The Soft Science of Dietary Fat
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.
When the U.S. Surgeon General's Office set off in 1988 to write the
definitive report on the dangers of dietary fat, the scientific task appeared
straightforward. Four years earlier, the National Institutes of Health
(NIH) had begun advising every American old enough to walk to restrict
fat intake, and the president of the American Heart Association (AHA) had
told Time magazine that if
All of this was apparently based on sound science. So the task before the project officer was merely to gather that science together in one volume, have it reviewed by a committee of experts, which had been promptly established, and publish it. The project did not go smoothly, however. Four project officers came and went over the next decade. "It consumed project officers," says Marion Nestle, who helped launch the project and now runs the nutrition and food studies department at New York University (NYU). Members of the oversight committee saw drafts of an early chapter or two, criticized them vigorously, and then saw little else.
Finally, in June 1999, 11 years after the project began, the
Surgeon General's Office circulated a letter, authored by the last of the
project officers, explaining that the report would be killed. There was
no other public announcement and no press release. The letter explained
that the relevant administrators "did not anticipate fully the magnitude
of the additional external expertise and staff
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.
As the Surgeon General's Office discovered, however, the science
of dietary fat is not nearly as simple as it once appeared. The proposition,
now 50 years old, that dietary fat is a bane to health is based chiefly
on the fact that fat, specifically the hard, saturated fat found primarily
in meat and dairy products, elevates blood cholesterol levels. This in
turn raises the likelihood that cholesterol will clog arteries, a condition
known as atherosclerosis, which then increases risk of coronary artery
disease, heart attack, and untimely death. 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
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
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
Indeed, the history of the national conviction that dietary fat is deadly, and its evolution from hypothesis to dogma, is one in which politicians, bureaucrats, the media, and the public have played as large a role as the scientists and the science. It's a story of what can happen when the demands of public health policy--and the demands of the public for simple advice--run up against the confusing ambiguity of real science.
Fear of fat
Nonetheless, by 1969 the state of the science could still be summarized by a single sentence from a report of the Diet-Heart Review Panel of the National Heart Institute (now the National Heart, Lung, and Blood Institute, or NHLBI): "It is not known whether dietary manipulation has any effect whatsoever on coronary heart disease." The chair of the panel was E. H. "Pete" Ahrens, whose laboratory at Rockefeller University in New York City did much of the seminal research on fat and cholesterol metabolism.
Whereas proponents of low-fat diets were concerned primarily about the effects of dietary fat on cholesterol levels and heart disease, Ahrens and his panel--10 experts in clinical medicine, epidemiology, biostatistics, human nutrition, and metabolism--were equally concerned that eating less fat could have profound effects throughout the body, many of which could be harmful. The brain, for instance, is 70% fat, which chiefly serves to insulate neurons. Fat is also the primary component of cell membranes. Changing the proportion of saturated to unsaturated fats in the diet changes the fat composition in these membranes. This could conceivably change the membrane permeability, which controls the transport of everything from glucose, signaling proteins, and hormones to bacteria, viruses, and tumor-causing agents into and out of the cell. The relative saturation of fats in the diet could also influence cellular aging as well as the clotting ability of blood cells.
Whether the potential benefits of low-fat diets would exceed
the potential risks could be settled by testing whether low-fat diets actually
prolong life, but such a test would have to be enormous. The effect of
diet on cholesterol levels is subtle for most individuals--especially those
living in the real world rather than the metabolic wards of nutrition researchers--and
the effect of cholesterol levels on heart disease is also subtle. As a
result, tens of thousands of individuals would have to switch to low-fat
diets and their subsequent health compared to that of equal numbers who
continued eating fat to alleged excess. And all these people would have
to be followed for years until enough deaths accumulated to provide statistically
significant results. Ahrens and his colleagues were pessimistic about whether
such a massive and expensive trial could ever be done. In 1971, an NIH
Science by committee
McGovern's committee was founded in 1968 with a mandate to eradicate malnutrition in America, and it instituted a series of landmark federal food assistance programs. As the malnutrition work began to peter out in the mid-1970s, however, the committee didn't disband. Rather, its general counsel, Marshall Matz, and staff director, Alan Stone, both young lawyers, decided that the committee would address "overnutrition," the dietary excesses of Americans. It was a "casual endeavor," says Matz. "We really were totally naïve, a bunch of kids, who just thought, 'Hell, we should say something on this subject before we go out of business.' " McGovern and his fellow senators--all middle-aged men worried about their girth and their health--signed on; McGovern and his wife had both gone through diet-guru Nathan Pritikin's very low fat diet and exercise program. McGovern quit the program early, but Pritikin remained a major influence on his thinking.
