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Assault on the System

Michael Glenwood

Assault on the System (continued)

Danger in a World of Plenty

In 1990, Chinese-born physician Jiang He approached Johns Hopkins' School of Public Health with a remarkable story.

For several years, he had been traveling to the remote mountains of southwestern China to study the Yi people, an ethnic minority for whom hypertension—very common elsewhere—was almost unknown. "The mean blood pressure for Yi farmers in their 60s was close to the mean for Yi farmers in their 20s," He remembers. The young researcher was invited to the School to complete his PhD, and produced a series of papers on the Yi, with epidemiologist Paul Whelton, MD, MSc, and then junior faculty member Michael J. Klag, MD, MPH '87.

The Yi were one of the extended tribes that in effect had been chased into the mountains 2,000 years before by the dominant Han ethnic group. In modern times, they had continued to live their traditional lifestyle, and as He had found, their diet was very low in sodium. He and his colleagues at Johns Hopkins subsequently compared the Yi farmers' blood pressures and sodium intakes—measured by collecting urine from subjects for a sample 24-hour period—with those of Yi migrants to lowland cities. It turned out that the Yi migrants, after exposure to a more modern, urban lifestyle, had begun to consume sodium and develop hypertension at rates like those seen for other Chinese.

"This Yi migrant study influenced my career," says Klag, now dean of the Bloomberg School. "It strongly suggested that our high rates of hypertension come from a real mismatch between how we evolved and how we live today."

For nearly all of the period in which modern humans evolved, sodium intake had been a small fraction—on the order of 100 mg daily—of what it now averages in modern diets. Only in the past few hundred generations (or past few generations, for some) had salt been used as a flavor enhancer and preservative. As for so many other things in modern life, a strong pleasure-sense that had evolved to steer us toward a scarce necessity now had come to endanger us in a world of plenty.

Convincing Evidence

Sodium is one of the essential elements used by cells, and its concentration in the human bloodstream is meant to be kept within a narrow range. When excess dietary salt enters the blood, more water is drawn into the bloodstream to compensate—which is why a salty meal is apt to make us thirsty. This increase in blood volume tends to raise blood pressure, especially for those whose blood vessels are less flexible. The kidneys can push blood pressure down again by filtering excess sodium from the bloodstream and excreting it via urine, but kidney function also tends to decline with age—and with hypertension—so that a chronic salt overload may trigger a vicious cycle of declining kidney function leading to rising blood pressure and further declines in kidney function.

Hundreds of experiments in humans and lab animals have confirmed that excess sodium intake is apt to raise blood pressure. Studies of pre-modern cultures such as the Yi have shown that low-sodium diets tend to be associated with low rates of hypertension. Studies of modern populations and patients also have concluded that hypertension is a major risk factor for cardiovascular disease, strokes and kidney disease—three leading causes of death in the Western world. Epidemiologists have estimated that even a slight downward shift in Americans' blood pressures, especially for those near or above the hypertension limit, would reduce these disease risks enough to prevent at least tens of thousands of deaths annually.

Why, then, has it taken so long for policymakers to consider restricting sodium's presence in the food supply?

One reason is that the medical profession hasn't always approached it as a public health problem. "Early on, there was more of an emphasis on treating people who have heart disease, for example, and then there was the idea of treating people who have risk factors for heart disease, such as hypertension," says Appel. "Only recently have we started to think in terms of preventing hypertension in the first place, using population-wide public health strategies."

And only recently has there been the convincing evidence needed to justify a public health approach. Population-comparison studies, for example, showed an association between a low sodium intake and a low rate of hypertension, but they couldn't untangle the effects of other dietary and lifestyle factors. To justify a strong sodium-lowering public health campaign, large-scale clinical trials of sodium reduction were needed. The three most prominent of these trials were all conducted from the late 1980s to the early 2000s.

The first was set up by Whelton with NIH funding in 1986, and included Klag and Appel, then a postdoctoral fellow. It was known as TOHP (for "Trials of Hypertension Prevention") and among other things, it indicated that for a group of people with moderately high blood pressure, even a slight reduction in their average pressure, via a reduced sodium intake, could markedly reduce the number of cases of hypertension.

A second study, known as TONE ("Trial of Nonpharmacologic interventions in the Elderly"), was set up by Whelton in the early 1990s, and included a sodium-reduction study designed by Whelton and Appel. It found that in several hundred elderly people, a one-third reduction in normal salt intake per day lowered blood pressure readings by 4-5 millimeters of mercury on average, compared to a control group. The improvement was sustained over 30 months and included a sizable drop in the relative occurrence of bad outcomes, such as cardiovascular problems or having to resume medication.

Then in the late 1990s came the "DASH-Sodium Trial" ( "Dietary Approaches to Stop Hypertension-Sodium Trial"). Appel served as one of its lead investigators. The study showed clearly that for people with higher-than-normal blood pressure or outright hypertension, moderate or strong reductions in sodium intake brought about correspondingly moderate or strong reductions in blood pressure—whether their diet emphasized fruits and vegetables or the meatier and fattier offerings typically found on America's tables.

"That turned out to be one of the most persuasive studies of sodium and blood pressure," says Appel, "because it showed a clear dose-response effect in which greater sodium reduction led to greater blood pressure reduction."

"The DASH diet studies that Larry Appel and others conducted were really the tipping point for a lot of people, in terms of justifying a more aggressive public policy on this issue," says Daniel Jones, MD, a prominent hypertension researcher who is now chancellor of the University of Mississippi.

Comments

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  • Adongo Amos Otieno

    Marsabit, Northern Kenya 10/16/2010 04:16:12 AM

    Hi, This indeed is very valuable information. Kenya is already into this trouble. Overweight among young people below 40 years is on the rise probably due to increase in consumption of processed foods. But attention now needs to shift research work towards nomads of Kenya who originally thought to be resistant to Hypertension despite red meat diet. Is there a professor in Johns Hopkins interested in the study of nutrition and public health among nomadic people? Let's be touch.

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