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G’bye to low-fat diets

Posted by medconsumers on July 21, 2011

Is it just me? Or are we all hearing less and less about the importance of the low-fat diet? First, it was advised only for people who’ve had a heart attack. In time, the low-fat diet would be recommended for everyone, even children, as a good way to avoid heart disease. Eventually, there would be some serious back-peddling where it concerned babies and toddlers who, as it turns out, need lots of fat in their diets. Then there were those hard-to-explain population studies that show a lower incidence of stroke among people on high-fat diets. here’s one  Over the last decade—without any official retraction from the government or the cardiology organizations—there is less emphasis on the importance of the low-fat diet. It’s long overdue.

A new Cochrane review of all relevant clinical trials shows that the evidence supporting the low-fat diet is thin…and always has been. In fact, the reviewers found no good research to answer the most basic of questions: Are monounsaturated (e.g., olive oil, canola oil, nuts, avocados, etc) or polyunsaturated fats (plant foods and oils) healthier than saturated (animal) fats? The review acknowledges the negative, but unintended, consequences of telling the healthy public to reduce fat intake. It led many Americans to consume more carbohydrates, particularly starchy, sugary refined foods.

Ten years ago, the first Cochrane review of all low-fat diet/heart disease prevention studies was published in the British Medical Journal. Here is how the findings were expressed at the time:

“Despite decades of effort and many thousands of people randomized [into clinical trials], there is still only limited and inconclusive evidence of the effects of modification of total, saturated, monounsaturated, or polyunsaturated fats on cardiovascular morbidity and mortality.”

The existing research hasn’t improved much in the intervening ten years. The new Cochrane review, which is an updated version of the 2001 review, was published recently in the Cochrane Library (online subscription-only library). The Cochrane reviewers identified 48 studies with a combined total of 65,508 participants, including studies published in the last decade. No studies proved that people would live longer, avoid a stroke or heart attack, or showed anything else that might motivate people to alter the fat content of their diets.

As with all Cochrane reviews, the best studies are identified and results are combined. In this case, all the bad outcomes you might like to avoid—heart and vascular disease, cardiovascular death, heart attack, stroke, angina, and the need for heart surgery—-were also combined. Once that was done, an unimpressive 7% reduction in bad outcomes was noted among the study participants on a reduced fat diet, compared with those randomly assigned to continue their usual diet. Generally speaking, diet studies are difficult to conduct and costly. And they usually don’t last very long. Two years is the longest duration of any study in this review and it largely accounts for the aforementioned 7% reduction. Yet the Cochrane reviewers saw this small reduction as a good sign that some form of a low-fat diet might be shown to be heart healthy had the participants been followed several years longer.

This updated Cochrane review entitled, “Reduced or modified dietary fat for preventing cardiovascular disease,” was conducted by Lee Hooper, PhD, University of East Anglia in the U.K. and colleagues at other British universities. The review did not disprove the idea that the low-fat diet is best for the heart. Rather, it found no strong evidence to support this longstanding public health message, which no doubt led many healthy people with high cholesterol to take cholesterol-lowering statin drugs unnecessarily. (Ever since the early 1990s, healthy people with elevated blood cholesterol are usually told to go on a low-fat diet.  It has long been known that two out of three of them will fail to get it down to normal with a low-fat diet and exercise. Statins are the next step—see current ads for Lipitor.)

What’s the take-home message from this Cochrane review? Uncertainty is the short answer. Unfortunately, the low-fat message to the public was delivered with a certainty that never existed. It has probably made many people take all other nutrition advice with a grain of salt.

In the early days of dietary fat studies, researchers relied heavily on the less definitive form of research called population studies, aka observational studies.  The researchers observed that people in some countries with low dietary fat intake also had low rates of heart disease and came to the conclusion that the low-fat diet is the key to avoiding heart disease.  As time went on, researchers tried to tease out factors, other than fat intake, that could account for a country’s lower rate of heart disease, for example, red wine drinking in France.  The next step is the type of study used in medical treatment research—participants are  randomly assigned either to go on a low-fat diet or stay on their usual diet, for example, the aforementioned two-year study in the Cochrane review.

Where do we go from here? You can’t go wrong eating real food. If you’re not sure what that means, read Michael Pollan’s Food Rules. If you want to know more about how the low-fat diet became standard medical advice, read our 2008 review of  Good Calories, Bad Calories by Gary Taubes.

Maryann Napoli, Center for Medical Consumers©

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