Does eating a lot of salt and fat kill people? "Of course," most Americans would respond. Many would even think salt and fat are almost as bad as tobacco. After all, isn't this what we've been told for years by the health-care professionals who set government dietary guidelines?
However, it turns out that the federal guidelines recommending restricted consumption of these substances are a perfect example of how the uncertainties of science get transformed into hard-headed government policies. The story of how salt and fat became bad has recently been revealed in a couple of superb articles by Science magazine correspondent Gary Taubes. Taubes details exactly how, why, and when the misperceptions passed into policy; his pieces are required reading for anyone interested in the vagaries of policy making.
First, salt. The theory is that the sodium in salt raises blood pressure, thereby increasing the risk of heart attacks and stroke. In 1976, when the salt wars were first gearing up, one researcher called salt "the most dangerous food additive of all." The battle really began in earnest when in 1972, the National High Blood Pressure Education Program (NHBPEP), a coalition of 36 medical organizations and six federal agencies, essentially declared that salt was an unnecessary evil. In 1978, the activist group Center for Science in the Public Interest began lobbying Congress to require the labeling of high-salt foods. In 1981, the Food and Drug Administration began programs aimed at reducing the nation's salt intake. In 1993, the NHBPEP recommended that the U.S. embark on universal sodium-reduction efforts.
The campaign against salt continues. The federal government's new edition of Dietary Guidelines for Americans, issued in May 2000, warns that "most people consume too much salt, so moderate your salt intake. Healthy children and adults need to consume only small amounts of salt to meet their sodium needs-less than 1/4 teaspoon daily." The government-mandated nutrition labels that appear on prepared foods lists a daily value of 2,400 milligrams of sodium, the equivalent of about 1 teaspoon (or 6 grams) of salt. Americans consume on average about 10 grams of salt daily. "The Dietary Guidelines are the gold standard when it comes to applying scientific research to what people should be eating," said then-Agriculture Secretary Dan Glickman when they were released.
But is salt a major health risk? Eating salt does cause a person to retain more water, which raises blood pressure until the kidneys can excrete the excess salt and water. The question is, Does it do this on a long-term basis? Given how broadly and strongly the anti-sodium message is promoted, one would think that the medical evidence was incontrovertible. But that is far from the case, as Taubes makes clear in "The Political Science of Salt," published in the August 14, 1998 issue of Science.
To make a long story short, the best evidence to date shows that, for most people, salt reduction offers only a modest health benefit, if any. Consider two dueling meta-analyses of salt studies. At reductions of roughly 6 grams of salt, the study headed by a proponent of salt reduction found a blood pressure benefit of 5.8/2.5 mmHg in hypertensives (those with high blood pressure) and 2.0/1.4 mmHg in normotensives (those with normal blood pressure). The other, headed by a salt skeptic found a benefit of only 3.7/0.9 mmHg in hypertensives and 1.0/0.1 in normotensives. (High blood pressure is defined as having a systolic blood pressure higher than 140 mmHg and diastolic higher than 90 mmHg [140/90 mmHg]).
In 1996, the Journal of the American Medical Association published a meta-analysis of 56 clinical trials done since 1980 in people with normal blood pressure. The conclusion: Extreme salt reduction offered little benefit. (That said, one should keep in mind that sodium can substantially boost blood pressure in some especially sensitive people.)
Possibly the most damning evidence against a strong relationship between salt consumption and high blood pressure comes from the Dietary Approaches to Stop Hypertension (DASH) study, published in April 1997 in The New England Journal of Medicine. In DASH, people ate a diet rich in fruits, vegetables, and low-fat dairy products. As Taubes reports, "In 3 weeks, the diet reduced blood pressure by 5.5/3.0 mmHg in subjects with mild hypertension and 11.4/5.5 mmHg in hypertensives--a benefit surpassing what could be achieved by medication. Yet salt content was kept constant in the DASH diets, which meant salt had nothing to do with the blood pressure reductions." DASH is now doing a study that varies the amount of salt intake, the results of which should be reported soon.
What about fat? Surely eating too much of it will shorten your lifespan, right? The federal Dietary Guidelines recommend that Americans "aim for a total fat intake of no more than 30 percent of calories," and that no more than 10 percent of calories come from eating saturated fats.
Taubes again shows that what everyone knows ain't necessarily so in his article "The Soft Science of Dietary Fat," published in Science this past March. The war on fat has been going on for nearly 50 years, but really took off when Senate's Select Committee on Nutrition and Human Needs, chaired by Sen. George McGovern (D-S.D.) issued its controversial dietary guidelines in 1977. The McGovern staffers who wrote the report regarded fat as the moral equivalent of cigarettes and the food industry as equivalent to tobacco companies, according to Taubes. The McGovern fat guidelines? They were identical to the ones just issued in 2000-- Americans should limit their total fat intake to 30 percent of the calories they consume and saturated fat intake to 10 percent.
The fat-is-bad hypothesis was based on a chain of inference that saturated fats are known to boost cholesterol levels that in turn are associated with higher rates of heart disease. All of these statements are true. But the question is, Does lowering fat intake, especially cutting back on saturated fats, increase overall life expectancy?
Taubes cites several independent studies that have concluded that following the federal dietary fat guidelines would not much prolong a devotee's life. According to Taubes, for example, a Harvard Medical School study concluded that "smokers…with high blood pressure--could expect to gain, on average, one extra year by shunning saturated fat. Healthy nonsmokers, however, might add 3 days to 3 months." Another study funded by the U.S. Surgeon General's office and done at the University of California at San Francisco concluded that "cutting fat consumption in America would delay 42,000 deaths each year, but the net increase in life expectancy would average out to only 3 to 4 months."
"Although there are undoubtedly persons who would choose to participate in a lifelong regimen of dietary change to achieve results of this magnitude, we suspect that some might not," noted the researchers who conducted the Harvard study.
Taubes reviews the results of the Harvard School of Public Health's three studies on diet and health; these studies tracked almost 300,000 Americans for 20 years. Their conclusions "suggest that total fat consumed has no relation to heart disease risk; that monounsaturated fats like olive oil lower risk; and that saturated fats are little worse, if at all, than the pasta and other carbohydrates that the Food Guide Pyramid suggests be eaten copiously." To be sure, some high-risk people may benefit from cutting back on fat consumption, especially if combined with new cholesterol-lowering medicines that are very effective in lowering cholesterol.
In the campaigns against salt and fat, Taubes has outlined how committed and responsible scientists can look at the same ambiguous evidence and come to very different conclusions. That's normal science. The problems start when such ambiguous s evidence is transformed into government policy. Unlike its scientific counterpart, the political process cannot accommodate ambiguity. The vote must be yes or no, not maybe. Sanford Miller, a former Food and Drug Administration staffer now at the University of Texas Health Sciences Center, told Taubes that the salt controversy is the "number one perfect example of why science is a destabilizing force in public policy." This same scientific-political process is responsible for launching the discredited synthetic-chemicals-cause-cancer scare and the global-warming-disaster campaign in which ambiguous science has been transformed into useless and costly government crusades.
Taubes' articles perform a real public service by documenting that when it comes to evaluating policy supposedly based on the latest irrefutable scientific findings, we do well to take such truths with a grain of salt.