How Low Should You Go? Experts Debate Latest Salvo in the War on Sodium

May 16, 2013

SAN FRANCISCO, CA — Earlier this week, the Institute of Medicine (IOM), after conducting a comprehensive review of the scientific literature, stirred up some controversy by concluding that the data are insufficient to recommend lowering sodium levels to what is currently suggested in federal dietary guidelines[1].

While the IOM said there is ample evidence supporting a positive relationship between higher levels of sodium intake and the risk of cardiovascular disease, which is based on past research showing the effect of high levels of sodium consumption on blood pressure, there are no advantages to lowering intake among individuals who consume a moderate amount of sodium.

In fact, the IOM went so far as to state that the evidence is not strong enough to recommend lowering daily sodium intake to the 1500- to 2300-mg/day range.

A number of organizations, including the American Heart Association (AHA), American Medical Association, and the American Public Health Association, support the reduction of dietary sodium, with the AHA stating that Americans should aim for 1500 mg of sodium daily. The 2010 Dietary Guidelines for Americans recommends a daily sodium intake of 2300 mg for the general population and 1500 mg for individuals 51 years of age and older, African Americans, or individuals with hypertension, diabetes, or chronic kidney disease.

So if you're between 2000 and 2400 mg of sodium, you're good to go.

A day after the report came out, the AHA criticized the IOM document, calling it "incomplete" and stating that it failed to take into account the vast evidence linking high blood pressure to excess sodium consumption. Moreover, the AHA said that blood pressure is a suitable proxy for hard cardiovascular disease outcomes.

The IOM report was released just a day before the kickoff here of the American Society of Hypertension (ASH) 2013 Scientific Sessions . Dr Suzanne Oparil (University of Alabama, Birmingham), a one-time president of the AHA and ASH, told heartwire that she agrees with the IOM report, stating that the drive to lower salt consumption as much as possible has been an easy-to-digest public-health message.

"It was never based very much in science," said Oparil. "The thinking was that we should advise people to do it because it's something that's relatively easy to do and it can do no harm. What's new now is that there are studies suggesting that severe salt restriction over time can be harmful. The evidence isn't perfect, because of concerns about reverse causality--people who are eating less salt might be eating less food, and they might be eating less food because they are sick to start. We have to acknowledge that, but still, there isn't evidence that going lower and lower with salt intake has benefits."

Interpreting the Message

To heartwire , Dr Brian Strom (University of Pennsylvania, Philadelphia), the chair of the IOM writing committee, said that individuals who consume large amounts of sodium, particularly those who continue to eat a large quantity of fast and processed foods, or individuals with hypertension, should aim to lower their sodium intake. However, there is no evidence to support lowering sodium levels beyond a "moderate range" for the reduction of hard cardiovascular events, such as MI, stroke, or mortality.

Dr William White (University of Connecticut Health Center, Farmington), the current ASH president, said that while he too doesn't dispute the conclusions of the IOM, there is a concern in how the public responds to the new message. "There is a potential harm that people will make some uneducated assumptions that they no longer need to worry about salt in their diet," White told heartwire .

Strom said the IOM is also concerned about such an interpretation, stressing that the report has two messages: the first being that there is a positive relationship between high levels of sodium intake and the risk of heart disease and the second being that there are inconsistent data showing that lowering sodium intake below 2300 mg per day reduces this heart-disease risk. The lack of studies linking sodium intake to an increased risk of hard events has long been a topic of contention. A number of modeling studies have estimated that excess salt intake is responsible for millions of deaths worldwide, as well as increased healthcare costs.

A Test of Political Wills

Speaking with heartwire , Dr George Bakris (The University of Chicago School of Medicine, IL), an expert in hypertension and kidney disease attending the ASH meeting, referred to the opposing opinions between the AHA and IOM as "political" differences. He agreed with the IOM data and had high praise for the researchers who conducted the review. He notes, however, that another IOM report within the last year also supported a very low sodium diet, but this different group, led by Strom, provided a new interpretation of the data.

There isn't evidence that going lower and lower with salt intake has benefits.

"I never prescribe less than 2 g of sodium for anybody," said Bakris. "For one, they won't eat it. And number two, a level teaspoon of salt is about 2400 mg, and 2400 mg is the original DASH diet. So if you're between 2000 and 2400 mg of sodium, you're good to go. There is no meaningful additional benefit going below that, and if you do there is potential harm. I fully agree with the IOM findings. I have no problems with their conclusions."