McGovern's committee listened to 2 days of testimony on diet and disease in July 1976. Then resident wordsmith Nick Mottern, a former labor reporter for The Providence Journal, was assigned the task of researching and writing the first "Dietary Goals for the United States." Mottern, who had no scientific background and no experience writing about science, nutrition, or health, believed his Dietary Goals would launch a "revolution in diet and agriculture in this country." He avoided the scientific and medical controversy by relying almost exclusively on Harvard School of Public Health nutritionist Mark Hegsted for input on dietary fat. Hegsted had studied fat and cholesterol metabolism in the early 1960s, and he believed unconditionally in the benefits of restricting fat intake, although he says he was aware that his was an extreme opinion. With Hegsted as his muse, Mottern saw dietary fat as the nutritional equivalent of cigarettes, and the food industry as akin to the tobacco industry in its willingness to suppress scientific truth in the interests of profits. To Mottern, those scientists who spoke out against fat were those willing to take on the industry. "It took a certain amount of guts," he says, "to speak about this because of the financial interests involved."
Mottern's report suggested that Americans cut their total fat intake to 30% of the calories they consume and saturated fat intake to 10%, in accord with AHA recommendations for men at high risk of heart disease. The report acknowledged the existence of controversy but insisted Americans had nothing to lose by following its advice. "The question to be asked is not why should we change our diet but why not?" wrote Hegsted in the introduction. "There are [no risks] that can be identified and important benefits can be expected." This was an optimistic but still debatable position, and when Dietary Goals was released in January 1977, "all hell broke loose," recalls Hegsted. "Practically nobody was in favor of the McGovern recommendations. Damn few people."
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.
Although the committee published a revised edition of the Dietary Goals later in the year, the thrust of the recommendations remained unchanged. It did give in to industry pressure by softening the suggestion that Americans eat less meat. Mottern says he considered even that a "disservice to the public," refused to do the revisions, and quit the committee. (Mottern became a vegetarian while writing the Dietary Goals and now runs a food co-op in Peekskill, New York.)
The guidelines might have then died a quiet death when McGovern's committee came to an end in late 1977 if two federal agencies had not felt it imperative to respond. Although they took contradictory points of view, one message--with media assistance--won out.
The first was the USDA, where consumer-activist Carol Tucker Foreman had recently been appointed an assistant secretary. Foreman believed it was incumbent on USDA to turn McGovern's recommendations into official policy, and, like Mottern, she was not deterred by the existence of scientific controversy. "Tell us what you know and tell us it's not the final answer," she would tell scientists. "I have to eat and feed my children three times a day, and I want you to tell me what your best sense of the data is right now."
Of course, given the controversy, the "best sense of the data"
would depend on which scientists were asked. The Food and Nutrition Board
of the National Academy of Sciences (NAS), which decides the Recommended
Dietary Allowances, would have been a natural choice, but NAS president
Philip Handler, an expert on metabolism, had told Foreman that Mottern's
The resulting document became the first edition of "Using the Dietary Guidelines for Americans." Although it acknowledged the existence of controversy and suggested that a single dietary recommendation might not suit an entire diverse population, the advice to avoid fat and saturated fat was, indeed, virtually identical to McGovern's Dietary Goals.
