What if bad fat isn’t so bad?

"In the nutrition field, it's very difficult to get something published that goes against  established dogma," says Mozaffarian. "The dogma says that saturated fat is harmful, but that is not based, to me, on unequivocal evidence." Mozaffarian says he believes it's critical that scientists remain open minded. "Our finding was surprising to us. And when there's a discovery that goes against what's established, it shouldn't be suppressed but rather disseminated and explored as much as possible."

Biased studies
Perhaps the apparent bias against saturated fat is most evident in studies on low-carbohydrate diets. Many versions of this approach are controversial because they place no limitations on saturated-fat intake. As a result, supporters of the diet-heart hypothesis have argued that low-carb diets will increase the risk of heart disease. But published research doesn't show this to be the case. When people on low-carb diets have been compared head-to-head with those on low-fat diets, the low-carb dieters typically scored significantly better on markers of heart disease, including small, dense LDL cholesterol, HDL/LDL ratio, and triglycerides, which are a measure of the amount of fat circulating in your blood.

For example, in a new 12-week study, University of Connecticut scientists placed overweight men and women on either a low-carb or low-fat diet. Those who followed the low-carb diet consumed 36 grams of saturated fat per day (22 percent of total calories), which represented more than three times the amount in the low-fat diet. Yet despite this considerably greater intake of saturated fat, the low-carb dieters reduced both their number of small, dense LDL cholesterol and their HDL/LDL ratio to a greater degree than those who ate a low-fat diet. In addition, triglycerides decreased by 51 percent in the low-carb group — compared with 19 percent in the low-fat group.

This finding is worth noting, because even though cholesterol is the most commonly cited risk factor for heart disease, triglyceride levels may be equally relevant. In a 40-year study at the University of Hawaii, scientists found that low triglyceride levels at middle age best predicted "exceptional survival" — defined as living until age 85 without suffering from a major disease.

According to lead study author Jeff Volek, Ph.D., R.D., two factors influence the amount of fat coursing through your veins. The first, of course, is the amount of fat you eat. But the more important factor is less obvious. Turns out, your body makes fat from carbohydrates. It works like this: The carbs you eat (particularly starches and sugar) are absorbed into your bloodstream as sugar. As your carb intake rises, so does your blood sugar. This causes your body to release the hormone insulin. Insulin's job is to return your blood sugar to normal, but it also signals your body to store fat. As a result, your liver starts converting excess blood sugar to triglycerides, or fat.

All of which helps explain why the low-carb dieters in Volek's study had a greater loss of fat in their blood. Restricting carbs keeps insulin levels low, which lowers your internal production of fat and allows more of the fat you do eat to be burned for energy.

Yet even with this emerging data and the lack of scientific support for the diet-heart hypothesis, the latest AHA dietary guidelines have reduced the recommended amount of saturated fat from 10 percent of daily calories to 7 percent or less. "The idea was to encourage people to decrease their saturated-fat intake even further, because there's a linear relationship between saturated-fat intake and LDL cholesterol," says Alice H. Lichtenstein, Ph.D., Sc.D., who led the AHA nutrition committee that wrote the recommendation.

What about Krauss's findings that not all LDL is equal? Lichtenstein says that her committee didn't address them, but that it might in the future.

It could be that it's not bad foods that cause heart disease, it's bad habits. After all, in Volek's study, participants who followed the low-fat diet — which was high in carbs — also decreased their triglycerides. "The key factor is that they weren't overeating," says Volek. "This allowed the carbohydrates to be used for energy rather than converted to fat." Perhaps this is the most important point of all. If you consistently consume more calories than you burn, and you gain weight, your risk of heart disease will increase — whether you favor eating saturated fats, carbs, or both.

But if you're living a healthy lifestyle — you aren't overweight, you don't smoke, you exercise regularly — then the composition of your diet may matter much less. And, based on the research of Volek and Dr. Krauss, a weight-loss or -maintenance diet in which some carbohydrates are replaced with fat — even if it's saturated — will reduce markers of heart-disease risk more than if you followed a low-fat, high-carb diet.

"The message isn't that you should gorge on butter, bacon, and cheese," says Volek. "It's that there's no scientific reason that natural foods containing saturated fat can't, or shouldn't, be part of a healthy diet."