Bakris echoed Strom and White, noting the fear is that if clinicians ease up on sodium restrictions the floodgates would open, with individuals now feeling free to add dangerous amounts of salt to their diet. Most important, Bakris told heartwire that the notion that a low-sodium diet will prevent the development of hypertension is completely false. A low-salt diet will delay elevations in blood pressure, but it will not completely eliminate them, he said. Hypertension is a genetic disease affected by environmental factors, and a patient with a strong family history of hypertension will develop elevations in blood pressure sooner than those without a family history.

"Hypertension is a disease of aging accentuated by the patient's genetic load," said Bakris. "Excessive salt consumption along with a genetic load accentuates the disease even more."

Eating at McDonald's? Reduce Your Salt!

The IOM report, which was requested by the US Centers for Disease Control and Prevention, even hinted at possible harm with reducing sodium levels to very low levels. For example, there is evidence that suggests low sodium intake might lead to risk of adverse cardiovascular effects among patients with mid- to late-stage systolic heart failure who are receiving aggressive treatment for their disease. There is also limited evidence showing any benefit of aggressively low sodium intake and clinical outcomes in various subgroups, including those with diabetes, chronic kidney disease, the elderly, and African Americans. The IOM states that the evidence is not strong enough to treat any of these subgroups differently from the general population.

"Realistically, 1500 mg per day is very difficult to achieve," Strom told heartwire . "Less than 1% of the population consumes that much sodium on a daily basis. It's not an easy diet to have. Therefore, to have such a goal as a national target doesn't make a lot of sense."

The initial recommendation to lower sodium levels to 1500 mg for the reduction of blood pressure was derived from highly controlled clinical trials, Bakris told heartwire . Even then, the reductions in blood pressure were small, and while this might be beneficial on a population level, the impact on the individual would be negligible. "Moreover, if you take in that small amount of salt, you're much more vulnerable to changes in volume," he added. "If you're in a hot climate, if you sweat more, you are more likely to become volume depleted, and if you become volume depleted you're much more susceptible to kidney injury."

To heartwire , Oparil said that it is nearly impossible for individuals to keep track of sodium consumption when preparing meals. "If you weigh 400 pounds and you're going to McDonald's and Kentucky Fried Chicken all the time, then you know you're taking in too much salt," said Oparil. "The campaign against conventionally prepared foods is also fair because there are huge amounts of salt in these processed foods."

While very high salt intake does tend to increase blood pressure, Oparil noted that Japanese individuals have some of the largest intakes of sodium but also have one of the lowest risks of cardiovascular disease. "It's an oversimplification, and it's a bad health message to simply try to get everybody to consume 1500 mg of sodium per day."

Heart-Failure Patients to Follow Low-Sodium Diet

White said that patients with heart and renal failure are generally recommended to consume less salt because it is felt they "could get into trouble" if they take in too much. Plasma volume expansion and hypertension are far more likely to occur when patients are eating a high-salt diet than a moderate-salt diet. For patients with hypertension or heart failure, he advises to not add salt to meals and attempt to eat fresh or frozen foods. He advises patients to consume 3 to 4 g of total daily sodium, including those with hypertension, but tries to get heart-failure patients down to 2 g of sodium per day.

Realistically, 1500 mg per day is very difficult to achieve.

"When you talk to patients with hypertension-related comorbidities such as kidney disease and heart failure, the thing I always tell them is that adding salt is only the smallest source of sodium in your diet," said White. "The biggest source is what's already in the food you're eating, particularly if it's processed or packaged. So we ask our patients to read the labels. Most of them don't realize that if they eat four pieces of pizza there's 2 or 3 g right there."

Bakris said that systolic heart failure patients have low ejection fractions and are susceptible to volume overload. These patients do need a low-sodium diet. However, because of their use of diuretics, clinicians need to be on guard against hyponatremia.

Oparil reports research support from AstraZeneca, Duke University, Merck, the National Heart, Lung, and Blood Institute, Novartis, Takeda, and Medtronic. She receives other research support from Daiichi-Sankyo, Medtronic, and Vivus and serves as a consultant/advisor to Backbeat, Bayer, Daiichi-Sankyo, Medtronic, Novartis, and Pfizer. Bakris is an advisor/consultant to Takeda, Abbott, Johnson & Johnson, Daiichi-Sankyo, GlaxoSmithKline, Boehringer Ingelheim, Medtronic, and CVRx. He receives grant/research support from Forest Labs, Takeda, Medtronic, and CVRx. White serves as an advisor/consultant to AstraZeneca, Forest Research, Palatin, Roche, and Takeda. His research is funded by the National Institutes of Health only.


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