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.

The reproduction of this article is intended for non-commercial, educational purposes only.

Part Seven: Dietary trade-offs

One inescapable reality is that death is a trade-off, and so is diet. "You have to eat something," says epidemiologist Hugh Tunstall Pedoe of the University of Dundee, U.K., spokesperson for the 21-nation Monitoring Cardiovascular Disease Project run by the World Health Organization. "If you eat more of one thing, you eat a lot less of something else. So for every theory saying this disease is caused by an excess in x, you can produce an alternative theory saying it’s a deficiency in y." It would be simple if, say, saturated fats could be cut from the diet and the calories with it, but that’s not the case. Despite all expectations to the contrary, people tend to consume the same number of calories despite whatever diet they try. If they eat less total fat, for instance, they will eat more carbohydrates and probably less protein, because most protein comes in foods like meat that also have considerable amounts of fat.

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 triglyceride 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 increases hunger later because insulin overreacts to the spike in blood sugar. "The high insulin levels cause the nutrients from the meal to get absorbed and very avidly stored away, and once they are, the body can’t access them," says David Ludwig, director of the obesity clinic at Children’s Hospital Boston. "The body appears to run out of fuel." A few hours after eating, hunger returns.

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 naive" 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."

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