Eat less saturated fat: that has been the take-home message from the U.S. government for the past 30 years. But while Americans have dutifully reduced the percentage of daily calories from saturated fat since 1970, theÂ obesity rate during that time has more than doubled, diabetes has tripled, andÂ heart disease is still the countryâ€™s biggest killer. Now a spate of new research, including a meta-analysis of nearly two dozen studies, suggests a reason why: investigators may have picked the wrong culprit. Processed carbohydrates, which many Americans eat today in place of fat, may increase the risk of obesity, diabetes and heart disease more than fat doesâ€”a finding that has serious implications for new dietary guidelines expected this year.
In March theÂ American Journal of Clinical Nutrition published a meta-analysisâ€”which combines data from several studiesâ€”that compared the reported daily food intake of nearly 350,000 people against their risk of developing cardiovascular disease over a period of five to 23 years. The analysis, overseen by Ronald M. Krauss, director of atherosclerosis research at the Childrenâ€™s Hospital Oakland Research Institute, found no association between the amount of saturated fat consumed and the risk of heart disease.
The finding joins other conclusions of the past few years that run counter to the conventional wisdom that saturated fat is bad for the heart because it increases totalcholesterol levels. That idea is â€œbased in large measure on extrapolations, which are not supported by the data,â€ Krauss says.
One problem with the old logic is that â€œtotal cholesterol is not a great predictor of risk,â€ says Meir Stampfer, a professor of nutrition and epidemiology at the Harvard School of Public Health. Although saturated fat boosts blood levels of â€œbadâ€ LDL cholesterol, it also increases â€œgoodâ€ HDL cholesterol. In 2008 Stampfer co-authored a study in theÂ New England Journal of Medicine that followed 322 moderately obese individuals for two years as they adopted one of three diets: a low-fat, calorie-restricted diet based on American Heart Association guidelines; a Mediterranean, restricted-calorie diet rich in vegetables and low in red meat; and a low-carbohydrate, nonrestricted-calorie diet. Although the subjects on the low-carb diet ate the most saturated fat, they ended up with the healthiest ratio of HDL to LDL cholesterol and lost twice as much weight as their low-fat-eating counterparts.
Stampferâ€™s findings do not merely suggest that saturated fats are not so bad; they indicate that carbohydrates could be worse. A 1997 study he co-authored in theJournal of the American Medical Association evaluated 65,000 women and found that the quintile of women who ate the most easily digestible and readily absorbed carbohydratesâ€”that is, those with the highest glycemic indexâ€”were 47 percent more likely to acquire type 2 diabetes than those in the quintile with the lowest average glycemic-index score. (The amount of fat the women ate did not affect diabetes risk.) And a 2007 Dutch study of 15,000 women published in theÂ Journal of the American College of Cardiology found that women who were overweight and in the quartile that consumed meals with the highest average glycemic load, a metric that incorporates portion size, were 79 percent more likely to develop coronary vascular disease than overweight women in the lowest quartile. These trends may be explained in part by the yo-yo effects that high glycemic-index carbohydrates have on blood glucose, which can stimulate fat production and inflammation, increase overall caloric intake and lower insulin sensitivity, says David Ludwig, director of the obesity program at Childrenâ€™s Hospital Boston.
Will the more recent thinking on fats and carbs be reflected in the 2010 federal Dietary Guidelines for Americans, updated once every five years? It depends on the strength of the evidence, explains Robert C. Post, deputy director of the U.S. Department of Agricultureâ€™s Center for Nutrition Policy and Promotion. Findings that â€œhave less support are put on the list of things to do with regard to more research.â€ Right now, Post explains, the agencyâ€™s main message to Americans is to limit overall calorie intake, irrespective of the source. â€œWeâ€™re finding that messages to consumers need to be short and simple and to the point,â€ he says. Another issue facing regulatory agencies, notes Harvardâ€™s Stampfer, is that â€œthe sugared beverage industry is lobbying very hard and trying to cast doubt on all these studies.â€