The Soft Science of Dietary Fat (i suggest reading the whole article which is quite long)
sciencemag.org
Gary Taubes, Science Magazine
Mainstream nutritional science has demonized dietary fat, yet 50 years and hundreds of millions of dollars of research have failed to prove that eating a low-fat diet will help you live longer
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During the past 30 years, the concept of eating healthy in America has become synonymous with avoiding dietary fat. The creation and marketing of reduced-fat food products has become big business; over 15,000 have appeared on supermarket shelves. Indeed, an entire research industry has arisen to create palatable nonfat fat substitutes, and the food industry now spends billions of dollars yearly selling the less-fat-is-good-health message. The government weighs in as well, with the U.S. Department of Agriculture's (USDA's) booklet on dietary guidelines, published every 5 years, and its ubiquitous Food Guide Pyramid, which recommends that fats and oils be eaten "sparingly." The low-fat gospel spreads farther by a kind of societal osmosis, continuously reinforced by physicians, nutritionists, journalists, health organizations, and consumer advocacy groups such as the Center for Science in the Public Interest, which refers to fat as this "greasy killer." "In America, we no longer fear God or the communists, but we fear fat," says David Kritchevsky of the Wistar Institute in Philadelphia, who in 1958 wrote the first textbook on cholesterol.
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By the 1970s, each individual step of this chain from fat to cholesterol to heart disease had been demonstrated beyond reasonable doubt, but the veracity of the chain as a whole has never been proven. In other words, despite decades of research, it is still a debatable proposition whether the consumption of saturated fats above recommended levels (step one in the chain) by anyone who's not already at high risk of heart disease will increase the likelihood of untimely death (outcome three). Nor have hundreds of millions of dollars in trials managed to generate compelling evidence that healthy individuals can extend their lives by more than a few weeks, if that, by eating less fat (see sidebar on p. 2538). To put it simply, the data remain ambiguous as to whether low-fat diets will benefit healthy Americans. Worse, the ubiquitous admonishments to reduce total fat intake have encouraged a shift to high-carbohydrate diets, which may be no better--and may even be worse--than high-fat diets.
Since the early 1970s, for instance, Americans' average fat intake has dropped from over 40% of total calories to 34%; average serum cholesterol levels have dropped as well. But no compelling evidence suggests that these decreases have improved health. Although heart disease death rates have dropped--and public health officials insist low-fat diets are partly responsible--the incidence of heart disease does not seem to be declining, as would be expected if lower fat diets made a difference. This was the conclusion, for instance, of a 10-year study of heart disease mortality published in The New England Journal of Medicine in 1998, which suggested that death rates are declining largely because doctors are treating the disease more successfully. AHA statistics agree: Between 1979 and 1996, the number of medical procedures for heart disease increased from 1.2 million to 5.4 million a year. "I don't consider that this disease category has disappeared or anything close to it," says one AHA statistician.
Meanwhile, obesity in America, which remained constant from the early 1960s through 1980, has surged upward since then--from 14% of the population to over 22%. Diabetes has increased apace. Both obesity and diabetes increase heart disease risk, which could explain why heart disease incidence is not decreasing. That this obesity epidemic occurred just as the government began bombarding Americans with the low-fat message suggests the possibility, however distant, that low-fat diets might have unintended consequences--among them, weight gain. "Most of us would have predicted that if we can get the population to change its fat intake, with its dense calories, we would see a reduction in weight," admits Harlan. "Instead, we see the exact opposite."
In the face of this uncertainty, skeptics and apostates have come along repeatedly, only to see their work almost religiously ignored as the mainstream medical community sought consensus on the evils of dietary fat. For 20 years, for instance, the Harvard School of Public Health has run the Nurses' Health Study and its two sequelae--the Health Professionals Follow-Up Study and the Nurses' Health Study II--accumulating over a decade of data on the diet and health of almost 300,000 Americans. The results 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. (The studies also suggest that trans fatty acids are unhealthful. These are the fats in margarine, for instance, and are what many Americans started eating when they were told that the saturated fats in butter might kill them.) Harvard epidemiologist Walter Willett, spokesperson for the Nurses' Health Study, points out that NIH has spent over $100 million on the three studies and yet not one government agency has changed its primary guidelines to fit these particular data. "Scandalous," says Willett. "They say, 'You really need a high level of proof to change the recommendations,' which is ironic, because they never had a high level of proof to set them."
