully representative of general older adult population. Finally, the lack of an association between dietary sodium intake and blood pressure or hypertensive status is surprising as blood pressure increase is the main mechanism by which sodium intake leads to increased cardiovascular disease and heart failure risk.
“There are further limitations listed in the paper, but in my view these four are the most important and call for a cautious interpretation of the main conclusion. In view of the above limitations, the recommendation that “there is a need for stronger evidence, preferably from rigorous controlled trials testing additional thresholds for sodium intake, before applying a policy of further sodium restriction to older adults beyond the current recommendation for the general adult population (2300 mg/dl)” is correct and fully warranted.
“I much prefer this conclusion to a more catchy comment such as “Salt intake not associated with mortality or risk of CVD and HF in older adults.” It is important to remember that sodium intake in the large majority of our UK population is much higher than currently recommended and this is true across all age groups.”
Prof. Francesco Cappuccio, Chair of Cardiovascular Medicine & Epidemiology, University of Warwick said:
“This study contains methodological flaws in many domains that have been repeatedly highlighted in the international literature and that make the study and the results unreliable and in danger of misleading researchers and the public.
“The study uses a poor method to assess sodium (salt) intake and has no statistical power to detect changes beyond overall mortality.
“It is disappointing that authors, reviewers and editors have allowed the citation of evidence of questionable scientific validity retracted or withdrawn in 2013 as support to their theories, suggesting, on this occasion, very poor scientific rigour.
“Using inaccurate measure of sodium intake and a self-selected population of older man and women, the study confirms that a higher salt intake is associated with higher mortality even in this group and that there is no evidence of harm for levels of sodium below the current global recommendation of 5g of salt per day (equivalent to <2,000mg of sodium).
Dr Tim Chico, Reader in Cardiovascular Medicine and consultant cardiologist, University of Sheffield, said:
“This study highlights how difficult it is to understand the relationship between lifestyle and health, and no single study can ever give a conclusive answer. The authors estimated sodium intake based on a single questionnaire. This needed people to remember and accurately report what they eat, and for this not to change too much over the 10 years that the study was conducted over. We know such questionnaires are usually inaccurate and this would reduce the chance of finding detrimental effects of a high sodium diet.
“People are understandably confused by the conflicting results of scientific studies on the relationship between diet and health. Instead of concentrating on individual studies (which often only get attention if their results are unusual or surprising) it is much better to look at the overall message from all these studies. These are pretty clear; a healthy diet is varied, contains a high proportion of vegetables and a low amount of processed food, and does not provide more calories than needed to maintain a healthy weight. At the end of the day, most people don’t need a doctor or a scientist to tell them that an apple is almost certainly healthier than a packet of crisps.”
Prof. Jeremy Pearson, Associate Medical Director at the British Heart Foundation, said:
“The study showed that risk of cardiovascular disease was not significantly different for people over 70 years old with a similar blood pressure but differing levels of salt intake. However, blood pressure can be influenced by a number of factors and the finding that people with a similar blood pressure had a similar risk of cardiovascular disease is not surprising. More research is therefore needed before recommending further restrictions on salt intake for older adults.
“This study does not contradict current dietary guidelines that people should consume less than 6g of salt per day. There is global agreement that lowering the amount of salt you consume will lower blood pressure which can significantly reduce your risk of cardiovascular disease.”
Expanded comment from Prof. Francesco Cappuccio, Chair of Cardiovascular Medicine & Epidemiology, University of Warwick:
Summary of study. “The present study reports the results of a 10-year longitudinal analysis of the association between sodium intake and morality in a population of 2,642 men and women, aged 71-80 years (mean 73.6 years).
“Sodium intake was assessed by Food Frequency Questionnaire (FFQ), outcomes were all-cause mortality, incident CVD and incident heart failure.
“Analyses were presented using sodium intake both as continuous variable and using three cut-off points of <1500, 1500-to-2300 and >2300 mg sodium per day, equivalent to <3.75, 3.75-to-5.75 and >5.75 g of salt per day.
“The study had a 80% statistical power to detect a 20% increase in all-cause mortality risk per 1g of sodium intake.
“The results show: (1) a linear association between dietary sodium intake and mortality (Figure 1) with estimates not improving by more complex non-linear modelling. Hazard Ratio was 1.09 [95% CI 1.04-1.16, p=0.001] for crude estimates and 1.03 [0.98-1.09, p=0.27] for fully adjusted model; (2) in group analysis (Table 2) sodium intake >2300 mg per day (>5.75 g salt per day) was associated with higher mortality than the mid group, whereas the group with sodium <1500 mg per day (<3.75 g salt per day) showed no significant increase in mortality; (3) the results for incident CVD and heart failure are only presented by sodium groups (Table 2) and follow the pattern of mortality.
“The authors conclude: (1) sodium intake was not associated with 10-year mortality, incident CVD or heart failure; (2) consuming >2300 mg sodium per day was associated with non-significant higher mortality
Comments. “This study contains methodological flaws in many domains that have been highlighted recently[1] [2] [3] and that make the study and the results unreliable and in danger of misleading researchers and the public.