Three months later, the NAS Food and Nutrition Board released
its own guidelines: "Toward Healthful Diets." The board, consisting of
a dozen nutrition experts, concluded that the only reliable advice for
healthy Americans was to watch their weight; everything else, dietary fat
included, would take care of itself. The advice was not taken kindly, however,
at least not by the
Hegsted now defends the NAS board, although he didn't at the time, and calls this kind of conflict of interest "a hell of an issue." "Everybody used to complain that industry didn't do anything on nutrition," he told Science, "yet anybody who got involved was blackballed because their positions were presumably influenced by the industry." (In 1981, Hegsted returned to Harvard, where his research was funded by Frito-Lay.) The press had mixed feelings, claiming that the connections "soiled" the academy's reputation "for tendering careful scientific advice" (The Washington Post), demonstrated that the board's "objectivity and aptitude are in doubt" (The New York Times), or represented in the board's guidelines a "blow against the food faddists who hold the public in thrall" (Science). In any case, the NAS board had been publicly discredited. Hegsted's Dietary Guidelines for Americans became the official U.S. policy on dietary fat: Eat less fat. Live longer.
The sixth study was the $140 million Lipid Research Clinics (LRC) Coronary Primary Prevention Trial, led by NHLBI administrator Basil Rifkind and biochemist Daniel Steinberg of the University of California, San Diego. The LRC trial was a drug trial, not a diet trial, but the NHLBI heralded its outcome as the end of the dietary fat debate. In January 1984, LRC investigators reported that a medication called cholestyramine reduced cholesterol levels in men with abnormally high cholesterol levels and modestly reduced heart disease rates in the process. (The probability of suffering a heart attack during the seven-plus years of the study was reduced from 8.6% in the placebo group to 7.0%; the probability of dying from a heart attack dropped from 2.0% to 1.6%.) The investigators then concluded, without benefit of dietary data, that cholestyramine's benefits could be extended to diet as well. And although the trial tested only middle-aged men with cholesterol levels higher than those of 95% of the population, they concluded that those benefits "could and should be extended to other age groups and women and ... other more modest elevations of cholesterol levels."
Why go so far? Rifkind says their logic was simple: For 20 years, he and his colleagues had argued that lowering cholesterol levels prevented heart attacks. They had spent enormous sums trying to prove it. They felt they could never actually demonstrate that low-fat diets prolonged lives--that would be too expensive, and MRFIT had failed -- but now they had established a fundamental link in the causal chain, from lower cholesterol levels to cardiovascular health. With that, they could take the leap of faith from cholesterol-lowering drugs and health to cholesterol-lowering diet and health. And after all their effort, they were eager--not to mention urged by Congress--to render helpful advice. "There comes a point when, if you don't make a decision, the consequences can be great as well," says Rifkind. "If you just allow Americans to keep on consuming 40% of calories from fat, there's an outcome to that as well."
With the LRC results in press, the NHLBI launched what Levy called "a massive public health campaign." The media obligingly went along. Time, for instance, reported the LRC findings under the headline "Sorry, It's True. Cholesterol really is a killer." The article about a drug trial began: "No whole milk. No butter. No fatty meats ..." Time followed up 3 months later with a cover story: "And Cholesterol and Now the Bad News. ..." The cover photo was a frowning face: a breakfast plate with two fried eggs as the eyes and a bacon strip for the mouth. Rifkind was quoted saying that their results "strongly indicate that the more you lower cholesterol and fat in your diet, the more you reduce your risk of heart disease," a statement that still lacked direct scientific support.
The following December, NIH effectively ended the debate with
a "Consensus Conference." The idea of such a conference is that an expert
panel, ideally unbiased, listens to 2 days of testimony and arrives at
a conclusion with which everyone agrees. In this case, Rifkind chaired
the planning committee, which chose his LRC co-investigator Steinberg to
lead the expert panel. The 20 speakers did include a handful of skeptics
--including Ahrens, for instance, and cardiologist Michael Oliver of Imperial
College in London--who argued that it was unscientific to equate the effects
of a drug with the effects of a diet. Steinberg's panel members, however,
as Oliver later complained in The Lancet, "were selected to include only
experts who would, predictably, say that all levels of blood cholesterol
in the United States are too high and should be lowered. And, of course,
this is exactly what was said." Indeed, the conference report, written
by Steinberg and his panel, revealed no evidence of discord. There was
"no doubt," it concluded, that low-fat diets "will afford significant protection
against coronary heart disease" to every American over 2 years old.
