Almost everything that we thought we knew about fat is wrong.

The “really bad fats,” according to Barry Sears of The Zone fame, are saturated fats, trans fats, and arachidonic acid. He says that the polyunsaturated fat known as arachidonic acid “may be the most dangerous fat known when consumed in excess.”

Everyone seems to agree that trans fats really are bad for us. But new research shows that the saturated fat and arachidonic acid that we eat can actually be a good thing.

But it depends. Specifically it depends on whether we are following a low-carb or a low-fat diet.

Most health authorities, including the American Diabetes Association, still recommend low-fat diets. This is the legacy of Ancel Keys, a doctor who was all too persuasive for our own good. He persuaded the American medical establishment that fat is bad and carbs are good when those experts bought into his analysis of the “Seven Countries Study.” That study seemed to show a strong association between cardiovascular disease and intake of saturated fatty acids.

It didn’t. One of the great strengths of the new book, Good Calories, Bad Calories by Gary Taubes, is how thoroughly he exposes the flimsy evidence linking dietary fat with heart problems. The emperor, Keys, figuratively wore no clothes.

But when we follow a low-carb diet – meaning that we correspondingly increase how much fat we eat – something paradoxical happens. “You can actually eat more saturated fat and see a reduction in saturated fat in your plasma,” says Jeff S. Volek, PhD, RD, associate professor of kinesiology at the University of Connecticut.

Dr. Volek is the lead researcher of a forthcoming study, “Comparison of Low Fat and Low Carbohydrate Diets on Circulating Fatty Acid Composition and Markers of Inflammation,” in the journal Lipids. The journal thought the results were so important that it published it electronically ahead of print and an abstract is free online. Dr. Volek kindly sent me a copy of the entire article and responded to my questions in a follow-up telephone interview.

Most people find this inverse relationship between dietary fat and the fat in our blood to be counterintuitive. I certainly did before I began my study of fat metabolism.

“This clearly shows the limitations of the idea that you are what you eat,” Dr. Volek says. “Metabolism plays a big role. You are what your body does with what you eat.”

Dr. Volek and his associates studied 40 overweight men and women for 12 weeks. They compared the results of the two groups on a very low-carb diet (35 grams of net carbs and 100 grams of fat, including 36 saturated grams) with a low-fat diet (191 grams of net carbs and 24 grams of fat, including 12 saturated grams). Both groups ate about 1500 calories per day.

Despite the three-fold greater saturated fat in the diet for the low carb group, saturated fat in the blood turned out to be higher in the low fat group. This is due to the process known as carbohydrate-induced lipogenesis, which I just wrote about here at “Eat Fat, Grow Thin.”