Crystal death ?
A low-salt diet is a key to better health, and salt -- mainly from processed foods, helped by the old salt shaker -- is killing us, says the NHLBI and an influential wing of scientists who study hypertension, or high blood pressure.

"The preponderance of the evidence indicates that there's a relationship between salt and blood pressure," says Edward Rocella, coordinator of the National Blood Pressure Education Program at the NHLBI. That was the message from a January, 1999 meeting sponsored by NIHLB and published in LINK April 2000 Hypertension.

Who should restrict salt intake?
-60 percent of hypertensives = 36 million

-children of hypertensives = 36 million

-patients with heart failure = 6 million

-kidney disease = 6 million

-diabetics = 10 million

-TOTAL = 94 million

The meeting was called to respond to some of the criticism we'll be covering. Our bodies need about 500 milligrams of sodium per day, Rocella says, equivalent to 1250 milligrams of salt, yet Americans eat between 2000 and 8000 milligrams of sodium daily. The program recommends 2400 milligrams, he says. "Twenty-four hundred milligrams is a lot of salt. We're not being abolitionist, not being teetotalers. Why do you need to put so much on?"

By cutting down on salt, according to this argument, millions of Americans would live longer, healthier lives. Heart attacks would decline, likewise strokes. We'd even see a reduction in deadly kidney diseases, since the delicate blood vessels in the kidney are damaged by high pressure, leading the blood to fill with junk chemicals.

Blood pressure is measured by two numbers: The first, called "systolic," measures pressure in the artery while the heart pumps. The second, or "diastolic," measures pressure between beats. The numbers are listed as "systolic/diastolic," pronounced, "120 over 65." Hypertension is usually defined as a sustained pressure above 140/90.

More evidence for a relationship between salt and hypertension comes from research by Denton, who fed chimpanzees a low-salt diet for a year, then raised the salt level in half of them for another year, until they were getting 12 grams of salt per day, as much as a human salt freak. Systolic blood pressure rose 33 millimeters, he says, and diastolic, about 10. Once the salt was withdrawn, the pressures moderated into the normal range. "It was quite a striking indication of a specific role for salt in blood pressure in these animals," says Denton, who observes that chimps are our closest relatives.

High-pressure headaches
Continuous high blood pressure causes disease by stressing the entire cardiovascular system:

The heart must work too hard, causing its muscle to expand and eventually get weak and flabby.

High pressure causes tiny breaks in arteries, and the natural repair process accelerates hardening of the arteries, which can impair blood circulation or cause strokes, complete blockages that starve the brain of oxygen.

Poor blood circulation can damage the eye, kidney, brain and other critical tissues.

Obese people are particularly likely to have hypertension, because each pound of fat must be served by miles of tiny arteries, all of which increases the pressure by raising resistance to the heart's pumping. Ditto for the elderly, whose arteries are already damaged by other effects of aging.

Despite the public-health obsession with sodium, the system that regulates blood pressure depends on many other factors, including other nutrients, genetics, hormones and the environment. Curiously enough, while high sodium concentrations do raise blood pressure in many people, they can also have the opposite effect. And that's only one reason salt studies are repeatedly shaken up by new interpretations of the data.

How long has this been going on?

Why add salt to the wound? The concern over sodium dates back to World War II when Walter Kempner of Duke University treated severe hypertension with a low-salt diet of rice and fruit. While many scientists credited the reduction in sodium for the success, the high-potassium, low-fat diet probably also played a role. Processed foods are a major source of sodium. This item contains close to half the recommended daily sodium intake. In the 1960s, Lewis Dahl of Brookhaven National Laboratory studied a rat that got hypertension with a high-salt diet. Dahl persuaded many scientists that high sodium caused hypertension, but critics later noted that, if scaled to human size, the rats would have been eating the equivalent of 500 grams of salt a day. That's beyond the famous potato-chip diet �?we're thinking pure road salt! And the rats were missing some of their kidney mass, so they were abnormally slow to get rid of extra sodium. But for a public eager for dietary advice, and a medical profession becoming more and more eager to dispense it, the sodium results were compelling enough. In 1979, the U.S. Surgeon General's report was the first blanket recommendation to reduce dietary salt. How convincing was the evidence at that point? Pretty flimsy, charges David McCarron, a persistent critic of the salt hypothesis from Oregon Health Sciences University. In 1979, he writes, the only "information that was supported by valid scientific data regarding the role of salt in blood pressure control was: Excessive sodium intake produced blood pressure increases under extreme conditions. In patients with renal [kidney] disease, blood pressure decreased with severe sodium restriction. Animal studies suggested a possible genetic link. McCarron says that because this data supported the expected relationship between sodium and hypertension, the recommendation was made despite the lack of answers to "a vast number of issues that bear far more heavily on the relevance and rationality of national dietary recommendations," including: Careful studies of the relationship between salt intake and blood pressure. Randomized controlled studies of sodium reduction in people. Ongoing studies of whether salt intake was related to cardiovascular diseases and deaths. Information about the interactions of multiple nutrients. In other words, the argument was inconclusive. What did the largest salt study find?

