2001 Science in Society Journalism Awards

Magazine

Gary Taubes

“The Soft Science of Dietary Fat”

NOTE: A .pdf version of “The Soft Science of Dietary Fat” is also available.

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Part Six: More fats, fewer answers

The original simple story in the 1950s was that high cholesterol levels increase heart disease risk. The seminal Framingham Heart Study, for instance, which revealed the association between cholesterol and heart disease, originally measured only total serum cholesterol. But cholesterol shuttles through the blood in an array of packages. Low-density lipoprotein particles (LDL, the "bad" cholesterol) deliver fat and cholesterol from the liver to tissues that need it, including the arterial cells, where it can lead to atherosclerotic plaques. High-density lipoproteins (HDLs, the "good" cholesterol) return cholesterol to the liver. The higher the HDL, the lower the heart disease risk. Then there are triglycerides, which contain fatty acids, and very low density lipoproteins (VLDLs), which transport triglycerides.

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 monounsaturated fats are not neutral. Rather, they raise HDL, at least compared to carbohydrates, and lower LDL. This makes them an ideal nutrient as far as cholesterol goes. Furthermore, saturated fats cannot be quite so evil because, while they elevate LDL, which is bad, they also elevate HDL, which is good. And some saturated fats — stearic acid, in particular, the fat in chocolate — are at worst neutral. Stearic acid raises HDL levels but does little or nothing to LDL. And then there are trans fatty acids, which raise LDL, just like saturated fat, but also lower HDL. Today, none of this is controversial, although it has yet to be reflected in any Food Guide Pyramid.

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 tended to die prematurely from heart disease. But below 160 mg/dl, the men tended to die prematurely from cancer, respiratory and digestive diseases, and trauma. As for women, if anything, the higher their cholesterol, the longer they lived (see graph on p. 2540).

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 Physicians, for instance, now suggests that cholesterol reduction is certainly worthwhile for those at high, short-term risk of dying of coronary heart disease but of "much smaller or ... uncertain" benefit for everyone else.

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 the HDL, LDL, and total cholesterol levels in the two groups remained virtually identical. Nonetheless, over 4 years of follow-up, the Mediterranean-diet group had only 14 cardiac deaths and nonfatal heart attacks compared to 44 for the "Western-type" diet group. The likely explanation, wrote de Lorgeril and his colleagues, is that the "protective effects [of the Mediterranean diet] were not related to serum concentrations of total, LDL or HDL cholesterol."

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.

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