Some low-salt treats.
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."
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...
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."
to the future?
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?
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