The last word?

By the early 1980s, the stage was set for a prolonged and expensive query into the role of dietary sodium. When it began in 1984, the Intersalt study was expected to provide the final word on salt and health.

With 10,000 participants from 52 communities around the world, it certainly had the potential to find a relationship between salt intake and blood pressure. More than anything, however, the study proved that salt's health impact remains elusive, debatable and sometimes contradictory.

As they expected, the Intersalt researchers found that higher salt intake was associated with higher blood pressure �?individuals and societies with higher average intake also had higher pressure.

But when considering societies as groups, the relationship broke down. True, when four primitive societies with extremely low salt intake and extremely low blood pressure were included, the relationship existed. But when they were excluded, the other 48 study groups had no relationship at all.

Does this prove that sodium contributes to hypertension? Perhaps. But blood pressure and salt intake were only two differences between the four societies and the other 48. The four had less stress, less obesity, and ate far less fat and processed foods, and a raft more fiber. And on average, they died too soon to get much cardiovascular disease.

Mining salt data?
Given these considerations, critics charge that Intersalt actually backfired because the four societies with low salt and low blood pressure were too dissimilar to be compared with the other 48. "I cannot calibrate you to someone in a Brazilian tribe," says David Freedman, a statistician at the University of California at Berkeley. "This is apples and string beans, it's not comparing apples and oranges."

"Intersalt expected to find that high-salt centers had high blood pressure," Freedman continues, "and that's true in a crude sense. But the low-salt centers are very different. Blood pressure does not rise with sodium in the other 48 �?in fact, as sodium increases, blood pressure decreases. The result in the 48 centers is exactly the opposite of what the Intersalt investigators expected to see," Freedman contends, and "the primary hypothesis of Intersalt was flatly contradicted."

The Intersalt investigators then reanalyzed their data and announced that eating salt was correlated with a faster rise of blood pressure with age. Yet Freedman argues that this question was not an original goal of the research, and therefore is a case of "mining the data" to prove preconceptions rather than hypotheses.

To Freedman, the message is simple: "You should not be drawing dietary advice from this type of analyses." The Why Files tried to ask Intersalt investigator Jeremiah Stamler at Northwestern University about the study, but he didn't call back.

Curiously, the Intersalt study SEEB (see "Intersalt: An International ... " in the bibliography), was followed in the journal by a study of 7354 people in Scotland (see "Urinary Electrolyte Excretion... " in the bibliography)

What about getting sick and dying?

Salty? Sure. Dangerous? Maybe.

The real goods
You could argue all day whether salt intake increases blood pressure in populations, but what does it do to rates of disease and death? Grim says a Dutch study that "feeding kids from birth to six months a high-salt formula made from cow's milk (compared to normal human milk) increased blood pressure by six months, and this persisted to age 15."

However, critics of the salt hypothesis have long charged that there was no evidence that lowered salt intake would improve health or average lifespan. Then, in December, 1999, scientists reported precisely that association.

According to the study (see "Dietary Sodium Intake and Subsequent Risk..." in the bibliography), people eating the most salt had 32 percent more strokes (and 89 percent more stroke deaths), 44 percent more heart-attack deaths, and 39 percent more deaths from all causes.