The test of time
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 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. The results of well-controlled clinical trials are consistent: People on low-fat diets initially lose a couple of kilograms, as they would on any diet, and then the weight tends to return. After 1 to 2 years, little has been achieved. Consider, for instance, the 50,000 women enrolled in the ongoing $100 million Women's Health Initiative (WHI). Half of these women have been extensively counseled to consume only 20% of their calories from fat. After 3 years on this near-draconian regime, say WHI sources, the women had lost, on average, a kilogram each.
The link between dietary fat and heart disease is more complicated, because the hypothesis has diverged into two distinct propositions: first, that lowering cholesterol prevents heart disease; second, that eating less fat not only lowers cholesterol and prevents heart disease but prolongs life. Since 1984, the evidence that cholesterol-lowering drugs are beneficial--proposition number one--has indeed blossomed, at least for those at high risk of heart attack. These drugs reduce serum cholesterol levels dramatically, and they prevent heart attacks, perhaps by other means as well. Their market has now reached $4 billion a year in the United States alone, and every new trial seems to confirm their benefits.
The evidence supporting the second proposition, that eating
less fat makes for a healthier and longer life, however, has remained stubbornly
ambiguous. If anything, it has only become less compelling over time. Indeed,
since Ancel Keys started advocating low-fat diets almost 50 years ago,
the science of fat and cholesterol has evolved from a simple story into
a very complicated one.
More fats, fewer answers
All of these particles have some effect on heart disease risk,
while the fats, carbohydrates, and protein in the diet have varying effects
on all these particles. The 1950s story was that saturated fats increase
total cholesterol, polyunsaturated fats decrease it, and monounsaturated
fats are neutral. By the late 1970s--when researchers accepted the benefits
of HDL--they realized that
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.
As for the scientific studies, in the years since the 1984 consensus conference, the one thing they have not done is pile up evidence in support of the low-fat-for-all approach to the public good. If anything, they have added weight to Ahrens's fears that there may be a downside to populationwide low-fat recommendations. In 1986, for instance, just 1 year after NIH launched the National Cholesterol Education Program, also advising low-fat diets for everyone over 2 years old, epidemiologist David Jacobs of the University of Minnesota, Twin Cities, visited Japan. There he learned that Japanese physicians were advising patients to raise their cholesterol levels, because low cholesterol levels were linked to hemorrhagic stroke. At the time, Japanese men were dying from stroke almost as frequently as American men were succumbing to heart disease. Back in Minnesota, Jacobs looked for this low-cholesterol-stroke relationship in the MRFIT data and found it there, too. And the relationship transcended stroke: Men with very low cholesterol levels seemed prone to premature death; below 160 milligrams per deciliter (mg/dl), the lower the cholesterol level, the shorter the life.
Jacobs reported his results to NHLBI, which in 1990 hosted
a conference to discuss the issue, bringing together researchers from 19
studies around the world. The data were consistent: When investigators
tracked all deaths, instead of just heart disease deaths, the cholesterol
curves were U-shaped for men and flat for women. In other words, men with
cholesterol levels above 240 mg/dl
These mortality data can be interpreted in two ways. One, preferred by low-fat advocates, is that they cannot be meaningful. Rifkind, for instance, told Science that the excess deaths at low cholesterol levels must be due to preexisting conditions. In other words, chronic illness leads to low cholesterol levels, not vice versa. He pointed to the 1990 conference report as the definitive document on the issue and as support for his argument, although the report states unequivocally that this interpretation is not supported by the data.
The other interpretation is that what a low-fat diet does to
serum cholesterol levels, and what that in turn does to arteries, may be
only one component of the diet's effect on health. In other words, while
low-fat diets might help prevent heart disease, they might also raise susceptibility
to other conditions. This is what always worried Ahrens. It's also one
reason why the American College of
This interpretation--that the connection between diet and health
far transcends cholesterol--is also supported by the single most dramatic
diet-heart trial ever conducted: the Lyon Diet Heart Study, led by Michel
de Lorgeril of the French National Institute of Health and Medical Research
(INSERM) and published in Circulation in February 1999. The investigators
randomized 605 heart attack survivors, all on cholesterol-lowering drugs,
into two groups. They counseled one to eat an AHA "prudent diet," very
similar to that recommended for all Americans. They counseled the other
to eat a Mediterranean-type diet, with more bread, cereals, legumes, beans,
vegetables, fruits, and fish and less meat. Total fat and types of fat
differed markedly in the two diets, but
Many researchers find the Lyon data so perplexing that they're left questioning the methodology of the trial. Nonetheless, says NIH's Harlan, the data "are very provocative. They do bring up the issue of whether if we look only at cholesterol levels we aren't going to miss something very important." De Lorgeril believes the diet's protective effect comes primarily from omega-3 fatty acids, found in seed oils, meat, cereals, green leafy vegetables, and fish, and from antioxidant compounds, including vitamins, trace elements, and flavonoids. He told Science that most researchers and journalists in the field are prisoners of the "cholesterol paradigm." Although dietary fat and serum cholesterol "are obviously connected," he says, "the connection is not a robust one" when it comes to heart disease.