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Like the flourishing American affinity for alternative medicine, an antifat movement evolved independently of science in the 1960s. It was fed by distrust of the establishment--in this case, both the medical establishment and the food industry--and by counterculture attacks on excessive consumption, whether manifested in gas-guzzling cars or the classic American cuisine of bacon and eggs and marbled steaks. And while the data on fat and health remained ambiguous and the scientific community polarized, the deadlock was broken not by any new science, but by politicians. It was Senator George McGovern's bipartisan, nonlegislative Select Committee on Nutrition and Human Needs--and, to be precise, a handful of McGovern's staff members--that almost single-handedly changed nutritional policy in this country and initiated the process of turning the dietary fat hypothesis into dogma.
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McGovern responded with three follow-up hearings, which aptly foreshadowed the next 7 years of controversy. Among those testifying, for instance, was NHLBI director Robert Levy, who explained that no one knew if eating less fat or lowering blood cholesterol levels would prevent heart attacks, which was why NHLBI was spending $300 million to study the question. Levy's position was awkward, he recalls, because "the good senators came out with the guidelines and then called us in to get advice." He was joined by prominent scientists, including Ahrens, who testified that advising Americans to eat less fat on the strength of such marginal evidence was equivalent to conducting a nutritional experiment with the American public as subjects. Even the American Medical Association protested, suggesting that the diet proposed by the guidelines raised the "potential for harmful effects." But as these scientists testified, so did representatives from the dairy, egg, and cattle industries, who also vigorously opposed the guidelines for obvious reasons. This juxtaposition served to taint the scientific criticisms: Any scientists arguing against the committee's guidelines appeared to be either hopelessly behind the paradigm, which was Hegsted's view, or industry apologists, which was Mottern's, if not both.
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Once politicians, the press, and the public had decided dietary fat policy, the science was left to catch up. In the early 1970s, when NIH opted to forgo a $1 billion trial that might be definitive and instead fund a half-dozen studies at one-third the cost, everyone hoped these smaller trials would be sufficiently persuasive to conclude that low-fat diets prolong lives. The results were published between 1980 and 1984. Four of these trials --comparing heart disease rates and diet within Honolulu, Puerto Rico, Chicago, and Framingham--showed no evidence that men who ate less fat lived longer or had fewer heart attacks. A fifth trial, the Multiple Risk Factor Intervention Trial (MRFIT), cost $115 million and tried to amplify the subtle influences of diet on health by persuading subjects to avoid fat while simultaneously quitting smoking and taking medication for high blood pressure. That trial suggested, if anything, that eating less fat might shorten life. In each study, however, the investigators concluded that methodological flaws had led to the negative results. They did not, at least publicly, consider their results reason to lessen their belief in the evils of fat.
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Such is the case, for instance, with the proposition that dietary fat causes cancer, which was an integral part of dietary fat anxiety in the late 1970s. By 1982, the evidence supporting this idea was thought to be so undeniable that a landmark NAS report on nutrition and cancer equated those researchers who remained skeptical with "certain interested parties [who] formerly argued that the association between lung cancer and smoking was not causational." Fifteen years and hundreds of millions of research dollars later, a similarly massive expert report by the World Cancer Research Fund and the American Institute for Cancer Research could find neither "convincing" nor even "probable" reason to believe that dietary fat caused cancer.
The hypothesis that low-fat diets are the requisite route to weight loss has taken a similar downward path. This was the ultimate fallback position in all low-fat recommendations: Fat has nine calories per gram compared to four calories for carbohydrates and protein, and so cutting fat from the diet surely would cut pounds. "This is held almost to be a religious truth," says Harvard's Willett. Considerable data, however, now suggest otherwise.
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To understand where this complexity can lead in a simple example, consider a steak--to be precise, a porterhouse, select cut, with a half-centimeter layer of fat, the nutritional constituents of which can be found in the Nutrient Database for Standard Reference at the USDA Web site. After broiling, this porterhouse reduces to a serving of almost equal parts fat and protein. Fifty-one percent of the fat is monounsaturated, of which virtually all (90%) is oleic acid, the same healthy fat that's in olive oil. Saturated fat constitutes 45% of the total fat, but a third of that is stearic acid, which is, at the very least, harmless. The remaining 4% of the fat is polyunsaturated, which also improves cholesterol levels. In sum, well over half--and perhaps as much as 70%--of the fat content of a porterhouse will improve cholesterol levels compared to what they would be if bread, potatoes, or pasta were consumed instead. The remaining 30% will raise LDL but will also raise HDL. All of this suggests that eating a porterhouse steak rather than carbohydrates might actually improve heart disease risk, although no nutritional authority who hasn't written a high-fat diet book will say this publicly.
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