New Research Finds Flaws in Studies that Claim Low Sodium Diets Increase Risk of Death
'Gold Standard’ Studies Prove High Salt Intake Leads to Higher Death Rates
A new Study by Queen Mary University of London, University of Calgary, Harvard TH Chan School of Public Health, US CDC and the Brigham and Women’s Hospital published in the International Journal of Epidemiology (22 June 2018)1 ‘Errors in estimating usual sodium intake by the Kawasaki formula alter its relationship with mortality ─ Implications for public health’ has for the very first time compared the relationship between sodium intake and death based on various methods to assess sodium intake. The findings indicate that inaccurate measurement of sodium intake could be an important contributor to the paradoxical ‘J-shaped’ findings between sodium intake and cardiovascular events and deaths (i.e. both low sodium intake and high sodium intake are associated with higher risk of death from cardiovascular disease) reported in some cohort studies.2
Measuring Sodium Intake
This study compared the relationship between sodium intake and death, based on the various methods to assess sodium intake – i.e. 24-hour measurement, spot urine estimation on one day, spot urine estimations over several days, and several 24-hour urine measurements. Data from the follow-up of the Trials of Hypertension Prevention (TOHP) were used for this study.3
Over 90% of sodium eaten in a 24-hour period is excreted in urine over the subsequent 24-hour period. Therefore, the most accurate method of measuring sodium intake is by measuring the amount excreted in the urine, and as sodium intake varies day to day, the ‘gold standard’ method is multiple 24-hour urine measurements.
Recent controversial cohort studies have used spot urine (i.e. one single sample of urine, usually collected in the morning), converting this to a 24hr urinary sodium using the Kawasaki formula.2 It has been hypothesised that a reason for the ‘J-shaped’ relationship found in these studies is the use of spot urine and the Kawasaki formula.
Relationship between Sodium Intake and Mortality
The results demonstrated that sodium intake measured by the gold standard method of several 24-hour urine collections, had a direct linear relationship with mortality - the more salt people eat, the higher their risk of death.
However, the average estimated sodium intake from several spot urines appeared to show a ‘J-shaped’ relationship with death (i.e. both high intake and low intake of sodium increase the risk of death from cardiovascular disease). This method over-estimated intake by 1,297mg/day (i.e. 3.2g salt) compared to the gold standard method. Furthermore, estimated intake from single spot urine weakened the relationship with death. The study concludes that estimated salt intake from spot urine by the Kawasaki formula finds a risk associated with low sodium intake, but accurate measurements find that lower intakes of sodium lead to a lower risk of death.
Need for Sodium Reduction
Accurate measurements of sodium intake reveal a direct linear relationship between sodium and death down to an intake of 1,200mg/day (3g salt) which is the long-term target for population sodium intake recommended by the National Institute for Health and Care Excellence, UK.4 This finding is consistent with the evidence that there is a strong direct linear relationship between sodium intake and blood pressure within the range of 1,200-4,800 mg/day (3-12 gram salt)5-7 and evidence also shows that as blood pressure increases, the risk of developing and dying from cardiovascular disease also increases.8,9
Sodium consumption has long been shown to have a negative impact on health, with studies of various types consistently showing a high dietary sodium intake is the major cause of raised blood pressure.10 Raised blood pressure can lead to cardiovascular disease (including strokes, heart attacks and heart failure), which is the leading cause of death and disability worldwide. A high sodium diet is also linked to a number of other conditions, such as osteoporosis, stomach cancer and kidney disease,10 with new evidence on the harmful effects of sodium emerging all the time.
Dr Feng He, lead author of the Paper and researcher at Queen Mary University of London, says: “Studies of poor scientific quality have unfortunately tried to cause controversy in what is recognised as a robust public health strategy. However, the totality of evidence shows reducing our salt intake will be immensely beneficial in preventing strokes, heart attacks and heart failure, the commonest causes of death and disability in the world. The question is not ‘should’ we reduce salt intake, but ‘how’.”
Professor Graham MacGregor, Professor of Cardiovascular Medicine at Queen Mary University of London, Chairman of Action on Salt and co-author of the Paper, says: “Salt reduction has been identified as the most cost-effective measure to reduce NCDs (Non-communicable diseases) for both developed and developing countries. Therefore, each country needs to develop and implement a robust coherent plan to reduce sodium intake in the whole population.”
Nancy Cook, Professor in the Department of Epidemiology, Brigham and Women’s Hospital, says: “Sodium consumption is very difficult to assess, and it is important to use the most accurate estimates to discover the truth about its effect on disease. The best evidence suggests that lowering sodium can lead to reductions in disease and premature deaths.”
David Clarke: email@example.com M: (+44) 7773 225516
Notes to editor:
World Action on Salt and Health (WASH) was established in 2005 as the international arm of Action on Salt (formerly Consensus Action on Salt and Health, CASH), and is a global group with the mission to improve the health of populations throughout the world by achieving a gradual reduction in salt intake. WASH has the support of more than 500 members from 100 countries, who are mainly experts in hypertension, cardiovascular disease or kidney disease, but also work in public health and nutrition.
- He FJ, Campbell NRC, Ma Y, MacGregor GA, Cogswell ME, Cook NRC. Errors in estimating usual sodium intake by the Kawasaki formula alter its relationship with mortality ─ Implications for public health. Int J Epidemiol 2018.
- Mente A, O'Donnell M, Rangarajan S, et al. Associations of urinary sodium excretion with cardiovascular events in individuals with and without hypertension: a pooled analysis of data from four studies. Lancet 2016; 388: 465-75
- Cook NR, Appel LJ, Whelton PK. Sodium Intake and All-Cause Mortality Over 20 Years in the Trials of Hypertension Prevention. J Am Coll Cardiol 2016; 68: 1609-17
- NICE (The National Institute for Health and Care Excellence). Guidance on the prevention of cardiovascular disease at the population level. http://guidance.nice.org.uk/PH25
- He FJ, Li J, MacGregor GA. Effect of longer term modest salt reduction on blood pressure: Cochrane systematic review and meta-analysis of randomised trials. BMJ 2013; 346: f1325
- He FJ, MacGregor GA. How far should salt intake be reduced? Hypertension 2003; 42: 1093-10999
- Sacks FM, Svetkey LP, Vollmer WM, et al. Effects on blood pressure of reduced dietary sodium and the Dietary Approaches to Stop Hypertension (DASH) diet. DASH-Sodium Collaborative Research Group. N Engl J Med 2001; 344: 3-10
- Lewington S, Clarke R, Qizilbash N, Peto R, Collins R. Age-specific relevance of usual blood pressure to vascular mortality: a meta-analysis of individual data for one million adults in 61 prospective studies. Lancet 2002; 360: 1903-193
- Ettehad D, Emdin CA, Kiran A, et al. Blood pressure lowering for prevention of cardiovascular disease and death: a systematic review and meta-analysis. Lancet 2016; 387: 957-67
- He FJ, MacGregor GA. Reducing population salt intake worldwide: from evidence to implementation. Prog Cardiovasc Dis 2010; 52: 363-82.