About Rationally Speaking

Rationally Speaking is a blog maintained by Prof. Massimo Pigliucci, a philosopher at the City University of New York. The blog reflects the Enlightenment figure Marquis de Condorcet's idea of what a public intellectual (yes, we know, that's such a bad word) ought to be: someone who devotes himself to "the tracking down of prejudices in the hiding places where priests, the schools, the government, and all long-established institutions had gathered and protected them." You're welcome. Please notice that the contents of this blog can be reprinted under the standard Creative Commons license.

Tuesday, February 14, 2006

Low-fat diets and the nature of science

So, apparently a low-fat diet does not help women prevent breast cancer, colon cancer, or heart disease. This was the surprising result of a eight-year study of 49,000 women, recently published in the Journal of the American Medical Association. And now people are confused. What do you mean low-fat doesn't make us healthy? Everybody knows that, and look at the huge low-fat products industry!

Of course, part of the beauty of science is that sometimes it contradicts “what everybody knows,” and it does so data in hand. One can bring up all the common wisdom and occasional anecdotes one wants, but eight years and 49,000 subjects is a lot of data to argue against. Or is it? Like any scientific study, the one published in JAMA has limitations. For one thing, it applies to women, and therefore says little or nothing about men's physiological reactions to the same diets. Second, the authors of the study looked at total fats, while the recent literature has argued that it is crucial to distinguish between unsaturated (good) and saturated (bad) fat, especially since it is the latter that causes increases in cholesterol, and presumably leads to related health problems such as heart disease.

Another point to understand in evaluating the results of the study is that even if (as it seems the case) a total low-fat diet does not prevent the diseases in question, it doesn't follow that a high (as in, humongous) fat diet will do no harm. The JAMA paper simply cannot be interpreted as a license to stop going to the gym and binge on fried chicken.

Fine, but what's the average educated reader of the New York Times to do with this information? Can't scientists figure things out once and for all and tell us what to do? Well, no, at least not most of the time. Mind you, at this point it is pretty much certain that the earth isn't the center of the universe, we can all count on that. But when it comes to complex problems, such as human physical and mental health, the best we can do is to read the latest findings, try to understand the inevitable limitations of the study, and mull over the possible implications for our personal choices. This is why science literacy is important in modern societies. This is why both scientists and journalists have an obligation to talk to the public and explain not just the specifics of a particular study, but the general nature of the scientific enterprise. Ignorance is not bliss, but poorly understood information can kill you almost as effectively.


  1. Good blog. My own personal experience is that low fat diets are not healthy. I personaly believe in the low carb diet. I know we have all heard of the cave man diets and such. But think about it, man has been around for anywhere from 2 to 8 million years. Carbs (like bread and pasta) were only invented approx 5 thousand years ago when we started to farm. What did the human eat in his latest part of evolution (from 2 million to 5 thousand years ago). Take a walk through the woods and try to find edible carbohydrates. You will see protiens and fats in every animal and fish scuddling about. But you wont find any bread. Even fruits and vegetibles are extreemly scarce in nature. unless we farm them they would be no sustaining supply for our ancestors to eat. So my guess is that man ate meat, and lots of it. My guess is that future scientific studies will show us that protiens and fats are healthy. All foods are made of one or more of three things, carbs fats or protien. So low fat means either high carb or high protien (or both) but the low fat foods we produce do not contain high protien (due to cost) so they are high carb. And as I said, I don't believe our bodies evolved to handle high carb. Since you really don't find carbs in nature. Unless you bend over and start eating grass like a horse. but we know were not equipped for that. Although humans are omnivors, My personal believe is that we are carnivors first and omni as a fall back.

  2. I was saddened to see you repeat the claims of low-fat advocates rather than look critically at the evidence. While this was supposed to be the study to end all studies and settle this once and for all, and the researchers carefully designed it well, and were certain it would prove claims of the benefits of low-fat diets (saturated fats were about 8% of the diet), it didn't. But for those who've followed the research for decades, this was not the least bit surprising because the bulk of the sound clinical and epidemiological research has not supported the benefits of low-fat (regardless of the type of fat) for heart disease or cancers or all-cause mortality. And when one carefully looks at the studies claiming to find a link between fat and disease, they are riddled with special interest bias and uncredible methodology. For example, the American Cancer Society studies that eliminate 90% of the population in their computer modeling in order to get the results they want (as a top CDC health expert has documented in medical journals) and other hyjinks. The body of sound scientific evidence doesn't support popular fears of fat or any "bad" food.

    Even the most skeptic among us can let our beliefs color our objectivity and be taken in by marketing and media spin, especially if it's a topic we don't work in.

  3. I am curious why you say I am repeating the claims of low fat advocates. I am saying the exact opposite. I approve of high fat high protien diets. Perhaps you missed my point.
    I said low carb not low fat. My wife and I did this low carb diet (high fat and protien) all the bacon you want. My wife has P.C.O.S. a disease similar to diabetes. she must take medication to get her period otherwise she won't. After starting the diet, she no longer needed the medication, as well as she lost all her other P.C.O.S. symtoms. For myself, I used to eat a roll of Tums antiacid every day. After about a week I no longer needed the Tums. Studies are great. But there is nothing like personal experience to make up your mind. Not to mention the weight loss that came with.


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