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Interact CardioVasc Thorac Surg 2007;6:793-798. doi:10.1510/icvts.2007.165415 © 2007 European Association of Cardio-Thoracic Surgery
Does reducing your salt intake make you live longer?Department of Cardiothoracic Surgery, James Cook University Hospital, Middlesbrough, UK Received 20 August 2007; accepted 21 August 2007
*Corresponding author. Tel./fax: +44-780-1548122.
A best evidence topic in cardiothoracic surgery was written according to a structured protocol. The question addressed was whether restricting dietary salt intake would provide protection from adverse cardiovascular events or mortality. Using the reported search, 462 papers were identified of which 14 papers represented the best evidence on the subject. The author, journal, date and country of publication, patient group studied, study type, relevant outcomes, results and weaknesses were tabulated. We conclude that restricting sodium intake to levels below 6 g per day as most international guidelines such as those of the AHA, the US Dietary Guideline Committee and the Scientific Advisory Committee on Nutrition recommend, clearly reduces blood pressure and in turn may reduce the need for antihypertensives by as much as 30%. However, the ability of dietary sodium restriction to reduce the incidence of cardiovascular events is more controversial due to the lack of adequately powered randomised trials or observational studies conducted with sufficient rigour. Some of the largest studies such as NHANES and TOHP, which do demonstrate a significant benefit, report a 20–30% relative reduction in adverse events which, due to the low rate of these events in the studies equates to an absolute risk reduction over 10–20 years in the region of 2–3% for protection from adverse cardiovascular events from sodium dietary restriction.
Key Words: Salt; Diet; Hypertension; Blood pressure
A best evidence topic was constructed according to a structured protocol. This protocol is fully described in the ICVTS [1].
You are seeing a patient at lunch-time 4 days after his coronary artery bypass grafts. He is well and has been walking around the wards. You are aware that he only gave up smoking three weeks ago, when he was admitted with a non-ST myocardial infarction. He thanks you for doing his operation and asks if it is okay to have some salt on his chips for lunch. You tell him that you are sure that it's okay and that he has to have a few pleasures in life. On walking away from the patient, your nurse practitioner tells you off. She had just told him that high salt intake is bad for him and that he should cut down. You sheepishly promise your nurse practitioner that you will look up the evidence.
Medline was searched using the OVID interface from 1985 to August 2007 [salt.ti OR sodium.ti] AND [diet.ti OR dietary.ti OR reduc$.ti] AND [exp Cardiovascular Diseases/OR exp Cardiovascular Abnormalities/OR Cardiovascular disease$.mp] LIMIT to human studies.
Four hundred and sixty-two papers were identified from the above-mentioned search from which 14 represented the best evidence on the topic (Table 1).
The INTERSALT study (n=10079) ran across 32 countries, and assessed hypotheses of blood pressure reduction. From these data, Stamler [2] and Elliott et al. [3] estimated that a 5.9 g per day (100 mmol/day) rise in salt intake equated to a 3–6/0–3 mmHg rise in blood pressure (systolic/diastolic). Similar changes in BP were seen in a meta-analysis of RCTs which compared salt reduction lasting more than 4 weeks with blood pressure, conducted by He and MacGregor [4] pooling together hypertensive (n=734) and normotensive (n=2220) subjects. In the former group, regression analysis suggested that a 100 mmol/24 h reduction of salt intake conferred a 7.11/3.88 mmHg drop in blood pressure (systolic/diastolic). In the normotensive group salt reduction conferred a 3.57/1.66 mmHg drop in blood pressure. The TONE study [5] found reducing salt intake to 1.8 g/day decreased the need for antihypertensive medication by 30%. Thus, repeated studies confirm that salt restriction reduces blood pressure and need for antihypertensive medication. The NHANES-I study [6] followed-up 9485 patients for 19 years after a single 24-h dietary sodium questionnaire. They found that in non-obese patients, there was no clinical increase in risk of any clinical outcome measures. However, for obese patients there was an increase in stroke risk, heart disease risk and mortality risk. However, between the lowest and highest salt groups, the difference in incidence of any of these risks over the 19 years was no more than 3%. In addition, their questionnaire did not include an estimate of added salt to food, which some authors estimate as contributing to 50% of dietary salt. Tuomilehto et al. [7] reported an increased rate of acute coronary events and an increased mortality in overweight men in a study of 2436 patients having a single measurement of 24-h sodium excretion. However, across all groups the 12–18-year mortality was 7% and an increase in mortality odds in the high salt excretion group was 1.36 and, thus, the incidence in this group will be around 10% giving a 3% increase in absolute risk. Morimoto et al. [8] measured sodium sensitivity in 350 patients by giving them a week of a high and then low sodium diet and seeing if the blood pressure changed. Patients were followed-up for a mean of 7.3 years. There were two events per 100 patient-years in the non-sensitive group and 4.2 events in the sensitive group. The most recent analysis of two randomised trials: Trials of Hypertension Prevention (TOHP I and II) in 2007 [9] reported that cardiovascular events were 25% lower in those given dietary and behavioural counselling in how to reduce their sodium intake as compared to the control patients. This is the most convincing paper to date making the link between sodium restriction and cardiovascular risk. However, on looking further into this study, this difference was 7.0% compared to 9.0% over 10 years for cardiovascular events (of which there were 200 across over 3000 patients). The 2006 Cochrane review [10] that sought to assess the impact of salt restriction on cardiovascular risk found only 11 RCTs of adequate quality for inclusion. In these studies, there were only 17 deaths spread evenly across groups and 46 cardiovascular events in the controls compared to 36 in the low sodium diet groups. They concluded that sodium restriction could be justified to reduce the need for anti-hypertensive medication but not for the reduction of cardiovascular events. Michael Alderman [11, 12] in his Presidential address to the 21st Scientific Meeting of the International Society of Hypertension in Japan in October 2006 stated that there is no question that sodium restriction can reduce blood pressure. However, the link between salt restriction and cardiovascular disease has not been adequately established and is heterogeneous at best from evidence from observational studies rather than randomised trials. There may actually be a J-shaped relationship between salt intake and risk and the contention that salt restriction will reduce cardiovascular risk is an argument of hope over reason. The AHA [13] recommends that the general public consume no more than 6 g of sodium chloride per day in support of the US Dietary Guideline Committee that also gives this recommendation. Slightly higher intakes (6.0–7.5 g/day) have not in their view been demonstrated to increase cardiovascular risk or raise blood pressure in normotensive persons without other cardiovascular risk factors. They state that the guideline is an arbitrary recommendation for avoiding excessive salt intake rather than an attempt to impose low salt intake. In the UK the Scientific Advisory Committee on Nutrition also recommend 6 g/day [14].
Restricting sodium intake to levels below 6 g per day reduces blood pressure and in turn may reduce the need for antihypertensives by as much as 30%. However, the ability of dietary sodium restriction to reduce the incidence of cardiovascular events is more controversial. Some of the largest studies such as NHANES and TOHP, which do demonstrate a significant benefit, report a 20–30% relative reduction in adverse events which, due to the low rate of these events equates to an absolute risk reduction over 10–20 years in the region of 2–3% for protection from adverse cardiovascular events from sodium dietary restriction.
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Abstract
1. Introduction