An Easy Measure of Potentially Harmful Salt Intake
February 28, 2023
By Michael H. Crawford, MD
Professor of Medicine, Lucy Stern Chair in Cardiology, University of California, San Francisco
SYNOPSIS: Those who rarely or never added salt to their food and strongly adhered to the Dietary Approaches to Stop Hypertension diet exhibited the lowest incidence of subsequent cardiovascular disease.
SOURCE: Ma H, Wang X, Li X, et al. Adding salt to foods and risk of cardiovascular disease. J Am Coll Cardiol 2022;80:2157-2167.
It is well established that high sodium intake is a major cause of hypertension, which is a significant risk factor for developing cardiovascular disease (CVD). However, the relationship between dietary sodium intake and the risk of CVD is not as clear. Part of the reason for this could be attributed to the difficulty in accurately measuring long-term sodium intake. Can the frequency of adding salt to food be a marker for more sodium intake generally?
Ma et al evaluated the relationship between the frequency of adding salt to food and CVD in a prospective cohort of subjects in the United Kingdom (UK) Biobank. In addition, the authors explored adherence to the Dietary Approaches to Stop Hypertension (DASH) diet and the frequency of adding salt to food in relation to CVD. The more than 500,000 UK Biobank subjects were recruited between 2006 and 2010. Those who completed at least one web-based dietary assessment were targeted for this study.
After excluding those with incomplete data, implausible data, or those with CVD before the last dietary assessment, 176,570 subjects were included. In stored study intake urine samples, researchers measured sodium concentrations and used those data to estimate 24-hour sodium excretion. The authors collected information on CVD events through the UK National Health System (NHS) and the UK death register. Total CVD events included ischemic heart disease (IHD), stroke, and heart failure.
After a median follow up of 12 years, there were 9,963 CVD events, the majority of which were ischemic heart disease (IHD). Compared to those who always added salt to foods, those who rarely/never added salt to foods recorded a lower frequency of CVD (adjusted HR, 0.77; 95% CI, 0.73-0.90; P < 0.001), with a graded response in between for those who usually (HR, 0.81) or sometimes (HR, 0.79) added salt to food. The strongest association for adding salt to food was found with heart failure, followed by IHD. The frequency of adding salt to food was not associated with stroke. Also, those who adhered to the DASH diet and rarely/never added salt to food were at the lowest risk for developing CVD (HR, 0.64; 95% CI, 0.56-0.73). The estimated 24-hour urinary sodium excretion confirmed the dietary questionnaire findings.
Interestingly, those with the lowest frequency of adding salt to food consumed more fruit, vegetables, nuts, whole grains, and low-fat dairy products, but less sugar-sweetened beverages and red or processed meats. In addition, the association of adding salt to foods with CVD risk was greater in those with lower socioeconomic status and current smokers. The authors concluded a lower frequency of adding salt to foods was associated with a lower risk of developing CVD, especially heart failure and IHD, and was additive to the beneficial effects of the DASH diet.
COMMENTARY
It is difficult to accurately measure sodium intake. Using a 24-hour urinary sodium excretion is helpful. Dietary questionnaires are common, but the results can be inaccurate. The novel feature of the Ma et al study is a simple question: How often do you add salt to your food? The authors postulated that this question on a semi-quantitative basis (always, usually, sometimes, rarely/never) would correlate with subsequent CVD events, and that adherence to the DASH diet would be of additive benefit. Not only did they demonstrate this, but Ma et al confirmed the accuracy of the salt frequency question — specifically, by measuring spot urine sodium levels to estimate 24-hour excretion by recognized estimation methods. That the association is strongest for heart failure and IHD is not surprising, as excess sodium can set off a chain reaction: fluid retention, which could precipitate heart failure and cause hypertension, which is a major risk factor for developing IHD. The lack of correlation with stroke is surprising, but the authors noted the number of strokes in the population studied probably was too low to make meaningful conclusions.
Despite these positive results, there are limitations worth considering. The salt intake data were self-reported, which could mean information bias. The frequency of salt intake was associated with other beneficial dietary habits and lifestyle choices, which may have played a large part in the reduction of CVD events. The study was observational and subject to residual confounding. Finally, this work was conducted among a UK population and may not apply to other groups.
Considering the difficulty of determining sodium intake in patients, the simple semi-quantitative question of the frequency of adding salt to food could be a surrogate for a more detailed dietary history or 24-hour urinary sodium measurements. Although table salt represents only a fraction of the sodium in the American diet, the frequency of its use may identify other poor dietary habits and lifestyle choices, as it did in the Ma et al study. I am incorporating this approach into my routine patient evaluation and counseling patients on the benefits of reducing or eliminating added salt in the diet.
Those who rarely or never added salt to their food and strongly adhered to the Dietary Approaches to Stop Hypertension diet exhibited the lowest incidence of subsequent cardiovascular disease.
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