Those are big numbers, particularly since they came from a large, long-term study, a follow-up on the National Health and Nutrition Examination Survey (NHANES I), which began tracking 20,729 Americans in 1971.

The report was a landmark in the long struggle over salt: "Our study is the first to document the presence of a positive and independent relationship between dietary sodium intake and cardiovascular disease risk in adults," said Jiang He, the lead author. The study not only had a large sample size, but it followed subjects for an average of 19 years �?time for plenty of people to get sick or die and provide hard data on the relationship between salt and death and disease.

(The emphasis on "hard end points" �?like death and disease �?is exactly the prescription of salt skeptic Alderman, an epidemiologist at the Albert Einstein College of Medicine in New York. Most of the sodium debate, he says, concentrates on blood pressure, which is only a "surrogate" for the real goals �?staying alive and healthy. Drugs that improve surrogate measures don't always improve the underlying disease, Alderman notes.) But there were some significant catches with the NHANES data: The increased illness and death occurred only among the overweight. The researchers wrote that "Dietary sodium intake was not significantly associated with cardiovascular disease risk in nonoverweight persons."

Obesity and hypertension
Although such a finding might seem to undermine a campaign for universal sodium restriction, Paul Whelton, a primary author of the study, said, "Our findings support existing recommendations for moderate reduction in salt intake, aimed at reducing blood pressure and cardiovascular disease risk in the overweight adult population."

Whelton, senior vice-president for health sciences at Tulane University, adds that "Because recent data show that 34 percent of the American population is overweight and most of us consume too much salt, these recommendations are of considerable importance to the individual and to our society as a whole."

But beyond exonerating salt among people with normal weight, the study had other catches. First, Whelton acknowledges, the NHANES research "was not specifically designed to answer" the question of sodium and health. Second, the entire study depended on one 24-hour recall of salt intake. "At best, the estimate for sodium is imperfect," Whelton admits. It's far better to gauge salt intake by the presence of sodium in urine, where excess sodium winds up. Collecting urine for 24 hours, Whelton says, is the "gold standard" of sodium research. Curious wording, but accurate...

A contrary view
The last catch is the biggest: When epidemiologist Alderman analyzed the same NHANES data, he concluded that the more salt you eat, the less likely you are to die! Although Alderman analyzed the same basic data, he selected a different number of subjects, cardiovascular deaths and total deaths (see "Dietary Sodium Intake and Mortality..." in the bibliography).

Alderman wrote us that five studies have now tried to link sodium intake to outcomes. "In two, no association between dietary sodium intake and morbidity was found, in one study (see 'Dietary Sodium Intake and Subsequent... ' in the bibliography), it was found that the minority of subjects who were obese had increasing mortality with increasing sodium, but in the non-obese minority, no association was seen, and in two studies an inverse sodium-to-outcome relationship was found."

In a study in the last category, Alderman says, "We looked at the relationship between sodium and ultimate outcome in hypertension with 3,000 patients in New York, and found that those who got the least sodium had the most myocardial infarctions [heart attacks] and cardiovascular disease" (see "Low urinary sodium..." in the bibliography).

In other words, and this may be the most disturbing part of the salt debate: Here's evidence that reducing salt can kill. Alderman says reducing sodium intake can reduce the responsiveness to insulin, raise levels of renin, a hormone that raises blood pressure. In one trial, it even reduced men's sexual activity.

To Alderman, the positive association between sodium, obesity and deaths actually indicates that "the effect of sodium on human health varies in different people." For the non-obese majority, he says, "there was no relation between sodium and outcome."

Backwards to the future?
The unexpected opposite relationship between salt and disease and death is not a reason to go out and suck salt tablets, but, Alderman says, "This suggests a low-sodium diet may not be a good thing."

Beyond undermining the universal prescription for sodium reduction and indicating that (gasp!) high sodium intake might offer health benefits, the Alderman study also shows that the same data can produce opposite conclusions depending on how subjects are chosen and their deaths are analyzed.

Thus as modern medicine probes such lifestyle issues as exercise and diet, the deadly dull and utterly inexplicable disciplines of statistics and epidemiology play a critical role. Need help distinguishing lies, damn lies and statistics?

What happens to blood pressure when you cut salt intake?