“It is an observational study and does not imply cause-effect relationship. Nevertheless these studies may be of interest if interpreted in the wider context of the available evidence and their numerous pitfalls acknowledged. Sadly the authors do not do satisfactorily address them.
Systematic error in sodium assessment. “The study relies on FFQ as a method for assessing sodium intake. The method is not only imprecise, but may lead to biased assessment. The authors accept that FFQ may be less accurate than 24h urine collection and that they are poorly correlated with it. Furthermore their validations (quoted in ref. 31 and 32) are very old and may be out of date given the significant reformulation of foods that has occurred globally since 2006, especially for sodium content. Finally, in Table 1 they describe 11% of the sample has having an intake of sodium <1500mg per day, at variance and in contrast with the evidence they quote that achieving this level is extremely difficult and that in the NHANES data only 1.3% of people over 51 years achieve that level. Clearly there is a problem with their assessment of sodium intake and/or with the selection of their population sample, admittedly selected on the basis of voluntary participation and good functional capacity, process that introduces concerning biases. Finally, discretionary use of salt is not factored in as contributor to total sodium intake.
Potential for reverse causality. “There is no mention of sensitivity analyses after exclusions of deaths in the first couple of years from sodium assessment. Table 1 suggests some selection bias in the characteristics of the three sodium groups that might be explained by several facts. Males are less represented in the low sodium group, possibly reflecting survival bias. People in the low sodium group eat significantly less calories than the other groups but have comparable body mass index, suggesting that the reason for lower calorie intake is not reduced size but likely ill-health or adopted life-style as a result of high risk.
Potential for residual confounding. “The referred imbalance in key confounders across sodium categories (Table 2) has the potential to alter the direction of associations.
Insufficient power. “The study had a statistical power of 80% (p=0.05) to detect a 20% increase in mortality risk per 1g of sodium intake (assuming a linear association). The authors accept (Figure 1) the assumption of a linear association, they are able to detect a highly significant 9% increased risk in the crude analysis (Table 2) and fail to detect statistical significance in the adjusted model which shows a 3% increase in mortality for higher sodium intake. These results indicate that the study was not powered to detect a smaller effect of 3%, still of population significance if it were true. Any additional analysis by incident CVD and heart failure, as well as by sodium groups is certainly underpowered and not helpful. In their discussion the authors often refer to ‘signals’ in their results, not a scientifically acceptable terminology. In conclusion, their results do not rule out any effect smaller than a 20% difference in risk for a 1g sodium difference and are unable to confirm the presence of a U or J shaped relationship between sodium intake and mortality.
“Finally, it is surprising that in support of the argument that there is ‘controversy’ on the efficacy of a moderate reduction in salt intake for the prevention of cardiovascular disease authors, reviewers and editors have allowed the citations of reference 49[4], that was withdrawn by the Cochrane Review Centre in June 2013. The existence of the raw data and the validity of the results published in the trials of heart failure have been questioned to the point that the journal Heart also had to retract a paper using the same trials and co-authored by Taylor RS[5]. It is important that the readers are fully aware of these facts, not acknowledged here but all into the public domain, that raise issues about the scientific rigour applied on this occasion by authors, reviewers and editors.
Conclusions. “Following previous recommendations, we should consider what the data show[6]: using inaccurate measured of sodium intake and a self-selected population of older man and women, the study confirms that a higher salt intake ia associated with higher mortality even in this group and that there is no evidence of harm for levels of sodium below the current global recommendation of 5g of salt per day (equivalent to <2000mg of sodium)[7].”
1 Cobb LK et al. Methodological issues in cohort studies that relate sodium intake to cardiovascular disease outcomes. A Science Advisory from the American Heart Association. Circulation 2014;128:1173-86
2 Cappuccio FP et al. Salt intake and cardiovascular disease: compelling evidence so hard to accept. Eur Heart J 2013; e-letter, May 8, 2013
3 He FJ et al. Salt intake and mortality. Am J Hypert 2014; 27(11):1424
4 Taylor RS, et al. Reduced dietary salt for the prevention of cardiovascular disease. Cochrane Database of Systematic Reviews 2011, Issue 7. Art. No.: CD009217. Withdrawn in June 2013 http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD009217.pub2/full
5 DiNicolantonio JJ et al. Low sodium versus normal sodium diets in systolic heart failure: systematic review and meta-analysis. Heart 2012; on-line 21 August; retracted in Heart 2013; on-line 12 March.
6Whelton PK et al. Sodium and cardiovascular disease: what the data show. Am J Hypert 2014;27(9):1143-4
7 World Health Organization. Guideline: Sodium intake for adults and children. WHO (Geneva), 2012
‘Dietary sodium content, mortality, and risk for cardiovascular events in older adults: The Health, Aging, and Body Composition (Health ABC) Study’ by Kalogeropoulos et al. published in JAMA Internal Medicine on Monday 19th January 2015.
Declared interests
None declared