The Truth Behind Fat in Your Diet

Dietary fat-fat in the food you eat-has acquired a really bad rep over the last few decades. Results from the largest clinical study in U.S. history, published February 8, 2006 in the Journal of the American Medical Association (JAMA), however, indicate that fat does not deserve all the negative attention it has received. According to the study, maintaining a low-fat diet apparently does not significantly reduce a woman's risk of developing heart disease, colon cancer, or breast cancer.

Part of the Women's Health Initiative (WHI), a project run by the National Institutes of Health, the study tracked about 49,000 women over the course of 8 years. Since such a massive study is hugely expensive, this one will likely serve as the last word on low-fat diets and health for the foreseeable future. "We usually have only one shot at a very large-scale trial on a particular issue," Dr. Michael Thun, director of epidemiological research at the American Cancer Society, told the New York Times. Realizing how high the stakes were, the study's architects were extra careful, trying to make the data they collected as close as possible to airtight. Thun called the WHI trial "the Rolls-Royce of studies."

Women who enrolled in the trial were divided randomly into two groups; 40% were assigned to the low-fat diet group, while the other 60% were instructed to stick to their regular eating habits. At the outset, all the women had similar eating habits, with fat making up about 38% of their total calories. Participants in the low-fat diet group were supposed to reduce their fat intake to 20%, and eat more fruits, vegetables and grains. Throughout the study the two groups consumed about the same number of calories; the goal was to control what people ate, not how much. To help the low-fat group change its habits and stick with the diet, the WHI sent members to group sessions with trained nutritionists-18 sessions during the first year of the study, and four sessions per year after that. At the sessions, women were instructed to record what they ate each day and learned to read food labels, what foods to avoid, and healthy habits like controlling portions and taking doggie-bags home from restaurants rather than eating entire dishes.

Few members of the low-fat group ever got down to the 20% mark, however, which some scientists note as a possible reason for the diet's failure to ward off disease. During the first year of dieting the group's fat intake averaged 24% of total calories, and by the end of the study it had climbed to 29%. This meant that the gap between the low-fat group and the comparison group narrowed over time-initially the low-fat group ate 10.7% less fat, but by the end the difference was only 8.1%.

Keeping tabs on how well participants maintain a diet regimen poses a serious challenge to every clinical study on nutrition. For the WHI study, researchers used a number of methods to track the women's adherence (how well they stuck, or adhered, to the diet). These included questionnaires, food diaries and blood tests. Questionnaires and food diaries, the standard tools clinicians use to track diet studies, have a high margin of error; they rely upon people remembering and accurately reporting what they ate, but dieters often forget some foods or underestimate food quantities. Blood tests, which were administered periodically throughout the study, measured levels of certain nutrients found in vegetables, as well as a chemical called y-tocopherol, a micronutrient found in fatty foods. Although these numbers give only a rough impression of a person's diet, researchers found that they overwhelmingly matched women's claims about their food intake, and thereby provided objective data to back up participants' reports. A number of experts in clinical study design agreed that, considering the difficulty of collecting good data on people's daily eating habits, the study was well-designed and should be taken seriously.

Sources
Beresford, Shirley et al. "Low-Fat Dietary Pattern and Risk of Colorectal Cancer." JAMA, February 8, 2006, page 643.

Couzin, Jennifer. "Study Finds Little Benefit From Low-Fat Diets." ScienceNOW, (February 7, 2006) sciencenow.sciencemag.org/ cgi/ content/ full/ 2006/ 207/2.

Davidow, Julie. "Low-Fat Diets Didn't Lower Cancer Risks." Seattle Post-Intelligencer, (February 8, 2006) seattlepi.nwsource.com/ health/ 258735_diet08.html.

Dooren, Jennifer Corbett. "Low-Fat Diet Doesn't Reduce Diseases in Women." Wall Street Journal, February 8, 2006, page D5.