This plus-minus problem suggests a different interpretation for virtually every diet study ever done, including, for instance, the kind of metabolic-ward studies that originally demonstrated the ability of saturated fats to raise cholesterol. If researchers reduce the amount of saturated fat in the test diet, they have to make up the calories elsewhere. Do they add polyunsaturated fats, for instance, or add carbohydrates? A single carbohydrate or mixed carbohydrates? Do they add green leafy vegetables, or do they add pasta? And so it goes. "The sky's the limit," says nutritionist Alice Lichtenstein of Tufts University in Boston. "There are a million perturbations."
These trade-offs also confound the kind of epidemiological studies that demonized saturated fat from the 1950s onward. In particular, individuals who eat copious amounts of meat and dairy products, and plenty of saturated fats in the process, tend not to eat copious amounts of vegetables and fruits. The same holds for entire populations. The eastern Finns, for instance, whose lofty heart disease rates convinced Ancel Keys and a generation of researchers of the evils of fat, live within 500 kilometers of the Arctic Circle and rarely see fresh produce or a green vegetable. The Scots, infamous for eating perhaps the least wholesome diet in the developed world, are in a similar fix. Basil Rifkind recalls being laughed at once on this point when he lectured to Scottish physicians on healthy diets: "One said, 'You talk about increasing fruits and vegetable consumption, but in the area I work in there's not a single grocery store.' " In both cases, researchers joke that the only green leafy vegetable these populations consume regularly is tobacco. As for the purported benefits of the widely hailed Mediterranean diet, is it the fish, the olive oil, or the fresh vegetables? After all, says Harvard epidemiologist Dimitrios Trichopoulos, a native of Greece, the olive oil is used either to cook vegetables or as dressing over salads. "The quantity of vegetables consumed is almost a pound [half a kilogram] a day," he says, "and you cannot eat it without olive oil. And we eat a lot of legumes, and we cannot eat legumes without olive oil."
Indeed, recent data on heart disease trends in Europe suggest that a likely explanation for the differences between countries and over time is the availability of fresh produce year-round rather than differences in fat intake. While the press often plays up the French paradox--the French have little heart disease despite seemingly high saturated fat consumption--the real paradox is throughout Southern Europe, where heart disease death rates have steadily dropped while animal fat consumption has steadily risen, says University of Cambridge epidemiologist John Powles, who studies national disease trends. The same trend appears in Japan. "We have this idea that it's the Arcadian past, the life in the village, the utopia that we've lost," Powles says; "that the really protective Mediterranean diet is what people ate in the 1950s." But that notion isn't supported by the data: As these Mediterranean nations became more affluent, says Powles, they began to eat proportionally more meat and with it more animal fat. Their heart disease rates, however, continued to improve compared to populations that consumed as much animal fat but had less access to fresh vegetables throughout the year. To Powles, the antifat movement was founded on the Puritan notion that "something bad had to have an evil cause, and you got a heart attack because you did something wrong, which was eating too much of a bad thing, rather than not having enough of a good thing."
The other salient trade-off in the plus-minus problem of human diets is carbohydrates. When the federal government began pushing low-fat diets, the scientists and administrators, and virtually everyone else involved, hoped that Americans would replace fat calories with fruits and vegetables and legumes, but it didn't happen. If nothing else, economics worked against it. The food industry has little incentive to advertise nonproprietary items: broccoli, for instance. Instead, says NYU's Nestle, the great bulk of the $30-billion-plus spent yearly on food advertising goes to selling carbohydrates in the guise of fast food, sodas, snacks, and candy bars. And carbohydrates are all too often what Americans eat.
