Lowering sodium intake has been a constant in dietary guidelines for decades and is widely recommended as part of healthy eating advice. The conventional view is that eating too much sodium increases blood pressure and thereby increases the risk for cardiovascular disease. Yet the benefits of sodium reduction are being hotly debated with experts around the globe trading arguments in journals, on-line debates and the mainstream media. What’s driving this debate?
In November 2011, O’Donnell and colleagues published a study on the relationship between urinary sodium and the risk for cardiovascular disease. Urinary sodium is a good way to estimate how much sodium people are eating as, generally speaking, ‘sodium in equals sodium out’. As higher sodium intake is known to be linked with higher blood pressure, the expectation might have been that the higher the sodium excretion, the higher the risk for cardiovascular disease. However, the relationship turned out to be J-shaped. In other words, high intakes of sodium were indeed associated with higher risk for cardiovascular disease, but so were low intakes of sodium. How can that be? Does eating too little sodium cause harm?
Just another essential nutrient?
In an on-line debate, Professor Michael Alderman from the Yeshiva University in the USA argued that the links between dietary sodium and health were similar to those of other essential nutrients – harm at low levels of intake (deficiency), optimal for health over a wide range of normal intakes, and harm at high intakes (toxicity). He argued that the notion that lowering sodium intake lowered risk for cardiovascular disease was simplistic and unproven. While not disputing that sodium intake was linked to blood pressure, he noted that sodium intake had many other effects on metabolism, neural systems, hormonal systems, insulin resistance and triglycerides. Some of these effects may have adverse implications for the risk for cardiovascular disease and could offset the beneficial effects on blood pressure. Alderman argued that intervention trials assessing the effects of lower sodium intakes on cardiovascular events were required.
An alternative view
In the same on-line debate Professor Lawrence Appel of DASH Diet fame argued that an intervention trial of sodium and cardiovascular events would never be conducted. It would require too many people, be too long and too expensive. However, he noted that the Trials of Hypertension Prevention (Phase 3) had shown a significant 25-30% fall in cardiovascular events in those who had participated in earlier studies to lower blood pressure by sodium reduction. And in the study by O’Donnell and colleagues higher intakes of sodium were associated with all measures of cardiovascular disease assessed – myocardial infarction, stroke, hospitalisation with congestive heart failure and cardiovascular death.
But what about the apparent increase in cardiovascular risk at low urinary sodium? Appel suggested that some of the apparently low intakes of sodium could actually be due to ‘under collection’ and had given rise to a spurious relationship. Others have suggested that perhaps these subjects were eating low sodium diets on purpose because they had cardiovascular disease and had been advised to adopt a low salt diet. If so, is it any surprise that they these sick people had more cardiovascular events? In other words, the low sodium intake may have been associated with their ill health but was not the cause of it. Maybe those with low sodium intakes just had low calorie intakes. If so, why? Again, were they sick?
Trends in sodium intake: more debate
There are no good data on trends in sodium intake in Australia though a recent US study provided some interesting insights and generated further passionate debate. Harvard researchers Berstein and Willett, reviewed a variety of US studies carried out in the past half century in which 24-hour urinary sodium excretion had been measured. They found there has been little change in sodium consumption over this period, findings that mirrored those of a similar study conducted in the United Kingdom. Surprisingly, the mean sodium excretion in both countries was almost exactly the same and also similar to those observed in the international INTERSALT study, despite the wide differences in food cultures.
As the period of investigation covered a time in which there was a significant shift from food preparation in the home to the consumption of more processed and fast food, the results came as a surprise. Isn’t processed and fast food supposed to be full of salt? The other take-home message from this study was that the rise in the prevalence of hypertension in the US is not being driven by changes in sodium intake. The likely cause is the rising prevalence of obesity.
An accompanying editorial took a very different line. Faced with the narrow range of urinary sodium levels and the lack of change over time, the authors suggested that human sodium intake may be a parameter that even the most well intentioned public policy cannot modify in most people. It was suggested that such policies are in fact ‘doomed to failure’. The authors cite sodium’s fundamental role in the regulation of extracellular fluid volume and the likelihood of failsafe mechanisms to ensure sufficient sodium availability. Indeed, when sodium intake falls below 120mmol/day a hormonal system switches on, reabsorbing more sodium from the urine. They argued that any recommended intakes below this level assume that the basic biology of humans can be ignored. The authors suggest that the range of sodium intakes in the US, between 120-210mmol/L should be considered ‘normal’. Current dietary recommendations are 100mmol/day.
Dietary sources of sodium in Australia
Researchers from Deakin University recently published a paper outlining the major sources of sodium in the diets of Australian children, based on the Australian 2007 Children’s Nutrition and Physical Activity Survey. Table 1 shows the data for 14-16 year olds.
Making sense of the sodium debate
I enjoy debates like this. It’s the stuff of science – established principles and policies being challenged and discussed as new data are published. There is seldom absolute certainty in nutrition as the ideal studies are often impractical to conduct. We are left to form a view based on the various lines of evidence that are available. But it has to be said that the evidence linking salt intake with blood pressure and that linking blood pressure with cardiovascular disease remains strong. Even if a reduction in population sodium intake to 100mmol/L may be out of reach at present, any fall in the current mean intake would be expected to bring benefit. It is not impossible that low sodium intakes have adverse health effects but these will have to be demonstrated, not assumed from epidemiology.
Table 1: % sodium intake by food category (14-16 year old children)
Food category % of total sodium intake
Cereal-based products and dishes 23.6
• Biscuits, cakes, pastries, etc (10.9)
• Pizza, hamburger, savoury noodles, etc (11.3)
Breads and cereals 20.2
• Breads, muffins, etc (16.1)
• Breakfast cereals/bars (4.7)
Meat, poultry, fish 16.7
• Sausages, processed meat (9.4)
Milk products 8.9
• Cheese (4.0)
Savoury sauces/condiments 8.4
Vegetable products 4.3
Snack foods 2.9
Fats, oils 1.1