Archive 2005 2006 2007 2008

Making Sense of the Low-Fat Diet News

In February a long-awaited study on low-fat diets made front-page headlines across America: "Low-fat diet does not prevent cancer or heart disease as promised" was the verdict. The study had sterling credentials, coming from the federal Women’s Health Initiative (WHI), the group whose findings a few years ago led many women to stop hormone therapy. The numbers were impressive—more than 48,000 women, followed for eight years, at a cost of $415 million. But almost immediately critics started pointing out the study’s shortcomings and limitations. Now that the dust has settled, here’s what we think you need to know.

The study’s three parts looked at the effect of a low-fat diet on the risk of breast cancer, colorectal cancer, and cardiovascular disease in women aged 50 to 79 at the start of the study. One group was put on a low-fat diet (with individualized counseling, fairly intensive during the first year), while a comparison group was able to eat whatever they wanted. The women were not encouraged to cut calories, even though most were overweight or obese. After eight years both groups had similar rates of breast and colorectal cancer as well as cardiovascular disease.

That seems pretty clear-cut. But there are some real problems with this picture:

• The low-fat diet was not really low in fat. The goal was for the low-fat women to cut their fat intake so that it provided 20% of their daily calories. When they started, 38% of their calories came from fat. After one year, fat contributed 24% of calories, on average, but by year six this rose to 29%—only 8% less than the control group. They were also supposed to eat more fruits and vegetables, but ended up eating only one extra daily serving. In the end, the differences between the two diets were relatively small. Perhaps a lower fat intake would have yielded health benefits, especially if coupled with more produce. But one thing was clear: it’s very hard to get people to make big changes and stick to them.

• The low-fat diet did not differentiate between "good" and "bad" fats (between olive oil and lard, for instance). When the study was designed in the early 1990s, many nutrition experts (including the Wellness Letter) focused mainly on reducing overall fat intake. Since then the emphasis has shifted to cutting down on "bad" fats (saturated fat found mostly in animal products, as well as trans fat in many processed foods and fast food) and replacing them with "good" fats (unsaturated fat, as in nuts, fish, and nonhydrogenated vegetable oils). That’s the advice from the federal dietary guidelines, which do not focus much on total fat intake. Yes, the women cut down slightly on saturated and trans fat, but they cut down equally on the potentially protective fats. Thus, the women had only small drops in blood cholesterol.

The big "what ifs"

No study, even one this large and expensive, can answer every question, but this one leaves some important questions unanswered. What if the women had changed their diet at younger ages? The reduction in fat may have been too late as well as too little. What would have happened had the women lowered their fat intake more? The women who lowered their saturated and trans fat the most did have the biggest drop in LDL ("bad") cholesterol. Would the results be the same in men, who tend to get heart disease earlier? The women were all relatively healthy. Would less healthy people benefit? Would there have been benefits if the women had been encouraged to lose weight (they lost only a pound after eight years) and exercise more? Diet without weight loss may not provide significant benefits in the overweight.

Finally, would a longer study have had positive results? Eight years is long, but cancer can take decades to develop. Researchers will continue to follow the women and further analyze the data, and it won’t be surprising if reductions in risk turn up.

If not low-fat, what?

Advice to cut way down on fat (or carbohydrates, for that matter) is nice and simple. But this study shows, as do many others, that it’s very difficult to follow such advice for long, even with counseling. Moreover, who can figure out the percentages of calories that come from fat in a day’s worth of food?

The following advice sounds more complicated, but it isn’t. It focuses on real foods and doesn’t require a calculator or nutritionist. It has been our standard advice for years.

Fruits, vegetables, and whole grains should be your main foods, along with low-fat dairy products, fish, beans, skinless poultry, and lean meats. Studies show that people who eat this way have a reduced risk of heart disease, diabetes, and possibly cancer. Such a diet is similar to the DASH plan, which is proven to lower high blood pressure. It can also help with weight control, especially if you limit portion sizes and start to exercise more.

•"Good" carbs: Eat plenty of fruits, vegetables, beans, and whole grains. These high-fiber, high-carbohydrate foods are nutritious, filling, and relatively low in calories.

• "Bad" carbs: Limit your intake of sugary foods, refined-grain products such as white bread, and salty snack foods such as chips.

• "Good" fats: Eat more fish and nuts, which contain healthy unsaturated fats. Substitute olive or canola oil for butter or trans-fat-containing margarine. But don’t go overboard, since all fats are high in calories.

• "Bad" fats: Cut down on animal fat (high in saturated fat). Cut way down on trans fats, supplied by the partially hydrogenated vegetable oils used in many packaged foods and fast foods. Limit cholesterol, found only in animal products.

• Keep portions moderate, especially of high-calorie foods. In recent years serving sizes have ballooned, particularly in restaurants.

• Exercise and be active for weight control and, ultimately, good health.

Bottom line: What you do eat is as important as what you avoid. How much you eat is as important as what you eat. And finally, when it comes to good health, being physically active is probably as important as what you eat.

UC Berkeley Wellness Letter, May 2006