Carbohydrates are what Harvard's Willett calls the flip side of the calorie trade-off problem. Because it is exceedingly difficult to add pure protein to a diet in any quantity, a low-fat diet is, by definition, a high-carbohydrate diet--just as a low-fat cookie or low-fat yogurt are, by definition, high in carbohydrates. Numerous studies now suggest that high-carbohydrate diets can raise riglyceride levels, create small, dense LDL particles, and reduce HDL--a combination, along with a condition known as "insulin resistance," that Stanford endocrinologist Gerald Reaven has labeled "syndrome X." Thirty percent of adult males and 10% to 15% of postmenopausal women have this particular syndrome X profile, which is associated with a several-fold increase in heart disease risk, says Reaven, even among those patients whose LDL levels appear otherwise normal. Reaven and Ron Krauss, who studies fats and lipids at Lawrence Berkeley National Laboratory in California, have shown that when men eat high-carbohydrate diets their cholesterol profiles may shift from normal to syndrome X. In other words, the more carbohydrates replace saturated fats, the more likely the end result will be syndrome X and an increased heart disease risk. "The problem is so clear right now it's almost a joke," says Reaven. How this balances out is the unknown. "It's a bitch of a question," says Marc Hellerstein, a nutritional biochemist at the University of California, Berkeley, "maybe the great public health nutrition question of our era."
The other worrisome aspect of the carbohydrate trade-off is
the possibility that, for some individuals, at least, it might actually
be easier to gain weight on low-fat/high-carbohydrate regimens than on
higher fat diets. One of the many factors that influence hunger is the
glycemic index, which measures how fast carbohydrates are broken down into
simple sugars and moved into the bloodstream. Foods with the highest glycemic
index are simple sugars and processed grain products like pasta and white
rice, which cause a rapid rise in blood sugar after a meal. Fruits, vegetables,
legumes, and even unprocessed starches--pasta al dente, for instance--cause
a much slower rise in blood sugar. Researchers have hypothesized
that eating high-glycemic index foods
If the theory is correct, calories from the kind of processed carbohydrates that have become the staple of the American diet are not the same as calories from fat, protein, or complex carbohydrates when it comes to controlling weight. "They may cause a hormonal change that stimulates hunger and leads to overeating," says Ludwig, "especially in environments where food is abundant. ..."
In 1979, 2 years after McGovern's committee released its Dietary Goals, Ahrens wrote to The Lancet describing what he had learned over 30 years of studying fat and cholesterol metabolism: "It is absolutely certain that no one can reliably predict whether a change in dietary regimens will have any effect whatsoever on the incidence of new events of [coronary heart disease], nor in whom." Today, many nutrition researchers, acknowledging the complexity of the situation, find themselves siding with Ahrens. Krauss, for instance, who chairs the AHA Dietary Guidelines Committee, now calls it "scientifically naïve" to expect that a single dietary regime can be beneficial for everybody: "The 'goodness' or 'badness' of anything as complex as dietary fat and its subtypes will ultimately depend on the context of the individual."
Given the proven success and low cost of cholesterol-lowering drugs, most physicians now prescribe drug treatment for patients at high risk of heart disease. The drugs reduce LDL cholesterol levels by as much as 30%. Diet rarely drops LDL by more than 10%, which is effectively trivial for healthy individuals, although it may be worth the effort for those at high risk of heart disease whose cholesterol levels respond well to it.
The logic underlying populationwide recommendations such as the latest USDA Dietary Guidelines is that limiting saturated fat intake--even if it does little or nothing to extend the lives of healthy individuals and even if not all saturated fats are equally bad--might still delay tens of thousands of deaths each year throughout the entire country. Limiting total fat consumption is considered reasonable advice because it's simple and easy to understand, and it may limit calorie intake. Whether it's scientifically justifiable may simply not be relevant. "When you don't have any real good answers in this business," says Krauss, "you have to accept a few not so good ones as the next best thing."
Volume 291, Number
5513, Issue of 30 Mar 2001, pp. 2536-2545.