A modest reduction in population salt intake worldwide would result in a major improvement in public health—similar to the provision of clean water and drains in the late nineteenth century in Europe.
—Feng J. He, Graham A. MacGregor1
Confused by the competing claims? I hope not, but then again, I wouldn’t be surprised if you were. Literally thousands of scientific articles have been published on salt, high blood pressure, and cardiovascular disease, so it can be confusing to dive deep into that sea of research, especially when the findings and the opinions of experts conflict so radically. And then the news media, websites, social media, and blogs, which sometimes have strong biases, convey some of that information to the public.
So how do expert committees reach their conclusions in the cases for and against sodium? Typically, by weighting a mass of diverse evidence by strength. Table 4.1 does just that by summarizing the strength of key research in support of or opposed to reducing sodium in the American diet.
In chapter 1 we saw that sodium intakes in human populations vary dramatically, from astonishingly small amounts by isolated, subsistence tribes, to excessive amounts in most industrialized countries, to enormous amounts in Turkey, parts of China, Japan, and certain other countries. But, bottom line, humans appear to be able to live quite well, virtually without hypertension or cardiovascular disease, with diets containing as little as a hundred milligrams of sodium per day—which is far less than the 1,500 to 2,300 mg of sodium recommended by the US Department of Health and Human Services, World Health Organization, and other public health authorities. Of course, it would be nearly impossible—and unnecessary—for healthy people living in a modern culture, with the temptation of salty prepared foods at every turn, to consume just a few hundred milligrams of sodium in a day.
Table 4.1
Strength of evidence for and against reducing sodium intake, rated from 1 (low) to 5 (high)*
| Key Evidence for Reducing Salt (strength) | Key Evidence for Not Reducing Salt (strength) |
|---|---|
|
• Controlled trials show that consuming more sodium raises blood pressure (5) • In trials and observational studies, higher blood pressure increases the risk of cardiovascular disease (5) • Limited trials found that lowering sodium reduces the risk of cardiovascular disease (2) |
• Some observational studies (based on NHANES, PURE, others) found that low sodium intakes were associated with higher mortality (1) • In limited trials, low-sodium diets together with restricted fluids and diuretics increased mortality in patients with heart failure (1) |
*Based on the number and quality of studies.
Chapter 2 provided clear-cut evidence that blood pressure rises as sodium consumption increases, more so in African Americans and people with hypertension and less so in younger people with normal blood pressure. Overwhelming evidence also supports a second fact: the higher the blood pressure, the greater the risk of heart attacks and strokes.
Experts have long connected those two undisputed facts to conclude that the risk of cardiovascular disease increases as sodium intake increases and that the general population should consume less sodium. While that logic is not airtight, trials on salt and cardiovascular disease strengthen my confidence that the relationship is real.
The Trials of Hypertension Prevention (TOHP) and a large trial at a veterans home in Taiwan found that lower sodium intakes were accompanied by lower risks of cardiovascular disease. Further support for lowering sodium comes from the United Kingdom, Finland, and Japan, where government campaigns to lower sodium intakes were associated with fewer, not more, deaths due to cardiovascular disease—though other factors, such as lower smoking rates, might have contributed to the benefit. But as critics of lowering sodium argue, none of those studies demonstrated benefits from slashing daily consumption to 2,300 mg or less per day.
Contradicting most expert authorities, as we saw in chapter 3, are researchers who disagree that consuming less salt leads to less cardiovascular disease. Those researchers agree that consuming more than 5,000 mg of sodium per day increases the risk of heart attacks and strokes—but contend that intakes lower than 3,000 mg per day also increase the risk. With most Americans (and people in many other countries) consuming about 3,000 to 4,000 mg per day, they argue that lowering average sodium intakes to 2,300 mg or less per day could be deadly. The National Academy of Medicine (NAM) and others, however, found that the reports suggesting harm suffered from disqualifying limitations, including inaccurate measurements of sodium intakes and reverse causation, and had a “high risk of bias.”2 In 2016, Frieden, then the director of the Centers for Disease and Control, said those reports have created a “false aura of scientific controversy around dietary salt,”3 and the NAM summarily dismissed them.
The argument boils down to interpreting a voluminous body of research, deciding where the balance of the evidence lies, and then judging how much evidence is enough to advise the public and call on companies to lower sodium levels. In the case of salt and cardiovascular disease, researchers have a rich lode of evidence to evaluate, including animal studies, comparisons of people in different countries and of people who migrated from one region to another, studies of aboriginal peoples, and controlled trials, most of which points toward cutting salt intakes.
Most of the world’s leading health organizations and cardiovascular disease experts have concluded that a large number of increasingly refined studies have established that lowering sodium would yield huge health and economic benefits. Public health advocates warn of the risks of waiting for perfect proof before taking action based on the weight of evidence. In 1968 former surgeon general William H. Stewart said, in reference to controlling noise pollution: “Must we wait until we prove every link in the chain of causation? . . . In protecting health, absolute proof comes late. To wait for it is to invite disaster or to prolong suffering unnecessarily.”4 Similarly, in 1986, when the Princeton historian Theodore Rabb testified in Congress about the ozone hole, he reminded legislators: “Scientists are never 100 percent certain. . . . That notion of total certainty is something too elusive ever to be sought.”5
Rabb also might have reminded legislators of a scientific aphorism popularized by Carl Sagan: “Extraordinary claims require extraordinary evidence.” To effectively challenge the informed wisdom that reducing sodium levels would yield enormous health benefits, skeptics need much more than a small number of studies that are flawed and effectively rebutted by other research.
Almost all authoritative health organizations and academic experts agree that the evidence for lowering sodium—by individuals and throughout the food supply—greatly outweighs the evidence that lowering sodium would be harmful or without benefit. Every year that we—consumers, health officials, and the food industry—fail to reduce sodium consumption condemns many thousands of people to premature illnesses and deaths and unnecessary costs. The next chapter provides a portrait of one of the causes of that failure.